View Diagrams  Download Paper  Download Images 


Angelino Dulceto produced some of the most beautiful Portolan Charts extant today. I have already written about them in previous texts, but, here-in concentrate upon the 1339 chart. It is thoroughly investigated through draughtsmanship techniques to unravel its inner workings, but now written in simple terms as so much is a repeat of the proofs already given vis a vis the Portolan Chart origins and construction. This chart has been used by other researchers to illustrate their findings and each of the four texts chosen is discussed separately in essay format, stand alone texts, and their findings judged against that which I have shown to be correct.
I have subdivided this first part of my text into separate topics by subsections which can again be stand alone essays. The 1339 chart is typical of portolans in covering the whole Mediterranean Sea basin and the Black Sea. It also has slightly more west N African coastline than others and extends northwards to the Baltic Sea. It is in fact a development chart.


Many researchers, including myself have noted and commented upon the fact that the Iberian Peninsula is normally drawn geographically NSEW matching the wind rose graticule and in part has a scale which varies from that of the main chart. Thus this first essay deals with the measurements used for the western seabord of Europe from Iberia to the UK as shown on the 1339 chart. The essay commences with a brief explanation of the various measurements and their interactions, before discussing those found on the 1339 chart in the second essay.


The original title in Greek is, “Geographike Hyphegesis”, or “Guide to drawing a World map”. The unit of distance used was the Stade understood by Ptolemy to be c185 metres, and following the work of Marinus the Tyrian the degree of latitude was given the equivalence of 500 Stades, and problems began which lasted for millennia. The Roman Stade was one eighth of a Roman Mile, and the Roman World measure for a degree of latitude was 600 Stadia or 75 Roman Miles; so very accurate and easily understood and used.
Thus we can immediately observe the dichotomy often discussed that the actual world is 600 Stadia per degree of latitude but Ptolemy chose to use a smaller world size of 500 Stadia per degree. The ratio of 6:5 is therefore writ large on the historical world measure.


It has been clearly shown that the basic measurement for distances on a Portolan Chart is the Millara of c1.233KM, derived from the Roman Mile, Mpm of 5000 paces, which is 1.4791KM. The ratio between the distances is therefore Roman Mile/ Millara = 6:5 and accurately is 1.4791Km to 1.232583Km, the Millara, which can be noted as c1.23 or c1.233Km.
Therefore it is obvious whence came the basic information for the 6:5 ratio.


The principal Miglio for road and sea distances throughout Italy, Sicily and Sardinia is derived from the Roman Mile of 8 Stadia, 1000 double steps totalling 5000 Pes or 1.4791Km. By the later middle ages many variants had arisen of which the following were the most important: PALERMO; 1.487Km consisting of 45 corde or 720 canne or 5760 palmi. In GENOA and ALBENGO it was 1.488Km consisting of 1000 passi or 6000 palmi. But importantly as we shall see at GENOA slightly later a Miglio Marritimo developed and was c1.852Km.
The difference between 1.4791, 1.487 and 1.488km is merely 8 and 9 metres and is thus negligible in our assessments of the distance measures used on Portolan Charts. But we can quantify them by ratio as follows;
1.233Km Millara to c1.852 M M = 2:3 ratio. Actual figures are 1.233 and 1.8495Km.
1.4791Km RM to c1.852 M M = 4:5 ratio, and thus the Marritimo Miglio is 1 ½ times the length of the Millara and 1 ¼ times the length of the Roman Mile. The overall ratios are, 5:6 – 2:3 – 4:5.


Roman Mile = 1.4791Km (c1.48Km) = 8 stadia of 184.8875metres (c185m)
Millara = 1.23258 Km (c1.233km) = 6.667 stadia
Ratio Roman Mile to Millara = 6:5
Roman World = 75 RM per degree of Latitude = 600 stadia (300BCE – 300AD)
Ptolemaic World = 62.5 RM per degree of latitude = 500 stadia (c150AD)
Ratio Roman World to Ptolemaic World = 6:5
Millara Geog degree = 90 ml. i.e. world = c111Km/degree = 90 x 1.233Km.
Millara short degree = 75ml. i.e. 92.5km/degree = 75 x 1.233Km.
Ratio = 6:5
Millara = 1.233Km or 6.667 stadia
Marritimo Miglio = 1.8495Km or 10 stadia
Ratio = 2:3
Marritimo Miglio per degree (MM)
Millara degree = 90ml but as MM = 60MM or 600 stadia
Short degree = 75ml but as MM = 50MM or 500 stadia.
The ratios are all naturally 6:5 as they are the same units of basic measure.

The first and only time the ratio 6:5 was introduced into world measures was by Marinus the Tyrian c100AD and then expressly used by Claudius Ptolemy c150AD in his “Geographia”, as he clearly states.
It is therefore quite clear that the Portolan Chart standard measurement of 1.233Km is actually derived from the Ptolemaic world degree of 500 stadia, and thus is 75 Millara and mimic’s the Roman World measure of 75 Roman Miles, and the later Marritimo Miglio evolved for ease of measurement to 10 stadia equalling 1.8495km.
Thus it is quite open to opine that the “Portolani” and its accompanying “Portolan Chart” were originally conceived in the period following the Roman Empire from Roman Itineraries, normally given in Stadia, and the Roman Map of the Mediterranean Sea basin, no doubt also scaled in Stadia which were converted to Millara via the known Ptolemaic reduction ratio. Thus the text of “Geographia” was probably known but not the maps that could be drawn from its data as they would have been shown to be false. Thus it is a simple progression of information from the Roman/Alexandrian geographers to the mariners.




It has been remarked by many researchers that the scale utilized for the western coastline of Iberia was different from the main scale of the chart. Thus it is necessary to subject not only this area but also the whole Atlantic coastline from N Africa to the UK to a quantitative analysis to find the putative latitudinal scale used. There are two obvious alignments which can be used to aid the research; firstly the Strait of Gibraltar at 36N and secondly the Sacred Promontory, Cape St Vincent which is set at 9W – 37N. One researcher has insisted on the use of 37.02N and 8.98W, thinking the difference on a Portolan Chart of such small scale will be capable of such finite measurement: I discuss later this later.
The Iberian Peninsula is latitudinally from those two, 36N or 37N to 43N, which passes through the north-western capes; Cape Finisterre being 42.87N and Cape Tourinan being 43.07N. The total peninsula actually stretches slightly further north with Cape Ortegal being at 43.77N. Thus we have simple distance measures from 36 – 43 or 37 – 43 to check as latitudinal measures. To do this it is necessary to assess the putative latitudinal alignments themselves as we have no guarantee they are equally spaced a consistent distance apart.
The first diagram, ChDUL/1/D01 illustrates the putative latitudinal parallels and using the main chart scale the distances apart can be measured. They are as follows; 75-90-90-75-65-55-70-85-75 millara and span from 35N to 44N latitudes. Bearing in mind they are just basic freehand alignments, not refined exactly to a coastal town of feature which we can evaluate from a geographical map, but merely a simple projection across the peninsula to gauge the average latitude, it takes little evaluation to note that the average is a calculated 75.5 millara as scaled.
However before evaluating that measurement the chart area for France and the UK must be studied and the putative latitudinal distances noted. Thus diagram ChDUL/1/D02 has appended the parallels from 44N and they can be read as follows; 44/45 = 140ml; then 9 degrees of 75ml to 54N; then 54/55 = 70; 55/56 = 80; 56/57 = 75; 57/58 = 70 and 58/59 = 80 millara. Thus from 45 to 59N it is possible to clearly state the latitudinal measurement is 75 millara per degree. However it is obvious that the coastline from Iberia to France and the Gironde estuary is incorrectly drawn. If the actual latitudinal degree of c111Km is converted to Millara it is either 90.24 at 1.23km or 90 at 1.233Km. Thus we should read from 35N to 45N, 900 or 902.4 millara, but instead we read 9 x 75 + 1 x 140 = 820 millara an average of 82 millara per degree. Please note the difference in length, 90 or 90.24 is irrelevant.
Ever curious about measurements, we have 75 – 82 – 90, and thus the 82 is nearly the mid-point. That will make sense when the following is studied, as the actual measurement of 75 millara per degree as measured on the chart, no maths involved, is accuracy and a curiosity itself.


A basic comparison can be made to explain it, simplicity itself!
The geographic latitudinal degree is 75 Roman Miles or Miglio. The ratio of 75 Millara to 75 Roman Miles is of course 5:6 as already determined. But what does 75 Millara represent? Quite obviously from the foregoing it is 5/6ths of the geographical degree which is itself 600 stadia. Thus the 75 millara is 500 stadia which is the distance Claudius Ptolemy assigned to the latitudinal degree in c150AD. The questions now to be asked are; 1) did the Millara evolve solely from the Ptolemaic ratio of 6:5, as it was known from the “Geographia” of Ptolemy and was obviously known in Italy from Roman times?; 2) and therefore, was the use of the degree of 75 millara merely an erroneous measure, the knowledge of the 75 RM degree being merely transcribed to 75 Millara?: 3) or, was the 75 millara degree a deliberate choice as it was known to be 500 stadia when 75 RM was 600 stadia?
Without a Portolan Chart having all scale bars appended I doubt that can be answered, but a theoretical argument is possible to mount regarding its usage. This I have already shown in the foregoing summation section as perpetuating the Ptolemaic 6:5 ratio of latitudes.
Return to diagram ChDUL/1/D02 and the putative latitudinal parallels; they are near perfect, represent the Ptolemaic degree with the added plus of a non-Ptolemaic form for the UK which is placed correctly in terms of latitude on the Atlantic Seabord of Europe, from 50N to 58.5/59N.


To assist this research I have included as diagram ChDUL/1/D03 a copy of a Ptolemaic map drawn in 1470AD of Iberia to the Ptolemaic co-ordinates given in “Geographia”, which obviously has all latitudes drawn as 500 stadia.


As the extant “LCN” type “Portolani” do not cover the western seabord of Europe it was probably up to individual draughtsmen to dimension this coastline from the data available. That data was no doubt originally Roman, used by Ptolemy (who as I have shown in my CP/1 to CP/4 texts obviously had mis-copied the longitudes of Iberia) and later drawn by Roman scribes. Therefore I do not believe that this is a medieval draughtsman’s work but the original scribe who drew the charts from the accompanying Roman Itineraries which morphed into the “Portolani” and Portolan Charts themselves. Thus it is certain that Rome had copies of Ptolemy, believed some but not that which their world survey c55BCE had confirmed as geographical, hence the UK profile which is not Ptolemaic.



To evaluate the foregoing I have decided to compare the first Portolan Chart to cover the whole of the Mediterranean Sea basin and the Black Sea available which is the Riccardiana ms 3827 Chart: it is reference C4 in Les Cartes Portolanes. I have drawn the matching chart sections for ms3827 and Dulceto 1339 as overlays and appended their distance measures as per their scale bars.


The two charts were obviously not originally the same scale and hence I have reduced C4 by c10% to align both and provide an accurate comparison. The results are simply unequivocal; both west coasts are drawn using the 75 millara/500 stadia degree for the Iberian Peninsula but then there is a spectacular divergence of measurement. Where-as Dulceto continues the 75/500 latitudes to cover the northern section from 43N, the C4 chart increases the degree to firstly 100 millara( query draughtsman’s error) and then 7 degrees at 110 millara; extra-ordinary but so very simple and very clever. (diagrams ChDUL/1/D04 and D05). Firstly the 110 millara is c135.3Km and the standard degree of c111km is 90/90.24 millara. However, via simple mathematics using the above units of Roman Mile and Millara, their 6:5 ratio, if it is applied to the 90.244Km degree and the 110 millara/135.3Km we have; via 5:6, 90.24 = 108.3. The chart scales 110 millara which per degree is only 1.7 millara, or over the 7 degrees 11.9 millara extra, and as such using the chart scale bars is un-noticeable and quite frankly irrelevant to the degree.( I use 90/90.24 to illustrate nothing!)
Thus the draughtsman of the C4, ms 3827 chart was fully aware of the error in latitude for the Iberian Peninsula, it should be 36/43N = 7 x 90.24 = 632 millara but is drawn as 7 x 75 millara = 525 millara and is some 107 millara short. (Diagrams ChDUL/1/D04 and D05). It is similar to the Cortona Chart (see text ChCOR/1) problem which has the mis-read measurement for the Black Sea and thus increases the original Millara length to that of Roman Miles. The draughtsman of ms3827 has merely increased the obviously wrong 75 millara twice as follows; 75 x 6/5 = 90.24 and then 90.24 x 6/5 = 108.3 millara.
Finally it should be noted that on ms3827 the latitude measure is as follows;
15 degrees latitude = 1420 millara = 1764Km
1764 millara = 1180 Miglio or Roman Miles
15 degrees latitude; Dulceto 1339 = 1180 millara
Simply put, the draughtsmen have over used the 6:5 ratio, perhaps inadvertently on their charts to correct perceived errors. This shows that there was a fundamental knowledge of all the measurements discussed but not of the actuality of the length of the Atlantic coastline. The sketches or map accompanying their “Portolani” were probably dimensioned in such a way as to be confusing with the shorthand terminology used: i.e.” ml”; does it mean Millara or Miglio, and could it even have meant the Genoese Miglio Marritimo?


Within many previous texts I have clearly indicated that the skewing of Portolan Charts is solely the result of scribal errors in noting down the Peleio distances and directions, mainly south of Sardinia. I have therefore reduced the explanations within this text merely showing the confirming distortions within the 1339 chart in the region of the western Mediterranean Sea south of the Balearic Isles from Gibraltar to Cape Bon.


Upon diagram ChDUL/1/D06 I have indicated the chart distance direction set against the geographical direction. As can be seen there is a large discrepancy vis a vis a constant deviation which could be expected if a single magnetic declination had been chosen and used. There is the usual mixture of geographical and erroneous directions, many of which can be traced to the “LCN” data.
Curiously the chart has a correct overall distance measure from Gibraltar to Cape Bon, but the individual sections are far from accurate and thus account for many of the spurious wind directions as measured and can easily be mistaken for magnetic readings if the whole research is not carried out properly. The diagram has the geographical plot of the N African coastline appended for comparative studies to be afforded to others.


However, if diagram ChDUL/1/D07 is studied, which has chart distances set against their geographical equivalents, given in Millara, the distortion of the chart is readily observed through the errors. Again we see excellent accuracy in some distance measures and completely nonsensical distances elsewhere. Returning to a theme I have previously noted, that of scribal error in copying Roman Numerals for distances, the distance along the Pyrenees given as 430 on the chart, geographically becomes 340 and thus we have CCCCXXX changing to CCCXXXX, or CCCXL, and I must wonder yet again if that is it.

A Dulceto 1339 Chart in Detail continued;

The following four diagrams illustrate the 1339 chart from the Balearic Isles to the Black Sea, and have appended the putative geographical graticule of latitude and longitude as derived from the chart itself.


Diagram ChDUL/1/D08 indicates that there are five latitudes, parallels, which are virtually East/West, being a mere 2 degrees out. However as “LCN” dictates from Cabrera to San Piero is due east/west, a geographical fact, and the wind rose graticule is perfectly aligned to that route.


On Diagram ChDUL/1/D09 I have indicated the Malta to Corfu alignment which varies from 52 on the chart to 48 geographically and is given as 45 degrees in “LCN”. The distance measure is accurate at 510 millara. This diagram however indicates the severe twisting of the Greek Mainland and the elongation of the latitudinal distance from 35N, Crete, to Thessaloniki. Reduce the charts 600 Millara distance to the correct 512 millara and the direction between Corfu and Thessaloniki, which is only 20 degrees geographical, reverts from 48 to being correct and not the visual continuation of the Malta to Corfu alignment.
Thus the distortion noted by researchers in this area can easily be quantified and thus dismissed as magnetic or other than scribal error.


The next diagram ChDUL/1/D10 illustrates the near correct Mediterranean Sea distance measures and the problem of the Aegean Sea excess latitudinal measure combining with the large errors in the latitudes given to Asia Minor which effectively skews the Black sea to the north-east even further than the general errors skew the Mediterranean Sea in the same direction.


In this section, the final diagram ChDUL/1/D11 illustrates the Black Sea, its putative graticule and confirms that it is drawn to the “LCN” distance measure of 1070 Millara, obtained when that text is used to draw the Black Sea.



If diagram ChDUL/1/D12 is studied, it indicates the Black Sea as drawn using the “Lo Conpasso de Navegare” distances and directions compared to the same scale A Dulceto 1339 chart. Naturally the correspondence is good, but, it also indicates that Dulceto was in possession of a variant “LCN” such that the area from the Danube Delta to the Crimea could be drawn correctly including the important River Borysthenes, the Europe/Asia boundary.


Being curious as to where the false distances for Asia Minor may have originated, I have analysed the Claudius Ptolemy data, as shown on diagram ChDUL/1/D13. The first point to note is that the length of the Black Sea at 43N geographical, 45N Ptolemy, is drawn correctly at 904 Millara as opposed to the charts and “LCN” distance of 1070 Millara. This c150AD map is not the origin of Asia Minor being drawn wrongly on the Portolan Charts, although it is not perfect and may have contributed slightly.


The chart follows the basic “LCN” data for its construction and is drawn by an accomplished artist who has made the data work when obviously faced with conflicting single distance measures and directions. The larger coverage of N Africa indicates new data arriving in Majorca via the Arab Traders and the northern European data that probably arrived with the Crusaders and the Hanseatic League formed around 1150AD. The ever expanding trade links were bringing new data, but let us not forget that even in Roman times trade existed with the Baltic area particularly for Amber. Thus the actual volume of overall data available probably far surpasses our knowledge of those times, that is what we have available today because of the many wars and document destruction.
The distance measures used are so consistent as to be self evidently the norm for Mediterranean mariners and hence the charts. Continuity of usage indicates ancient origins which extends across City States and the Maritime States, fractious in their dealings with each other, but it indicates in all probability the Roman World was well remembered.
Thus I believe the origination of these “Portolani” and “Portolan Charts” is with the Roman State and their data was copied and expanded over several hundred years by a burgeoning Church that became a state itself.


Prior to analysing the four texts noted it is necessary to state plainly and clearly my objections to the use of “mathematical” analysis by formulae etc of Portolan Charts. Firstly the much over used “MapAnalyst” tool with the various formulae used to illustrate the mathematical prowess of an author which actually does nothing to analyse a chart drawn by human hand.
Many texts ago I indicated that the rather excellent “MapAnalyst” tool was not appropriate for Portolan Charts as there was a lack of basic points that could be used by virtue of the fact that on a Portolan Chart very few places are actually marked definitively and thus any researcher had to nominate the position, thus it is subjective not objective. The bland averaging of any graticule found by this method must surely be visible to all, when by simply drawing the lines freehand the bland flow of the graticule is shown to be rather more linear.
Make no mistakes, “MapAnalyst” is a marvellous tool used correctly; that is its function and output are understood; if not it can be GIGO.
The use of formulae, when a Portolan Chart has a scale bar (or several) and everything can be measured and judged against known geographical distance/directions, should in a researchers mind give cause for concern when using one, it is probably unnecessary and incorrect to use.
The following should perhaps be engraved on all researchers’ minds, it is not my text.
An elementary warning should be written with letters of fire on the mind of every researcher that makes use of mathematical and statistical techniques, or the conclusions drawn with their aid by some-one else.
It has been clearly evaluated as follows;
It is true that it is extremely difficult to interpret figures when they relate to some concrete problem. Here-in lies the real difficulty; averages can be calculated to 19 places of decimals with astonishing ease. When the job is done it looks very accurate. It is an easy and fatal step to think that the accuracy of our arithmetic is equivalent to the accuracy of our knowledge about the problem in hand. We suffer from “delusions of accuracy”. Once an enthusiast gets this disease, he and all who depend on his conclusions for their welfare are damned. How easy it can be to become bemused by the apparent magic power of numbers are legion and are perhaps symbolised for the innumerate by the sight of an equation in a page of text. The sense of inadequacy produced by the failure to comprehend its meaning instantly is matched by an equal sense of awe at the mysterious and often proved power of the language of the wise to reveal the secrets of nature.

The final comment must be, “because you can use a mathematical tool, it does not mean you should as it may not prove the best solution. Ask yourself if it is necessary and bear in mind “Occam’s razor”, the simplest answer is always best.
I hope that the analysis of the following texts will illustrate the above points and ensure research is carried out systematically by analysis of the charts inner workings without resorting to spurious tools, but by the same methodology as the original author who drew the chart; he used draughtsmanship to create the chart, analyse it the same way.



I have lost count of the number of texts which imply, but never prove that the Portolan Chart circa 1300/1500 was drawn using a magnetic compass bearing, a declination to align sailing routes and thus the chart itself. The subject of the variation in the magnetic north direction is used to infer that because these charts have such a variety of distortions that they must have all been drawn using a variation of the declination across the Mediterranean.
Many so called experts who purport to have carried out serious research into the form of a Portolan Chart have concluded that the skewing of the chart is from that perceived magnetic declination.
Thus the first text written in 2008 and full of mathematical jargon will be used to illustrate the above points as it chose to use the 1339 chart just analysed for its subject.


Quote,summary; For more than two centuries much has been written about the origin and method of construction of the Mediterranean Portolan charts, still these matters continue to be the object of some controversy as no one explanation was able to gather unanimous agreement among researchers. If some theory seems to prevail, that is certainly the one asserting the medieval origin of the Portolan Chart, which would have followed the introduction of the marine compass in the Mediterranean, when the pilots start to plot the magnetic directions and estimated distances between ports observed at sea. In the research here presented a numerical model which simulates the construction of the old Portolan Charts is tested. This model was developed in the light of navigational methods available at the time, taking into account the spatial distribution of the magnetic declination in the Mediterranean, as estimated by a geomagnetic model based upon paleomagnetic data. The results are then compared with two extant charts using cartometric analysis techniques. It is concluded that this type of methodology might contribute to a better understanding of the geometry and methods of construction of the Portolan Charts. Also, the good agreement between the geometry of the analysed charts and the model’s results clearly supports the a-priori assumption on their method of construction.
I will not now comment upon the above until the paragraphs chosen to illustrate the whole text have been noted and commented upon. However my objections must be already clear.
I will commence with an extract taken from the papers introduction, as follows;
The oldest known chart, the Carta Pisane (Pisan Chart), was made around 1285 and its accuracy and detail are so striking, when compared with the symbolic representations of the known world made at the time, that we are tempted to believe that the techniques used in its construction were already known for some decades before.
My first criticism is that nowhere in any text does there appear to be a comprehensive study of the methodology that may have been used to ascertain the compass bearings that may have been obtained and hence how the bearings were modified over the length of the Mediterranean Sea to read as one bearing per chart, nor is there any research to quantify the distances which are at times so very accurate and very hard to measure as such given the methodology of the age.
A simple example of using a compass at sea would be from Majorca to Alexandria will suffice to explain the difficulties. Bearing in mind this is to formulate a route plot and distance measure before a “Portolan Chart” was available; that is to enable it to be drawn. Fortuitously Yale University, Beinecke Library ms 557 is a copy of Benedetto Cotrugli, “de Navigatione, 1464” and folio 62r-v states;


Yale University Library, Beinecke Library Ms 557
“Volendo aguistare la carta de navigare o vero cognoscere se l’e giusta, vidi in prima da Porto Petro de Majolica fini ad Alexandria se va directo scapulando li scogli et le isole, et le isule per la quarta de Sciloccho verso Levante, et da Venetia ad Monoploi per Scilocho, et da Tenedo a cavo Malio pure scapulando l’isule et gli scogli del Arcipelago. Et pero benche alcuna volta falla questa regula, ell’e pure la commune prova che costumano experimentare li marinara.”
“Wanting to correct the navigation chart or indeed know whether it is correct, look firstly whether a straight line goes from Porto Petro on Majorca to Alexandria skirting the reefs and the islands, and the islands, in an easterly direction via the “quarta de Sciloccho verso Levante”, and then from Venice to Monopoli via the “Sciloccho”, and from Tenedos to Cape Malea also skirting the islands and the reefs of the archipelago. And, although this rule might sometimes fail it is still the common check by which sailors are accustomed to test (them).”


The whole of “De navigatione has been transcribed by Professor Piero Falchetta, (Biblioteca Nazionale Marciana Venezia), and in his text “The use of Portolan Charts in European Navigation during the Middle Ages1”, he makes the following points (quote);
It is obvious that such variety of figurative models, in which reliability and precision could not be verified by their authors, represented an obstacle to the use of cartography as a practical instrument essential to navigation. I believe that this condition of uncertainty was evident to medieval men, and also that they had very clear awareness of the difficulty of establishing a definite “technical” relation between geographical space and maps. we find an explicit reference to this problem in Benedetto Cotrugli’s treatise ‘De navigatione’, an important fifteenth-century work that we will examine below, Cotrugli, a wealthy merchant and ship owner from Ragusa (today’s Dubrovnik) who had acquired considerable navigational experience on board Genoese, Venetian and Catalan ships, provides some instructions for testing the precision and exactitude of cartographical drawing on the carte da navigare (Portolan charts), He states that it is possible to know whether a Portolan is right (giusta) simply by tracing one line from Mallorca to Alexandria, another line from Venice to Monopoli, and another line from Tenedos to Cape Maleas. If these lines do not cross any islands (scapulando li scogli et le isole), the map is right, otherwise it is wrong.
This is of course a very empirical and rough method, but it is also a very useful way for reckoning the accuracy of a map at first sight. However, Cotrugli warns his readers that although very commonly used by sailors, such method sometimes worked and sometimes didn’t: fala questa regola, he writes. The acknowledgement that this system is fairly unreliable is essential because it makes clear that medieval men’s attitude towards maps was very different from our approach to modern maps. Actually, the doubtful nature of medieval cartography is a direct consequence of uncertainty about geographical space, its shape and extension.
Nevertheless, Cotrugli describes the use of Portolan charts in detail. This description is without precedent in other texts of previous centuries – I’ll discuss later the reasons of this strange uniqueness (particularity) – and this makes it a document of extraordinary importance. Cotrugli writes that directions (venti), distances (migla), and place names (nome delli lochi) are marked (e signa) on Portolan charts. Directions are necessary to compare information given by the compass during navigation with the actual route of the ship. A route is not a straight line, but the result of a series of boards, and only sailors experience could determine the difference between the true (intended) course and the course made good (actual course sailed). Then, he continues, there are distances that depend on the speed of the ship and on the current. In order to appraise the distance covered in one day by one ship, it is therefore necessary to turn over the sand-glass (mezarole) night and day, and mark each time with a spot of wax. This is the method for calculating how many miles the ship travelled in an hour, and in which direction. At that moment, it becomes possible to establish the point where a ship is located. Once you have established that point, it is possible to calculate your route using dividers, and to know the distance of your ship from the coast.
A question is raised here: why is Cotrugli’s text the only pre-modern text – as far as I know – that contains such a detailed description of the use of Portolan charts in Navigation? If charts were commonly used by sailors, why are there not references to this practice in other contemporary nautical texts? (end quote).


Firstly it is necessary to reiterate that from Majorca to Alexandria is a 22 ½° South of East course and hence the wind is; outward ESE Maestro-Ponente; return WNW Levante-Sirocco. This of course indicates a quarter wind, 11 ¼° difference to that quoted by Cotrugli as already indicated. But, the use of the SE by E or Quarta Sciricco verso Levante must be a mistaken or rather mis-copied wind direction. Given the setting out of the “Map within the Portolan” it is likely to be East by South, Quarta Levante verso Scirocco, merely a transposition of the wind names.


If the magnetic declination diagram for 1300AD is studied (diagramChUg/1/D11) it shows that over the length of the Mediterranean Sea there is a change in magnetic north from 5 degrees east tilt to 16 degrees east tilt, and thus a complete quarter wind deviation. Thus any magnetic bearings taken will indicate the above deviation. Simply put there will be a marked curvature to alignments longitudinally on the Mediterranean Sea length. Sail from Majorca with an 8 degree magnetic tilt; as you pass south of Sardinia that tilt becomes 11 degrees; at Gozomalta it is 13 degrees; at Gozocreti it is approaching 16 degrees until at Alexandria it is the full 16 degree tilt.
I now return to the text being analysed.

Figure 5

Now study the diagram, figure 5 from the 2008 paper which have the cartometric analysis geographical grid appended and no such deviation, an extra tilt of 11 degrees is apparent. Therefore if the author is to be believed the mariner taking the readings was able to convert them all to a single deviation long before the magnetic compass had any Fly capable of giving accurate readings. It is complete rubbish to suggest the compass was used. But sail north from Alexandria along the 30 degree longitude and you will arrive close to the island of Megeste, or as it was known to Portolan users, Castell Rossi. Use a magnetic compass from Alexandria which will indicate north as 16 degrees east and you will arrive in Cyprus. Study a Portolan Chart, Castell Rossi is not drawn 16 degrees west of Alexandria to obviate the magnetic declination by twisting the chart position and not one Portolan Chart is skewed by 16 degrees.
Thus my suggestion to those proponents of the magnetic basis for the formation of a Portolan Chart is; draw one out properly and prove it so that we all may witness the basis. Commence with a geographically based map and show how it can be transformed into a Portolan Chart by the use of Magnetic bearings being applied, magnetic bearings taken via a needle skewered through a piece of straw floating on oily water, the norm for the age.
The next sentences to be examined relate to map projection.
Also the possibility that some map projection (especially the plate carre or Mercator projection) was deliberately used in its drawing has lost most of its credibility, being generally accepted that the underlying projection of Portolan Charts must have been the accidental result of the method of construction. That is indeed the conclusion reached in the pioneering works of Lanman (1987) and Loomer (1987), though the author’s don’t agree on the charting techniques used.
There are no projections used on a Portolan Chart, they are merely the resultant of distance and direction plotted on a flat plane. It takes very little error, objective error that is, to obviate the necessity to curve any grid plot and thus form a chart or map particularly over such short distances such as are involved in the Mediterranean Sea basin.
The following is a necessary interjection in the text analysis.



I have included a simple diagram of explanation for the following text, ChDUL/1/D14.
The following are simple examples of the calculations required to determine the geographical great circle distance between two places, the shortest distance possible on the face of the earth, and the basic triangular calculation using the Pythagorean Triangle maths.
Example 1; Majorca to Alexandria ( the Cotrugli route)
From 3E/39.5N to 30E/31N, the great circle calculated distance is 2610.644Km with one degree of latitude equal to 111Km. the same distance calculated using the Pythagoras Theorem gives three distances, that calculated at 31N, 39.5N and the mid-point 35.25N. They are thus, 2736.71Km; 2497.62Km and the average is 2623.637Km. Thus the average calculated distance is only 13Km adrift, less than 0.55 error and as such a perfect measure.
Example 2; Gozomalta to Acre
From 14.267E/36N to 35.083E/32.9167N is 1931.765Km via the great circle calculation. At 36N the distance calculated is 1900.35Km; at 32.9167 the distance is 1969.6Km and at the average latitude of 34.5N by the Pythagoras theorem the distance is 1934.72Km, a mere 3Km actual difference. But the max/min difference is only 34Km in 1930Km and given the scale and accuracy of these charts it is irrelevant.
It is therefore quite ludicrous to consider that given the minor errors in the distance measures I have shown exist between the “LCN” and geographical fact that anything but a flat earth plane projection would be achieved by the draughtsman of a Portolan Chart.


Following on in the text, these sentences require being included as they lead to the list of comments the text author has made later.
“It will be shown that its geometry can be replicated by plotting directly on a plane, with a constant scale, the magnetic directions and estimated distances between places observed by the pilots at sea, as if the Earth were flat.”
This sentence towards the end of the texts first paragraph follows directly from the previous quote. However, nowhere has a single magnetic direction or estimated distance been shown or implied or explained, basically because the first does not exist and the second is incorrect as the distances are taken from “LCN” and attest by their similarity throughout.
The text second paragraph deals with world projections, totally at odds with the simple methods required to plot the Mediterranean Sea and are thus a completely unnecessary foray into the subject and are just a method of obfuscation by the author.
However the 4th paragraph does mention the work of W R Tobler some of whose work is analysed later. But the text now turns it attention to Cartometric analysis as follows;
“Two Portolan Charts will be simulated: the Angelino Dulcert’s chart of 1339, and the Jorge de Aguiar’s chart of 1492. (note I subject this chart to scrutiny in another text). The former is of Genoese origin, most certainly by the same author of an older one, signed Angelino Dalorto (c1330). It is among the oldest known Portolan Charts and it has great historical importance, being the first to represent the Atlantic archipelagos of Madeira and Canary. The second is of Portuguese origin, the oldest dated and signed Portuguese nautical chart, and was drawn in the period when the portolan model was been replaced by the latitude model, the so-called plane chart. However it is clear that this chart is still based on the older portolan model. In figures 5 and 6 the grids of meridians implicit to both representations, as estimated and drawn with MapAnalyst, on the basis of about 100 control points, are shown. The visual inspection of these grids suggests the following comments;
– Both grids are tilted counter clockwise. The rotation angle is about 10 degrees in Dulcert’s chart and 8 degrees in Aguiar’s chart;
– Meridians and parallels are roughly straight and normal to each other, but the grid of Aguair’s chart is more regular;
– There is a slight convergence of the meridians in both cases, which can be assessed by comparing the lengths of the upper and lower parallels;
– An irregularity in the orientations of the parallels occur, in both charts, in the area 38-42N, 20-28E. This is common to many other Portolan charts and it is probably due to local magnetic anomaly, affecting the behaviour of the compasses;
– There is an east-west scale variation in Dulcert’s chart, summing up to about 15%, which can be assessed by comparing the lengths of the parallel and meridian segments in the western limits of the grid. As asserted by various authors and referred by Campbell (1987, pp 383-384), this is an indication that the first Portolan Charts might have been piecemeal creation combining representations of independent origins.”

I have already analysed the Dulcerto 1339 chart and suggest that if this author had actually carried out a thorough examination as I have shown the results would be so very different than those he has posited. The above comments are thus unacceptable.
The crux of this paper is, “a mathematical process to evaluate the charts, prove a magnetic compass was used to plot routes and discuss the change of magnetic declination.” However, one single section in the text comprising two sentences indicates a lack of cogent research. I quote, “No accurate conclusions about the lengths of the tracks used to make the charts can be drawn from these results alone. The use of the maritime routes of the time as a model input, obtained from the available “Portolani”, is the obvious next step for achieving more precise and reliable results on this matter”.
If the author considers the results obtained to be unreliable, it is beyond belief that the attempt was made in the first place, and the abstract gave such a glowing report of these results. The “Portolani” were available then and thus surely should have been used instead of leaving future researchers with the unreliable data that has been produced.
The next subhead is, “Magnetic declination and the Mediterranean skewing.” As the last section of this text prior to its concluding remarks it only remains for me to reiterate the fact that there is no magnetic skewing of the Portolan Charts, it is solely the errors caused by the scribal copying of what this author has called Pilots Notes.

Concluding Remarks concerning this 2008 paper.

Any researcher can assert a theory, but in this day and age perhaps excellent circumstantial evidence or clear proof should be the order of the day. Where is the proof that a single mariner pre 1300AD had a compass of sufficient accuracy that a route could be aligned, such as Cabrera to San Piero, geographically due east/west, “LCN” due east/west, and determine magnetically it is actually a variation from 8 to 11 or 11 to 8 degrees south of east/north of west. Where is the written attestation of a mariner of the magnetic route from any place to any other place? No such data exists and if it does it is incumbent upon this texts author to provide it and prove his statement regarding mariners providing such magnetic bearings and the ultra precise distances on the charts.
Finally please note this texts author has conflated via its footnote 10 the fact that in 1514, the Portuguese pilot Joao de Lisboa wrote that the Flemish and Genoese compasses were adjusted according to the value of the declination at the place they were made.
I feel certain the needle in the straw used in the 1200’s was similarly adjusted!


The next text to be analysed is that by W R Tobler, “Medieval Distortions: The Projections of Ancient maps”, American Assoc Geogs 1966 and his talk given in 2007 entitled “Properties of Portolan charts”.

Within the sub-heading Portolan charts the following text is extracted for discussion; “Several authors have come to the conclusion that the portolan charts are not based on a map projection15. This view has already been rejected here on a priori grounds, in accordance with the modern interpretation of map projections16.
This is especially true of Wagner’s otherwise excellent study18. Wagner sketched lines of latitude and longitude on detailed tracings of several portolan charts and demonstrated that the length of mile employed differs along the west coast of Europe from that employed in the interior of the Mediterranean Sea19. He also illustrated an abrupt jog in the path of the parallels through Greece on the map and attributed this to the method of construction in which the map is made by piecing together separate, local charts. He concluded that the portolan charts are not based on a map projection, thus rejecting the notion of an implicit map projection. Wagner presented a careful analysis of all of these points. Steger proceeded in a less detailed but similar manner and concluded, on the basis of map interpolations that the meridians and parallels are straight lines when sketched in on the charts. He thus refuted Breusing’s suggestion. Steger’s demonstration, however, can be disputed on two grounds. His study did not include the Black Sea, which is where the curvature might be most noticeable, and his meridians and parallels are virtually determined by only two points. A more recent study of the portolan charts by Clos-Arceduc compared them to a Mercator chart at approximately the same scale. His reasoning is that Mercator’s projection provides a better fit than does a square projection. He further illustrated that the eastern portion of the charts is too far north, only, it must be noted in relation to Mercator’s projection.”
15; Among these are Nordenskiold, Wagner, Bagarow and M Eckert.
17; Cf Clos –Arceduc
18; Wagner, also The Origin of the Mediecal Italian Nautical charts.
19; for a plausible explanation of the phenomena that differs from that given by Wagner, see Clos-Arceduc.

The first point to be discussed is the “map projection”, I have already opined and clearly shown in various papers and from their diagrams that the standard portolan chart is projectionless, being drawn at such a small scale and having “normal” inaccuracies which cancel out any necessity for a curved or graduated graticule. These charts are entirely derived from the instructions given in the basic “Portolani” and its accompanying chart.
The fact that Wagner demonstrated the length of the mile differed along the west coast of Europe should come as little surprise to researchers, but like many other points it seems to have been ignored. The problems within the Greek graticule are easily demonstrated to be the mis-measurement of the latitudes from Crete to the Dardanelles and thence to Thessaloniki.
The Black Sea comments are not sustainable as it is determined solely from the “Portolani”, having a length of 1040 Millara and is generally skewed to the north-east because of the latitudinal errors of Asia Minor.
Thus the following discussion in Tobler’s text of a variety of differing projections should be dismissed on the grounds of non-research via the draughtsmanship method which would have shown the distortion caused by the distance measures of the “Portolani” being copied wrongly by the various copy scribes to such a degree that the errors can be evaluated from the Roman Numerals used.
It would be in the interest of all future researchers if this text was no longer used to evaluate portolan charts and their projections.
Is that statement fair? In April 2007 Professor Emeritus of Geography, Waldo Tobler gave a lecture to the American Association of Geographers entitled, Properties of Portolan charts.
He illustrated his lecture with the Carte Pisane, a 1456 portolan Chart (un-named), the 1482 Benincasa Portolan Chart, The Compass Rose ( a black and white redraw), the reverse of some charts and then a page entitled; What projection is used for these Portolan Charts?
He writes; Thus far proposed have been;
None (Eckert, Nordenskiold and others)
Azimuthal Equidistant (Fiorini)
Oblique Stereographic (several times)
Oblique conformal Conic
Oblique Mercator (Clos-Arceduc, Tobler)
Empirical Projection (Lanman, Tobler)
He then discusses Dr J T Lanman’s work (see my ChLCN/1 text), compares the 1468 Petrus Roselli to an Oblique Mercator and finally asks the most pertinent questions on page 13.
Which projection is the wrong question!
It is extremely unlikely that any now-known projection was used. So maybe NONE is correct. None in the sense that no now-known projection was used. But we know that ALL maps require a projection, thus a projection constructed from empirical data is much more plausible, as described by Lanman and Tobler. It is reasonable to ask which modern projection comes closest to fitting the old chart?
But, that is not a terribly interesting question. It is, however, of interest to know how good, quantitatively, the charts are. Especially in contrast to the contemporary ecclesiastical maps of the time. Modern computing techniques should allow some answers.

I cannot fathom why the Professor did not actually take a Portolan chart and dissect it; he is a geographer and should be able to draw and give himself a definitive answer.
However, he then continues and discusses the work of Scott Loomer, by stating ”Loomer compensated for the geometric distortion as it might occur on an old portolan chart”. Did nobody investigate the reason for the distortion?
The lecture then descends into a theoretical and computer based research project by the use of Displacement Theory and a Map Machine followed by page, 29, headed,” The Bidimensional Regression Computer Programme. The distortion on Portolan Charts is then calculated and conclusions are given as follows;
Tissot’s measures indicate that this portolan chart is neither conformal nor equal area. Suggestions that it is based on a now-known map projection are erroneous. It is more likely that it was based on empirical measurements (loxodromic distances and directions) in the Mediterranean, and was thus constructed to fit an unconscious hypothesis that the earth is flat, resulting in an empirical projection, and was put together from several separate sectional maps.
I conclude with simple comments; firstly there is as just surmised no projection but, the idea that the chart is built from sectional maps must be dismissed. A Portolan Chart is a single item drawn from a “Portolani” and its accompanying map for the coastlines.

Perspectives on the Origins and Uses of the Portolan charts; J E Kelley jr. Cartographica, vol 32 (3). 1995.

The abstract of this paper is perhaps a timely reminder that “tools” can be used to assist research, but unless the programme is perfectly attuned to the task we have GIGO.
I quote; ”This paper suggests proposed hypotheses be tested by simulating the detailed processes involved, using a computer. A simulation run produces an outline map of the area studied, typically the Mediterranean, which, when compared with surviving charts, provides a measure of credibility of the hypothesis.”
In the introduction, paragraph 2, we read; “Most speculations on the subject include descriptions of, or allusions to, some processes or activities that may have been, or should have been, underway in order to produce the charts. For instance, those who see the magnetic compass as essential to the development of the chart make an implied assumption, untested to this writer’s knowledge, that selecting and maintaining reasonably accurate bearings in the Mediterranean cannot be done without the instrument. Even if this assumption is rigorously true – and it is not in the practical case – the law of large numbers suggests the possibility of generating a map more accurate than the units of data from the many individual voyages used to generate it.”
These words, quite prophetic actually, are followed by a short discourse upon how a mariner may have recorded a voyage and the chart that may there-from have been generated. He states; ”In practice, this simply stated programme becomes involved- possibly the reason so little has yet been done2.
2 = Most of the quantitative work to date on portolan charts has concerned fitting a grid of latitudes to existing charts, (most recently Bremner 1985, Loomer 1986, Duken 1988, Messenburg 1988, 1990), or generating a chart from “portolani” data (Lanman 1987). The results are then interpreted as implying some class of chart-generating process, or some operational or physical conditions that must have existed. This work is valuable for its insights into processes to be used for simulation studies.”

The text then discusses; “1) the many uses of the chart; 2) For planning voyages; 3) For visualizing the ship’s position at sea; 4) For figuring optional headings to recover a planned course, or to seek shelter on land; 5) For recording observed errors in the chart and providing an archival record of the voyage.”
Following those paragraphs the Raxon del Marteloio is discussed, then the Proto-compass and early improvements in the compass.
We then arrive at; “The anti-clockwise Rotation of the Mediterranean”, with the second sentence stating; “the earliest portolani or pilot books (late 13thC) thought to be derived from ship’s logs, give directions in terms of compass bearings and distances49.”
49 = See eg, Motzo 1947, Kretschmer 1909, Campbell 1987. To this writer’s knowledge, no ship’s logs survive from the early period. Possibly one may assume some such records were kept in order to partially fulfil the reporting requirements of voyage patrons to their shareholders, a requirement codified in the sea laws; See eg Jados, 1975, 141f.”

The text continues; “The most compelling evidence is in the portolan charts, the northern displacement of the eastern Mediterranean of almost a compass point relative to Gibraltar. This is seen as reflecting an eastward variation of the magnetic compass at the time the charts were standardized.”
There follows a discussion and comparative study of the various charts rotation Which came first pilot book or chart?” angles, which I have shown to be totally incorrect when thought of as magnetic as they are solely caused by the “LCN” scribal errors of distance/direction. We are then asked to consider “other sources for the false orientation” and the theoretical development of a trilateral grid to set out the Mediterranean Sea. After this point the text reduces to the use of formulae, average positioning methods and computed results for magnetic variations.
All of the above have been shown conclusively to be a nonsensical evaluation of a Portolan Chart, but the final section, “requires noting.
There are many points raised which if properly researched would have completely changed the foregoing text and negated the magnetic argument. However the most glaring error comes when he discusses, “many Pieleghi, (long distance courses running over hundreds of miles) in the Portolani are in error by multiples of 50 miles, the major division of the typical mile scale of Portolan Charts.”
This is a completely fallacious statement, If the “LCN” had been properly studied, it is after all quoted in the references – Motzo 1947- and then those distance measures checked against not only the charts but also the geographical distance, the greater number are correct and those in error rarely 50 millara out.
Therefore, I can only conclude that had this author actually studied “LCN” and compared that texts distance and directional data in the “Pieleghi” sections to actual charts the foregoing text would not have been written. The distances are in Millara, 1.23Km and the directions are given by winds and there are no references to magnetic courses within the portolani texts.
Thus the first quote I utilized which finished with the words; “provides a measure of the credibility of the hypothesis”, gives me the opportunity to state clearly, that there is no credibility in the given hypothesis and the author (now unfortunately deceased) has by virtue of computer GIGO provided a flawed text.

Assigning map projections to Portolan charts. ePerimtron, vol 1 no 1, 2006, C Boutoura.
Figures 1, 2 and 3 are included for information

Figures 1, 2 and 3

The summary commences with the sentence; ”In spite of the widely spread impression that the portolan-maps are projection-less, some authors, during the last two centuries, treated the relevant problem, mainly from a theoretical or intuitive point of view”. It continues later with; “In this paper a digital raster processing environment is used in order to test empirically best fitting projections of some portolan-maps of the Mediterranean and the Aegean Sea. We then move onto, ”The underlying theory” and the first sentence says it all; ”Once an old map with unknown projective properties is set in digital form, a known map projection model can be used in order to test its optimal fitting to the old map”.
Before commenting it is worth including sentences from the next subsection, “Testing the theory”, and particularly the notes on the selected test maps and their characteristics;
(a) The western side of the old map-frame is parallel to the western coasts of the Iberian peninsula, which is actually extended along the 7 degrees and 47 minutes meridian.
(b) The 36 degrees parallel2 is slightly tilted north wise along the west-east direction (figure 1) by a c10 degrees rotation, a figure confirmed by other authors as well (see eg Astengo 2000)
(c) If a new frame, equal to the old map’s, is placed in various projection-systems in a way of keeping its left side parallel to the west coasts of the Iberian Peninsula, e.g. in a normal conic projection (figure 2) or in a normal cylindrical projection (figure 3), then the old map either should rotate a great deal in order to come to its regular position or the 36 degrees parallel takes certain shape, not relevant to its original image on the old map. For example, it is transformed into a line of strong curvature (figure 2) or onto a straight line (figure 3).

Why a 16th century map should be chosen when the majority of Portolan charts date from the 14th C is quite curious. However, the first question to be asked regarding the research is surely; Why was the map not analysed using its scale bars to ascertain any accuracy or errors, any distortions due to those errors as distortions produce illogical positioning? Surely if you do not know the basic facts about the map being studied how can you analyse it. How can you accept it should be digitized and set upon a geographical map for comparison to test its inherent or supposed projections? A check on the simple distance measures of the constituent parts of the map would have indicated the distortions caused by those errors.
The paper then produces various figures which indicate these distorted maps, distortions due to the “LCN” text data and from these distortions a curved 36N parallel will always be found, varying in curvature because of the variations in the accuracy and copying of the “LCN” data. The “Discussion of the results”, concludes that an “oblique conic projection” satisfies the projective content of a variety of maps, both regional (Med Sea) and local (Aegean) covering almost two centuries from the early 14th to the early 17th C.


If the author instead of opting for a computer based research package (Professor of Digital cartography no less) had analysed these maps via their scale bars and compared them to each other, it would have been immediately apparent that they are all based upon the same or very similar distance /direction data and hence they would all exhibit such a projection when analysed. But the analysis is false, the basic data from which they are drawn is flawed and induces a skewing of the Mediterranean Sea which is not the resultant of any projection as these maps are projectionless, but the result of basic scribal errors in copying an original text which was used to draw the original portolan type maps.
Thus this text should be re-assessed before its basic premise of just fitting a digitized portolan Chart into various geographical projections is taken up but her researchers. It is a totally unjustified method of testing Portolan Charts.


However, I include part of a previous text which discussed the above chart and the subject matter of the four texts in a question and answer paper. This following text is taken from, “Three Statements, Three Answers” text

STATEMENT 2: “The requirement to set a chart at an angle equivalent to a ‘supposed’ declination was unnecessary. The Wind Fly atop the needle could be set to alleviate the discrepancy on a geographical chart for magnetic deviation”.

RESPONSE 2: J Alves Gaspar stated, “It is not true that the Mediterranean pilots knew about magnetic declination”. The referee to my ChPo1 text stated,” {the charts north is now generally considered to represent the state of actual variation in the 13th C; there after the lack of change would have been the result of the general, literal copying}”.
Absolute fact; the wind rose is always drawn foursquare, NSEW to align with geographical north and the Iberian Peninsula which is normally on its western sea border due north/south. Study Vesconte 1311 and 1321 plus Dulceto 1330/1339 for the proofs. Geographic north is always a feature of the Portolan charts, and the Wind Rose cannot indicate the magnetic deviation to which the chart has been drawn. Thus to be able to use the magnetic compass with the Portolan Chart the chart must be pivoted to align Wind Rose North with the Magnetic Arrow Pointer and by so doing the ‘Map within the Portolan’ becomes readable by the Fly atop the compass, but, the Wind Rose automatically becomes out of alignment by the magnetic deviation and cannot be used with the compass as aligned to the twisted map. This actually negates the use of the Portolan as a navigation tool by Wind.
But a new methodology now exists to indicate the latitudinal and longitudinal deviations on the Portolan; cartometric analysis. This methodology produces flowing graticules across the face of a chart which is completely at odds with the drawing method. There are no curved lines of latitude or longitude on a Portolan chart. They are all straight lines as can be shown by a simple analysis and drawing method (see ChPo2 and ChPo3).
In his PhD dissertation (2010) J Alves Gaspar2 within the pages of chapter 4, (85/128), illustrated what he called the “geographic grid implicit in a chart”. Unfortunately the charts used (see diagram 2) have so few node points that they do not allow for a comprehensive analysis by cartometric means. They produce spurious information as large swathes of the charts can be no more than a simple averaging.
The originator of MapAnalyst was very specific concerning this point as my text ChPo2 fully explains by quoting Bernhard Jenny3, and ending with his words, “the desert effect”.
Thus it is necessary to illustrate why the statement was made in my text.
There are Portolan charts which exhibit not just one apparent declination by tilting the eastern Mediterranean northwards, but two or three declinations. These are quite deliberate and specific, having the junction of the declinations clear and precise.

Diagram 2

This chart has been chosen to illustrate the above comment regarding declinations solely on the basis that it is one of the earliest extant and it features in Alves Gaspar’s 2008 paper. Study the cartometric plot carefully shown above, (Figure 5,pp 196/197), and then follow the actual inherent plot drawn by hand. The Dulcert Chart was subjected to careful scrutiny, as Diagram 4 illustrates with attention paid to geographical alignments that could be seen there-on. One example is the 9E longitude which passes through Sardinia, Corsica and then Genoa. This is one of many such alignments which can be used to test a chart. The Iberian Peninsula provided excellent latitudinal alignments and these allowed the west to east parallels to be plotted. Eventually the chart had there-on longitudes from 9W to 43E and latitudes from 27N to 45N solely by using the information upon the chart.

Figure 5

Diagram 4

The information indicated that from 9W to 3E there was a geographical alignment for longitudes and a changing declination alignment for latitudes. From 3E to 17E a second declination is observed and then from 17E to 43E a third declination. Thus one chart, a very early chart, has three declinations of 0 degrees, c6.69 degrees and 11.25 degrees. If this had been a basic chart sans decoration to be used at sea, how many compass fly positions would have been used and if only one was chosen, the central naturally, confused directions occur.

Diagram 12

In my ChPo1 text I quoted J H Parry4 in the section headed ‘Portolano; Chart or Library adornment’. The last paragraph from Parry is; “Most of the surviving medieval charts are highly decorated examples which probably were never used at sea, but graced the offices of shipping firms or libraries of great men, no doubt, is why they have survived. They differ from working charts, however, only in their wealth of decorative detail.” Previously in my text I had commented, “Add to this the flowery designs on most examples” to which the referee interposed, “{not even ‘most’ surviving examples, and Pujades and Campbell5 are in agreement that what has survived is overwhelmingly biased in favour of the elaborate over the everyday; possibly at least several hundred lost plain charts to each one that survives, e.g. the contract to Vallseca (see Pujades) to provide 24 ‘good and sufficient navigational charts’ in 6 months}”. (Read the 3rd statement for the refutation of this comment by the referee).
Thus if one version of the Dulcert 1339 chart was the same as that which is extant minus the decoration, it is difficult to accept that the Mediterranean Sea pilots could be anything but aware of a declination change across the extent of the chart. Set the wind fly for the eastern Mediterranean and sail near Majorca and the pilot would find himself not only one wind adrift but a large error in his sailing course which is not as described a few miles out and easily corrected.
But J H Parry also stated, “all charts were drawn to bring the lines indicating magnetic north into the vertical: these lines being parallel in all compass roses on each chart.” This implies that at no time would the discrepancy matter and thus there was no reason to alter the geographical chart by slewing it to suit a perceived declination. Either chart was as false as the other when set against the magnetic compass (see diagram 12).
Thus I see no necessity to draw a Portolan chart with any declination slewing. If the chart were geographic then the wind directions would be perfect, they are after all drawn NSEW, and the fly atop the compass would be set to remove the declination problem. It is doubtful that many pilots sailed from the Iberian Peninsula to the Levant and hence the declination between Spain and Italy or Italy to the Levant would be simply accommodated.
Thus I consider that the pilots did know that declination existed and dealt with it.
Thus we can dispense with the idea that Portolan Charts are drawn to a specific projection, are deliberately slewed by magnetic declination and are anything to do with being formed from magnetic bearings and pilot’s manuals, but just “Portolani Texts and its Chart”.

1 Falchetta Piero, “The use of the Portolan charts in European navigation during the middle ages”.
2 Alves Gaspar, J, 2010, From the Portolan Chart of the Mediterranean sea to the Latitude Chart of the Atlantic, Cartometric Analysis and Modelling. Dissertation for PhD in Information Management, Geographic Information systems, Universidade Nova de Lisboa.
3 Jenny, B. 2006, MapAnalyst- A digital tool for analysis of the planimetric accuracy of historical maps.
e-Perimetron, Vol 1. No 3, summer 2006, pp 239-245.
4 Parry, J H, The Age of Reconnaissance, Readers Union, Weidenfield and Nicolson, London, 1964.pp101/102.
Campbell Tony, Portolan Charts from the late 13th century to 1500. Chicago, HOC, vol 1, chapter 19. And http://www.maphistory.info/PortolanFutureResearch.html.

M J Ferrar April 2016.