PORTOLAN CHARTS OF 1413, 1436, 1448 & 1463/9; THEN CAPE VERT AND 3 LETTERS FROM 1495

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Obviously the explanation of West Africa from Hercules Pillars to Cape Verde and beyond is a tale of many attempts at exploration and of many diverse texts describing the knowledge gained, but all are tinged with the basic lack of real geographical evidence given the timescale, over two thousand years is involved. Thus it is necessary to follow the time line of historical fact rather than discuss it by a looser framework of just the later texts and charts involved. The charts/atlases have been chosen for possibly being the first drawn examples of the exploration southwards down the coast, or should I state those available I consider show more than others. The historical texts speak for themselves and I am merely copying from my original research of years ago, which is still totally valid for this research as it illustrates the ancient knowledge. The distance measure the Millara is then quantified.

THE TEXT OF MARINUS THE TYRIAN AND CLAUDIUS PTOLEMY ‘GEOGRAPHIA’, BOOK 4, CHAPTERS 1, 6, 7 AND 8, The West coast of Libya explored and the zero longitude determined.

Various papers by this author concerning Eratosthenes, Strabo, Marinus the Tyrian and Claudius Ptolemy have concentrated upon their descriptions of the Mediterranean Sea (and also Britannia), but largely ignored the west coast of Africa. However, the research to explain the variety of geographical differences had been carried out. Text length limitations were generally the reason for the sub-division of the afore-mentioned geographer/historians work and its non inclusion. Then, in writing about the English cartographer, Christopher Saxton and his 1597 map of Britannia, it was necessary to discuss the prime meridian he had utilised, and thus this paper was abstracted from the MT/CP original papers. That prime meridian was shown to be the Cape Verde Islands, and, as the Saxton map was also shown to be a Ptolemaic construct, it thus accorded with their prime meridian, or zero longitude. That zero longitude was stated as passing through Inaccessa Island, Pluvialia Island, Capraria Island and Ninguaria Island, as recorded in Book 4, chapter 6 of the ‘Geographia’.

This paper, the fourth in the series, analyses the west coast of Libya (our Africa), in a similar manner to the afore-mentioned papers, by aligning the map plots, geographical and Ptolemaic at a single point, Sacrum Promontory, and also to a single scale. Thus the prime meridian used by MT/CP is confirmed.
The east coast of Libya (Africa) is also examined to ascertain the most southerly point.
Caution; The text translated by Stevenson8 has significant errors and should always be tested against another of the (10) translations. It is normally copyist error which can be shown as the prime error, but not always. This author has used Codex Lat V F 328 from the Library of Naples as a check document.


Herodotus, (c485-425BCE)1, called by Cicero and others, ‘the Father of History’, wrote in his epic work, ‘The Histories’, Book4-34 to 42, as follows:
‘I cannot help laughing at the absurdity of all map-makers- there are plenty of them- who show Ocean running like a river round a perfectly circular earth, with Asia and Europe of the same size’. And later, ‘As for Libya, we know that it is washed on all sides by the sea except where it joins Asia, as was first demonstrated, so far as our knowledge goes, by the Egyptian king Neco, who, after calling off the construction of the canal between the Nile and the Arabian Gulf, sent out a fleet manned by a Phoenician crew with orders to sail round and return to Egypt and the Mediterranean by way of the Pillars of Heracles. The Phoenicians sailed from the Red Sea into the southern ocean, and every autumn put in where they were on the Libyan coast, sowed a patch of ground, and waited for next year’s harvest. Then, having got in their grain, they put to sea again, and after two full years rounded the pillars of Heracles in the course of the third, and returned to Egypt. Those men made a statement which I do not myself believe, though others may, to the effect that as they sailed on a westerly course round the southern end of Libya, they had the sun on their right- to the northward of them.’


Then in 4.196, Herodotus states that the Carthaginians traded beyond the Pillars of Heracles with the peoples of Libya.


In confirmation of the last statement, in the first half of the sixth century BCE, the Carthaginian admiral Hanno sailed on a voyage along the west coast of Africa, probably as far as Gabon. Pliny the Elder3 stated that Hanno actually circumnavigated Libya (Africa) and arrived back at Arabia; i.e. an opposite voyage to that previously described.
The text concerning the voyage of Hanno is a Greek translation of a Punic inscription that had been displayed in the Temple of Ba’al Hammon in Carthage. At present there are two copies of that text (probably abridged) and dating from the 9/10th and 14th century. One is held in Heidelberg, and the other is split between London and Paris. The text includes sailing times, generally given as a number of days, and mentions known places, such as ‘Lixus’, in Mauritania Tingitana, and the ‘fiery region filled with vapours from which great torrents of fire flowed to the sea. The land could not be approached because of the heat’. It continues, ’we sailed away from there quickly, being struck with fear. Then having sailed from there for four days we sighted at night a land full of flames. And, in the midst of it there was a fire higher than the rest which seemed to touch the stars. By day we discerned it to be a mountain of great height named Theon Ochema.

This is confirmed as Mount Cameroon, 4° 13’N: 9° 10’E, the only active volcanic area of West Africa.

PSEUDO-SKYLAX4 (Skylax of Karyanda)

In the 3rd or 4th century BCE appeared another text, ‘I’. There within chapter 112 appears a description of the west coast of Libya, with sailing distances as follows;
Coastal voyage from Pillars of Heracles to Cape Hermaia = 2 days
Coastal voyage from Cape Hermaia to Cape Soloeis = 2 days
Coastal voyage from Cape Soloeis to Kerne Island = 7 days
This text appears to be based upon the voyage of Hanno.


The texts of Eratosthenes, (c285-194BCE), particularly the ‘Geographia’, are known mainly from the text of Strabo6 (C64BCE-AD21). Libya (now Africa) is mentioned but briefly in Strabo’s text, 2.5.15 and 17.1 and 2, where-in Strabo indicates that there is a coastline south of the Detroit of Gibraltar, but endeavours to curtail the knowledge, to a small section finishing at the Atlas Mountains.
Thus we enter the first century of our Common Era, the times of Marinus the Tyrian7 and Claudius Ptolemy8. As mentioned previously, there are other writers such as Pliny the Elder3, who mention Libya, but their text is mostly a reworking of older sources already mentioned.


In any study of the cartography of the oikoumene, the first named is synonymous with the second. We know little about Marinus the Tyrian, as Ptolemy addresses him, but the text written by Ptolemy, his ‘Geographia’, is acknowledged by him as a near copy of the work of Marinus the Tyrian.
In Book 4, chapter 6, ’The location of interior Libya’, there is a detailed description of the west coast of Libya from the terminus of ‘Mauritania Tingitana’, south to the ‘Hesperius or Great Bay, located in 14E:4N, on the west by the western ocean’. There is an interior point, the Arances Mountains, 470 30’E: 10 30’N, closer to the Equator, but in Book 4, chapter 8, (discussed later) ‘Location of Interior Aethiopia’ (east coast), we read of recorded land points south of the Equator.
The furthest west in geographical terms or the zero longitude of MT/CP is to be found at the end of Book 4, chapter 6, as follows:
The islands near Libya in the western ocean are;

Cerne island 5E25° 40N
Junonia island or Autolala 8E 23° 50N
and the six Beatorum islands;
Inaccessa island 0E 16° 00N
Junonia island 1E15° 15N
Pluvialia island 0E14° 15N
Capraria island 0E1° 30N
Canaria island 1E11° 00N
Ninguaria island 0E 10° 30N

Thus it is these islands which establish the longitude of the world.



In previous papers, this author established that there were zero points which could be aligned upon a geographical map and a matching scale Ptolemaic map. The projection of the geographical map must of course reflect the Ptolemaic projection, and for small scale maps the system of Meridional points is more than adequate.
Thus when the two maps are overlaid, with the coincident point aligned, and not a geographical graticule, other than NSEW, the survey details of the MT/CP map can be evaluated.
The zero point for the western Mediterranean was determined as Sacrum Promontorium, now Cape St. Vincent. It is the south west corner of Iberia. This point is given the co-ordinates by MT/CP of 2° 30’E: 38° 15’N, and geographically 9° West: 37° North. This point had already been utilized by’ as the base to dimension his oikoumene. It is also from this point that Eratosthenes’, so we are informed, added 2000 stadia to the length of his oikoumene (at each extremity), no doubt in preparation for the discovery of more land, habitable land, to the south. This would have ensured his desired proportion between the length and breadth of the oikoumene was maintained at 2:1.
Thus if a pair of maps are prepared as Diagram 2 illustrates, the one being the Libya map by MT/CP covering the west coast or Atlantic littoral, and the other a corresponding geographical map of Ptolemaic projection. They can then be aligned using the single coincident point of Sacrum Promontorium.



The most striking feature of the MT/CP plot is that the west coast of Libya is plotted as a basic southerly progression from the Detroit of Gibraltar, whereas, the geographical west coast is a south-south-west, and then south alignment forming a rather large bulge which then sweeps eastwards into the Gulf of Guinea.
With this form of comparison it is possible to project the coastal points of MT/CP onto the geographical map plot and to cross refer each point thus enabling actual places to be determined.
The most important cross reference is that of the Islands noted by MT/CP as lying on the zero longitude. The alignment is quite perfectly the Cape Verde Islands which are geographically between 15/170 N and 23/250 W.
The fact that one of the islands named by south-south-west, and then south alignment forming a rather large bulge which then sweeps eastwards into the Gulf of Guinea.
With this form of comparison it is possible to project the coastal points of MT/CP onto the geographical map plot and to cross refer each point thus enabling actual places to be determined.
The most important cross reference is that of the Islands noted by MT/CP as lying on the zero longitude. The alignment is quite perfectly the Cape Verde Islands which are geographically between 15/17° N and 23/25° W.
The fact that one of the islands named by MT/CP is ‘Canaria’, has led to the assumption by many authors that the zero longitude is in fact the Islas Canarias which vary from 1.5° to 5° west of the African coastline. However, as has been indicated they are 15 degrees to the north of the islands shown as on the zero longitude. is ‘Canaria’, has led to the assumption by many authors that the zero longitude is in fact the Islas Canarias which vary from 1.5° to 5° west of the African coastline. However, as has been indicated they are 15 degrees to the north of the islands shown as on the zero longitude.


The west coast of Libya is terminated by I, 14E: 5° 15’N, although in the interior are noted the Arualtes Mountains, 33E: 3N and the Arancas mountains 470 30’N: 10 30’N.
But, the text of I indicates he reached at least to the Equator, and the shoreline of Gabon.
From the Detroit of Gibraltar, Hanno describes sailing west and south, where-as MT/CP have a slightly east of south bearing for their coastline. The difference is some 17 degrees of longitude between the MT/CP plot and the geographical plot. But, the difference in longitude for the Cape Verde Islands is 13 degrees. Perhaps having confused the Islas Canarias with the Cape Verde Islands the mistake was inevitable, and thus the west coast of Libya, was, or had to be wrongly drawn.



In AD1579, Christopher Saxton9 published a map of Britannia as the finale to an Elizabethan era landmark publishing feat, a complete set of maps of the counties of England and Wales.
It was published first without bordure scales and had only the centre latitude and longitude markers. But, a careful examination of the map (a British Library facsimile copy) indicated they were not perhaps what they purported to be.
When the map was published with its bordure scales, the defining point of Britannia according to many ancient writers, Cantium Promontorium was indicated by the c25° East longitude. Geographically Cantium Prom., the South Foreland, is 51° 07’ N and 1° 23’ East. The Cape Verde Islands, and in particular their capital Praia is 23° 34’ West. Hence there is by addition the longitude of 240 57’ from one to the other. Thus, it can be fairly stated that Christopher Saxton utilized the Cape Verde’s for the zero longitude.
Why should this be? A full analysis of the Saxton Map sans bordure indicates that it is drawn to a Ptolemaic projection using a graticule of 11:20 ratio.
There is, therefore, a 16th century English map drawn to the ratio set down by MT/CP for Britannia in the middle of the 2nd century of our common era, and it has the precise zero longitude as listed in MT/CP Book 4, Chapter 6, The Cape Verde’s.
It would therefore appear to be a fact that the early sailors, possibly Phoenician, or Carthaginian, logged the position of these islands on one of their many voyages of exploration. The natural sea current of this area on the coast of Africa, is the North Equatorial Current, and it would sweep craft sailing south away from the coastline of Africa and out past the Cape Verde Islands and St. Vincent.


The following text is from the Stevenson8 translation;
‘Aethiopia, which is below this land (Aethiopia below Egypt) and entire Libya, is terminated toward the north by the indicated southern boundary lines of the land which we have treated, which extends from the Great Bay of the Outer Sea to Rhaptum Promontory as we have said, and is located in 730 50’East and 80 25’South, then by part of the Western Ocean which is near the Great Bay: by unknown land toward the west and the south; toward the east by the Barbaricus Bay, which near the shallow sea is called Breve, from the Rhaptum Promontory even to the Prasum Promontory and the unknown land’.

Prasum Promontory moreover is located in 80E and 15S
Near this is an island, Menuthias, 85E and 12° 30’S
Mountains of the Moon from 57E and 12° 30’S
(Its water flows to lakes of the Nile) to 67E and 12° 30’S
High Mountains and unknown land,
Dauchis mountains (middle of) 15E and 13S
Ion mountains (middle of) 10E and 8° 25’S
Zipha mountains (middle of) 25E and 8° 25’S
Mesche mountains (middle of) 25E and 13S
Bardiyus mountains (middle of) 45E and 6S

The final text of Book 4 is: ‘Toward the south from the inhabited land to the South Pole the degrees are not definitely known, 73° 35’ or full 74 degrees’.
Thus spread across the longitude of Libya from 10E to 67E, are the mountains of the interior including the source of the river Nile, via snow melt-water in the Mountains of the Moon. The two lakes referred to are given at 57E: 6S, and the eastern at 65E: 7S. Thence the river Nile flows in two streams northwards.
(End of original text included here.)

Therefore I concluded that the Cape Verde Islands were known in c150AD and their general latitude correctly noted. Other researcher’s papers concerning this coastline have not changed that opinion and thus it is opportune to investigate how the Portolan Chart Cartographers viewed the coastline in question, and discover the distance measurements used.




Theirs was not the first exploration in the 14th century, but the most methodical. Arab traders had certainly visited the coast for centuries, generally in search of Gold and Slaves. The Canary Isles were known prior to 1339, but in the mid14th century Portuguese, Italian and Catalans led expeditions there, hence the Catalan Atlas of 1375 shows the islands and Cape Bojador. Later Prince Henry, known as the navigator, directed numerous expeditions to circumnavigate Africa from the Canary Isles southerly. This they did from c1420 onwards annexing the Azores in 1431 and gradually exploring to Cape Padrone and the Cape of Good Hope in 1488. For our research the period 1444 to 1447 is the most pertinent as it covers the exploration of Senegal, Gambia and Guinea but also the 1456 exploration when the Venetian Captain Alvise Cadamosto, under Portuguese command explored the Islands of Cape Verde. Having set the scene, we can now explore the Portolan Chart depictions of this area of West Africa, but it is worth noting that as additional new lands were discovered at different times, there was little agreement to the actual names of places on various charts and even charts by the same cartographer do not agree in some of these areas.


Illustrated is the western section of this chart, which by Portolan standards is quite large at 850 x 1150mm. It illustrates Europe from the Baltic Sea to West Africa around the 16th parallel, the Senegal River. I have appended some latitudinal lines illustrating the geographical positions, not those as drawn on the chart, which occurs on the second diagram.
The chart illustrates that from the far north to the Canary Isles it constitutes an excellent chart drawn from well founded data. But below the 28th parallel the coastline may appear to be a guess, but from a geographical portrayal it is acceptable as an examination of the actual West African coastline south of Cape Blanc shows that it is basically north/south until Cape Verde when it starts to assume a S.S. Easterly routing. However from Cape Bojador, which is named along with the corresponding red coloured town name, southwards there is a Cape and island shown (opposite the caravel) at a point equating to c21N. This is the approximate position of Cape Blanc and the Island of Arguin, as named on the chart. But more importantly just west of Cape Blanc are the two Islotes of Piedra Galha. These have been transferred southerly, perhaps mis-placed by the Island at the mouth of the Senegal River, as on the south western extremity of the chart we read of the Islas Gades, two curiously shaped half moon isles resembling a circle cracked in two, positioned just south of the Senegal River mouth. Thus from Cape Bojador to the Senegal River, c25N to c16.5N by chart scale is 7 degrees, but 8.5 degrees geographically.
Therefore it is quite open to opine that well before any European exploration of this coastline, the Arab traders, who may well have had Dhows at their disposal plotted features on the coastline and this data was transferred to, among others, Mecia de Viladestes along with the work of the Catalan Cartographers and that noted below.
In her book “The Haven Finding Art” page 156, EGR Taylor states;
“On the great Catalan Atlas of 1375, too, one James Ferrar was painted in his sailing galley setting out southwards beyond the Pillars to seek the River of Gold which lay somewhere on the way round Africa. Actually the Moors were already fishing and trading along the West African coast as far as the Gulf of Guinea (where on one occasion a Spanish Friar accompanied them), while in East Africa Arabs were now settled as far south as Sofala. But this did not appear on the maps, although one or two carried a strange, nameless, featureless South African peninsula.”


The Senegal River and its conflation with others is described thus;
”It is from the Arabs that Mecia de Viladesters has borrowed the Ued Nil (Nile), alias “riu de lor”, which combines the Nile, Niger and Senegal Rivers in a single course. He interrupts it with a gold-spangled lake, fed by 5 tributaries descending from the “montanies dellor” and an “Insula de Bronels”, “where gold is collected in the river”. Already represented in this way by Idrisi in the 12th century, these would appear to be the gold-panning zones of the upper Niger and upper Senegal River’s respectively near the gold mines of the Boure and the Bambouk”.

“The Sahara, however, is considerably foreshortened in latitude; obviously the cartographer has only hypothetical knowledge of its oceanic fringe beyond Cape Bojador (his cap de Buyeter). He has therefore worked hard at enriching it, adding manuscript notes in this region, and he does know of a more westerly trail starting at Ofan (Ofren) more and more used by Caravans in the 15th century”. (Basically taken from “Sea Charts of the Early Explorers”)
Thus the above 1980’s text requires to be read such that it is probable the Arab Traders had a great deal more information than is thought likely, and it belies the traditional fact the Gil Eanes, a Portuguese, was responsible for confirming Cape Bojador by sailing past it for exploration, as the southerly coast was already here-on and on the 1375 Catalan Atlas chart.
If we study the putative geographical grid appended to the 1413 chart we can understand its construction. Firstly, using the scale bar the “circle” radius of the Windrose is 1150 RM and the subdivisions into the 35/30/20/7 sections equate to 437.5/375/337.7RM and are thus 6.5/5.5/5.0 degrees; that is 17 degrees = 1150 miles or 1 degree = 67.5RM. This figure is generated from the logical latitudinal spread of the Iberian Peninsula from 37N to 43N, Cape St Vincent to Cape Torinana/Finisterre. But if we project the latitudes north and south it produces a varying set of measures for the Mainland of Britannia, but curiously at the 35/37N position the latitudinal scale reverts to 75RM, as it does from 43N to 49N. But Ireland and the Madeira/Canary isles are misplaced by them, although the Azores are correct latitudinally. However, if a putative latitudinal scale is set out for the Madeira/Canary Isles it becomes apparent that the 31N latitude sets out this section of the chart and there is a complete change in the latitudinal scale to 100RM per degree (a figure we will meet again, which is in all probability 90 millara but called a RM). Thus although the chart may appear to have a regular latitudinal scale if the Windrose is utilized, when studying the coastline minutely there are variations in all areas delineated.
Thus the Mecia de Viladesters 1413 chart is the first to include so far south and indicates that the coastline was well known below Cape Blanc, the Senegal River and probably to the Gambia River in the late 14th century, well before the explorations began.




Chart 5 illustrates the Iberian Peninsula and the West African coastline from Cape Spartel to Cape Bojador and includes the Atlantic Islands of the Azores, Madeira and the Canaries as well as “Antillia” (see Pizzigano 1424) and another fragment of land “Y (sol) to de la satanaxio man”. The scale of the latitudes is 75RM and these are marked and can be checked against the Iberian Peninsula 6 degrees of latitude. The Windrose is 575RM setting out radius with the graticule at 35/30/20, thus ignoring the outer marker. This gives a nominal setting out for the sections as 219/187.5/125/43.5RM. The 575RM is obviously 7.667 degrees at 75RM/degree with a setting out point of 35.5N. Hence the putative Cape Bojador is at 35.5 – 7.667 = 27.833, the corresponding latitude for Fuerteventura which is thus correctly shown. That indicates that Cape Bojador is some 2.5 degrees north of its geographical position which is actually 26.13N.
Putative longitudes are set out at 62.5RM by the scale bar, which would represent a geographical latitude ratio for 33.5N. However the 35.5N centreline would have a longitudinal measure of 61RM and hence given the age, the methodology of the draughtsman is for an excellent portrayal of geographical facts.
Thus with the exploration of the West African coastline, the Portuguese traveller Gil Eanes passed Cape Bojador (buzidor by Bianco) in 1434, but the correct positioning of Cape Bojador would have to wait a little longer.


The chart is illustrated in three sections to outline the overall content from Scotland in the north to Cape Verde in the south. There are numerous islands dotted in the Atlantic Ocean, including the mythical and mystical Island of St Brendan. But generally it covers the Azores, Madeira and the Canary Isles until when the southern bordure is reached we have;“Ixola A(n)tarticha, Xe longo a ponente 1500 mia,” which will be discussed later.




The general chart has Cape Buxedor at the latitude of Feurteventura as the 1436 chart, but then proceeds south to Cabo Verde and Cabo Roso with the two isles, Dos Ermanes, to the west. The Senegal River is shown as on the Mecia de Viladesters 1413 chart complete with island and 5 rivers from the mountains. Most importantly Cabo Brancho and the islands to its south comprising the Bay of Arguin are well noted. There was a trading factory on the Isla de Grain (Arguin) in 1443 and thus the Bianco chart of 1448 indicates the Portuguese expansion over the 5 years. Hence from the beginnings of exploration of this West Coast which commenced on charts with the Vesconte chart of 1321 and Mogador, Dulcert 1331 and the Catalan Atlas of 1375 with Bojador and the Canary Isles, we moved onto the 15th century with Viladesters 1413 up to and including Benincasa 1469 (see later).


Utilizing the profile of West Africa on the 1448 chart I have endeavoured to translate the place names and provide a geographical location for each. I am not a palaeographic expert and have written out the Andrea Bianco names as best possible and translated accordingly. Any errors are thus entirely mine and should be corrected with my thanks. I have included later an expert’s evaluation of one simple sentence which will explain the difficulties.
Most should be incontrovertible and they illustrate a new accuracy, such as at Cape Blanc we have the name Pedra Galha which refers to the two Islotes Piedra Galha, the islands noted on the Viladesters 1413 chart. Thus we arrive in the southern portion of the chart where Andrea Bianco still has Viladesters two isles but names them “Dos Ermanes” and situates them opposite Cape Verde. However he shows just south of Cape Verde (14.75N geog) Cape Roso, which has been previously interpreted as Cape Roxo some 2 degrees south of Cape Verde when in fact it is correctly drawn just around the bay from Cape Verde and is in fact Cape Rouge at 14.633N geog.






The background to any Portolan Chart is the Windrose and its construction determined by the original work of Petrus Vesconte for his 1318 atlas which is crucial if it is to be used as a draughtsmanship guide. Following on from the 1436 Atlante Nautico, which had a centreline at 35.5N and terminated at c27.5N, this chart extends to the newly discovered Cape Verde, nominally at 14.75N (15N) and is set out from the 35N latitude.
Within texts ChWR1 and ChWR2, the basic parameters and construction of the Windrose are explained so that a draughtsman could draw one without recourse to compasses. It is a matter of simple geometric proportion confirmed by the scale bars. Any Windrose that has the standard 22.5/45/67.5° divisions will in fact be a basic 92 unit (circle) construct and the subdivisions will fall at 35/30/20/7 units. Thus with a putative radius of 1600RM, the basic subdivisions are 608.7RM or 8.116 degrees; 521.74RM or 6.96 degrees; 347.83RM or 4.64 degrees and finally 121.74RM or 1.62 degrees.
Thus we must ask the question; did Andrea Bianco use 1600RM so that he could actually consider his subdivisions as 8/7/4.667/1.667 = 21.33 degrees? We must always bear in mind the fact that at this time (as will be adequately proven later) it was common place to adjust measurements etc to suit mathematics. They had quite a cavalier attitude to it all.
Thus with the Windrose at 1600RM or 21.33 degrees radius the chart is extended north and south by differing amounts as the diagram illustrates. Thus the northern limit is 59.576N (60N) and the southern limit is 7.17N (7N). Other than the standard graticular lines at 35/30/20 sections there is an extra N/S line to the east of the centreline which splits the 35 unit section into 21 and 14 units. Its position is determined by a geometric construct and is thus for an 11.25 degree locator when drawn from the centreline across the 35 unit square and can be calculated as the diagram illustrates. This is of course the 7 unit section giving a Tangent ratio for 11.25 degrees as 7:35 or 1:5 the standard appertaining then and hence the 35 units are split 3:2 or 21:14 units as shown.




I have subdivided the chart into 2 sections, north and south of its Windrose centre line at 35.0N. The northern section has a latitudinal scale of 75RM with a longitudinal scale of 62.5RM as per the 1436 chart. There are minor aberrations in the draughtsmanship but generally as with all of Andrea Bianco’s work it is excellently constructed.
However when we study the construction below the 35N latitude, a completely different picture emerges which is probably due to the transmission of the data from Portugal to London where Andrea Bianco was quartered and thus drew the chart. In this section we see a gradual revision of the latitudinal degree length from 75RM to the 31N latitude- precisely as occurs on the 1436 chart- then to a 100RM degree of latitude. I have appended scales to the diagram for comparison, but must state it is predicated upon a single fact that Cape Verde is correctly positioned at 14.75N (15N). However when the putative 100RM degrees are studied it becomes obvious that there is possibly a single major cause for the expansion, the latitudinal spread of the Bay of Arguin, Cape Blanco to Ras Timirist. Naturally with the increased latitudinal scale the longitudes, marked in the south are proportionally larger. Thus we must ask in what form the data arrived in Andrea Bianco’s hands. Was it a combination of distance measures from sailing or was Cape Verde actually positioned by latitudinal reading. This may have occurred at the 1443 first voyage of discovery. Naturally any attempt at the actual latitudinal positioning via the Pole Star, which had 3.5 degree circumpolarity at the time, was possibly a problem and accurate to only a single degree of reading.
There is in this change of distance measure the obvious problem encountered so often, that of the figures being taken as Roman Miles when in fact they are Millara distances and hence an expansion of 120% from 75 to 90 units. Thus given the scale bars length and the probability it was drawn on the chart after the actual coastal profile there can be a small discrepancy such that we should be measuring 90 units not 100 units, and it is all Millara distances. The fact that we have seen the same distance measure used on the Viladesters 1413 chart tends to lend weight to this theory of a conflict of names for miles, Mile to Millara.


When I first studied this chart many years ago I was unable to fully determine the actual letters which formed the above and hence uncertain as to its actual meaning. Then in 2002 Gavin Menzies book, “1421, the year China discovered the world”, was published, within which, CH15 = Solving the riddle; CH16 = where the earth ends; CH17 = colonizing the new world; CH18 = on the shoulders of giants, there is to be found a systematic analysis of these Portolan Charts, the Portuguese explorations and an explanation of the Antilles Island of Pizzigano and the Satanazes Island. But on page 377 I read, “Andrea Bianco’s map of 1448 referred to, “Ixola Otinticha. Xe longa a ponente 1500 mia” with a translation reading “a genuine island is 1500 miles west of here (West Africa)”. My own attempt had told me that it probably read as “Xe longo a ponente 1500 mia” but the islands name was unfathomable. Thus when I decided to look at this chart again because of the research which now culminates this text, I wrote to the one person who is probably the expert of experts, Ramon J Pujades y Bataller and as usual his kindness meant a full response (not the first time) and was for me a master class in palaeography. I thus reproduce his response fully:
In my opinion, and as far as I can see on the image that I have, it could be read “Ixola A(n)tarticha. Xe longo a ponente 1500 mia”, that could be translated from Venecian to English as: “The Antarctic Island. It has 1500 miles long to the west”. Anyway, that reading should be checked on the original. Initially it seems that it could be read “Ontarticha, due to the first letter of the name place, but if you compare with the last one, which is clearly an ”a”, it seems very probable that the descendant very small line that differences “o” and “a” in this handwriting was just not so marked in this case. About the central “a” it happens something similar. If you compare again with the last “a” you will realise that the author writes the “a” by adding to a curved line (similar to C) a descendent straight line, as it is normal in this kind of late gothic handwriting. Due to this, it was also normal that sometimes the second descendent line was written a bit separated of the first curved line, and, due to this we find what palaeographers call an open “a”. Thus, and in the context of the Italian Renaissance and, specifically, on a chart produced by an author that has already copied Ptolemy’s mappa mundi. I think it is the most suitable reading, although it should be checked on the original, of course. This also suits with its position on the very extreme of the map (to the south of the mythical “dos ermanes”) and with its enormous size. As “dos ermanes” it was just a speculative introduction from the geographic theories of the time (= Terra incognita australis). The other proposals of absolutely unknown name places do not seem reasonable to me. That is my opinion and just my opinion, of course.


Thus there is a slight frisson of doubt in my mind as to the meaning apropos “longa a ponente”. Is it actually saying “1500 miles long to the west” or “1500 miles away to the west”. The one produces a massive land mass stretching from just south of Cape Verde to the west and the other an island of unknown size 1500 miles away. Thus has Andrea Bianco just chosen this point on the southern bordure to mark an island not knowing if anything is located there or not; or, is it to hint of lands and islands closer than the “Terra Incognito Australis”, which surely Andrea Bianco would understand must be at least 60 degrees south, below the Equator.

However one simple point came to mind; 1500 miles is 375 leagues and the final research of this paper explains why it may be so important.




The Benincasa 1467 chart is part of an Atlas, of which there are at least two extant copies; one in Paris and the other London. It was drawn in Venice and basically repeats the 1448 Bianco chart and is an extension to the 1436 Atlas of Bianco even to the numbering of the Atlas pages. But there are not as many toponyms included and they are more akin to geographical names for meaning with some places the name of actual occurrences, such as Ponto Cavalier where Antao Goncalves was dubbed Knight. I have looked at both atlas pages and assessed the toponyms which indicate that they are not in agreement in some places and the spelling varies from chart to chart. They appear to be by the same hand and perhaps it was being copied from a master i.e. Bianco and not with total compliance for all the names.
Again though we can see a split in the charts distance measures for latitude. Above 28N the chart has 75RM per degree, but below that it is by scale bar 56.9RM, but surely meant to be 56.667RM or 56 2/3rds, the Arab Mile degree. There is a reasonable accord with the putative parallels for the construction of the chart. However on the second diagram, the Windrose is analysed, it being 650RM radius or 8.667 degrees at 75RM for the northern part and thus 11.42 degrees at 56.923RM (11.47 at 56.667). The longitudes are far more accurate and thus it is an improved version of the Andrea Bianco 1448 chart, although perhaps the distance measures are awry, but naturally, as Andrea Bianco was Venetian, working there etc. we should expect the correlation of the charts.



The four diagrams which interlink are from two pages in the atlas covering the west coast of Iberia and Africa from Portugal to Guinea. They clearly show the discoveries made along the coast and the knowledge gained of the latitudes involved. The Cape Verde Isles are clearly shown, although not all are named and positioned correctly, but are very indicative of the exploration and the data being transferred back by the mariners to the geographers.






This is an investigation into the distance measures quoted in the letters and NOT an attempt at delineating the Line of Demarcation longitudinally. That has been attempted too often without a specific attempt to resolve the measurements that have been used, other than to quote the standards expected from the Spanish data. To illustrate the storyline I am including a very basic resume of the actions leading to the determination of the distance the line of demarcation should be from the Cape Verde Islands.


Pope Sixtus IV confirmed it in his bull, “Aeterni Regis, 21/6/1481” and the 8th article established spheres of influence in the Atlantic Ocean.
Portugal = “all the trade, lands and traffic in Guinea, with its gold mines and whatever other islands, coasts and lands” that may be discovered as well as Madeira, the Azores, the Cape Verde Islands and any other islands that might be found and conquered below the Canaries and opposite Guinea. The Canary Isles were explicitly reserved to Castile. The problem occurred when Columbus returned from his epic voyage to the New World and under this Treaty Portugal claimed it all. Thus Fernando and Isabel of Castile appealed to Pope Alexander VI to set down the line of demarcation between Portuguese and Spanish areas of discovery. The next treaty eventually resolved these issues, well nearly!


Trouble brewed as Portugal challenged Spanish rights, but Spain appealed again to Pope Alexander VI who agreed with Spain and issued a bull, “Inter Caetara, 3/5/1493”, but it was too vague and another was issued on 4/5/1493 in which he drew a line of demarcation from north to south assigning lands 100 leagues west and south of the Azores and Cape Verde Isles to Spain, provided they were not subject to other Christian rulers. Still later, on 26/9/1493 the Pope issued another bull “Dudum Siquidem” entitling Spain to all lands discovered by sailing westward or southwards toward the Orient and India.
Portugal was very dissatisfied by these Papal decisions and negotiated directly with Spain and thus on 7/6/1494 the Treaty of Tordesillas was agreed, to be ratified by Spain, 2/7/1494 and Portugal 5/9/1494. But, most importantly it was not submitted for Papal approval. The Treaty has 4 clauses ;

  • 1) a stripe or straight line from pole to pole 370 leagues west of the Cape Verde Isles. All to the east, north and south would belong to Portugal and everything west to Spain.
  • 2) they are not to explore in each other’s zones
  • 3) 10 months to establish the meridian between the zones
  • 4) permission for Castilian subjects to cross the Portuguese zone on the way west.

There was an exception that if Columbus before June 20th 1494 should discover any lands beyond the first 250 leagues, they would belong to Spain, any other east of that point would be reserved for Portugal.


This is therefore 370 leagues from the Cape Verde Isles. That also caused problems as at first it was from Boa Vista, the central and easternmost isle (at c16N/23W), but was then argued as being from the westernmost isle, Santo Antao (at 17N/25W). Those differences geographically being important on the coast of Brazil, but were more important on the other side of the world, 180 degrees away, when the Spice Isles were being apportioned. But at no time is the League length confirmed by reference to any known measurement in a treaty.


The complete correspondence comprising three letters is an appendix as I am only analysing the final response, which contains all of the measurements and problems.
The basic facts contained in the paragraphs of the response letter by Jaime Ferrer, 1495, with some comments contained within (brackets) for info only.

  • 1) 370 leagues start from the Islands of Cape Verde (league unknown but probably 4RM)
  • 2) Cape Verde and Isles 15N; 370 leagues = 18 degrees; 1 degree = 20 5/8ths leagues at 15N
  • 3) Sail along a parallel and the Pole Star is always at the same elevation (altura sailing)
  • 4) Sail from Cape Verde Isles on a course 11.25 degrees north of west until the Pole Star elevation reads 18.333 degrees. Then turn south and sail until the Pole Star elevation lowers to 15 degrees and you are precisely 370 leagues west of Cape Verde Isles. (Tan 11.25=20%)
  • 5) —– (problem of marking a line on the ocean)
  • 6) Cape Verde, a quarter wind by 370 leagues at 20% = 74 leagues which is 3.333 degrees latitude. (nominal ratio is taken from the Tavola de Marteloio)
  • 7) Each degree has 700 stadia, although Ptolemy says 500 stadia (sleight of hand involved)
  • 8) and 9) are just a methodology which is fraught with the difficulty of involving sailors.
  • 10) Ptolemy, 500 stades/degree; 8 stades/mile = 22500 miles or 5625 leagues for circle
    • each degree = 15 and 225/360 leagues. i.e. 15 5/8ths leagues
    • Tropics; 164672 stades, 20584 miles, 5146 leagues
    • each degree = 14 and 106/360 leagues (14.2944)
    • Eratosthenes, 252000 stades, 31500 miles, 7875 leagues
    • tropics = 7204 2/5ths leagues
    • Rule of Three; 22500 Equator = 7875 (i.e. 7875/22500 = 0.35)
    • 20584 = 7204.4 (i.e. 20584 x 0.35)
    • therefore, the tropics = 670.5 leagues or 2682 miles less than the Equator.
    • (Calculating at 700 stades / degree, but 7875 leagues = 20 7/8ths per degree and thus 87.5 Millara per degree. J F has used 20 5/8ths but it could be one of many miscopies)
  • 11) Equator = 21 .625 leagues (but 360 x 21.625 = 86. 5 x 360 = 31140 and 87.5 x 360 = 31500)
    • Tropics = 20 4/360 = 80.044 leagues
    • (80. 044 x 360 = 2885.84 divided by 4 = 7203.96 i.e. 7204.)
  • 12) Cape Verde, 370 leagues = 18 degrees at 15N therefore a degree at the Equator = 20.625.
    • (but, 370 divided by 18 = 20.555 or 20 5/9ths not 20 5/8ths.)
  • 13) Note. This paragraph is probably the most important of the whole letter and thus it is worth inserting it here-in.
    • From Cape Verde to the Grand Canary Island are 232 leagues of four miles per league, and it lies from the said Canary on a meridian almost at a third of lebeix or south western quarter, and is distant 15 degrees from the Equator, and the middle island of those which lie in front of Cape Verde lies in the quarter of the west towards northwest 177 leagues (away), which are equal to 5 2/3rds degrees; and from this middle island commences the terminus of the 370 leagues towards the West which terminus is 18 degrees towards the West from the said middle island, and on that parallel each degree is 20.625 leagues, counting 700 stades to a degree, according to the above cited learned men, although Ptolemy uses a different calculation.
      Discussion; firstly and unfortunately, a third of Lebeix can be read as meaning 15 degrees west of south or 15 degrees south of South West (Lebeix). But the quarter of West is only 11.25 degrees north of west. Thus the two diagrams illustrate the possibilities and also how the middle island’s position is determined. Thus we can indicate that from Cape Verde to Gran Canary it is probably 12.37 degrees and this places Gran Canary at 27.37N (OK).
    • However, it is evident that the Leagues are Millara based as can easily be demonstrated.
    • Thus we have; 75RM = 90 Millara = 720 stades at 8 per millara
    • Therefore 700 stades = 87.5 millara and 1 League = 21.875 or 21 7/8ths millara
    • But 117 leagues = 468 millara = 577.044Km or 5.382 degrees at 15N (5.667 by JF)
    • And, 370 leagues = 1480 millara = 1824.84Km or 17.02 degrees at 15N (18 by JF)
    • Return to the diagrams and Cape Verde is 62.164 leagues west of Gran Canary which is 248.656 millara or by JF figures 247.5 millara/305.17Km/206.32RM, which is the equivalent of 1.924 degrees at 15N and in fact the two are 2 degrees apart longitudinally.
    • The alternative when the two are set out from Gran Canary at 30 degrees actually indicates that Boa Vista the central island would be the correct locator for the measurements. I have included a Mercator projection plot of the Canary to Verde route which geographically confirms the text above.
  • 14) Ptolemy Equator = 15.667 leagues and the Tropics = 14.33 leagues per degree. (Therefore 370 leagues at 15N = 25.227 degrees {nearly 25.33}).
    • (Therefore 370 leagues at 15N = 25.227 degrees {nearly 25.33}).
    • 15.667L = 62.668M = 501.344 stades i.e. 62.5 miles
    • 500 x Cos 23.75 = 457.656 = 14.302
    • 500 x Cos 15 = 482.963 = 15.093 (15.667 x cos15 = 15.133)
    • Therefore 370 divided by 15.133 = 24.45 degrees.
  • 15) Cape Verde 9.25 degrees north (according to Columbus). The 180,000 equals 252,000 stades of differing lengths.

It is worth noting that EGR Taylor, page 177 of “The Haven Finding Art”, wrote;
“Such a north/south line appeared, too, on some charts to show how the Pope was considered to have partitioned the world’s discoveries, although a commission summoned to Badajoz in 1524 to plot it precisely had broken up in confusion, for there was no way of fixing it over the ocean. One of the delegates had indeed put forward the idea that it could be done by the Rule of Marteloio. Sail out from the Canaries along a certain quarter of the wind, he said, and by the table you can calculate when you are 370 leagues west, and so find where the line is to run. “And supposing you found it, how could you mark it on the sea?” they replied.”
The problem of Badajoz was amplified by the ideas of other Seamen who specified another distance from Santo Antao, the most westerly of the Cape Verde Isles, as being 22 degrees plus 9 miles. However this led to the Treaty of Saragossa and a further division.


It is worthy adding a paragraph here-in dealing with the figures given by Columbus as to latitudes and thus we gain an understanding of his very limited navigational expertise.
Firstly his method for establishing the measure of a degree was to compare the ship distance travelled with the altitudes of the sun irrespective of any change of declination. In his Guinea voyage text he records that in the Isles of Los Idolos he found he was only 5 minutes latitude above the Equator (but he states different latitude in his notes as my text ChMEA/1). These isles are c9N and the 5 minutes is the sun’s northerly declination. At Elmina he therefore assumed he was on the Equator when in fact it is at 5.5N and the declination was 5.617 degrees, hence no reading. Off the coast of Cuba he thought he was at 42N when in fact declination was 17 degrees and thus 42-17 = 25N, and is OK.
Therefore it is fair to opine that cartographers using his data were sorely misled.


In the Middle Ages there was a definite trend to alter distance measures, particularly for degrees of latitude multiples to suit the mathematics of the age. Thus what they saw as numbers which would be hard to deal with mathematically they massaged to a number more compatible with the methodology being used. The variation in the Cosine and Tangent ratios is a case in point. Thus when these figures are being researched it is advisable to look at slight variations for the probable original unit as that used may be a “ringer”.
This is a short summation of the mathematics within this paper, but it is based upon the accepted norm which I will dispute later in the text.

  • 1)There are 600 stades of c185m in a degree of latitude which may be expressed without significant error as 111Km. The actuality for those who consider equations necessary in the research of Portolan Charts would be as follows;
    • 1 Roman Mile = 1.4791Km. 1 Stade = 184.8875m and thus 600 Stades, a degree of latitude = 110.9325Km or 75 Roman Miles.
  • 2)For reasons best known to them, Mariners and Cartographers in the Middle Ages chose to introduce another measurement into the system, that of the Millara. It has been assessed as 5/6ths of a Roman Mile, which is in itself quite strange as neither the 1000pm or 5000p readily divide to the 1/6th part. There are other ideas which reduce the Roman Mile to the Digitus level but I think that is too farfetched for credibility; why do that?
    Thus accepting for now that the Millara is 5/6ths x 1.4791KM or 1.232583 Km (for those who desire accuracy, but surely 1.233 is adequate, but please read the last section regarding the Millara before leaping to judgement).
    However, for the degree of latitude we can work it in the opposite; thus 6/5 x 75 = 6 x 15 = 90 Millara, with a single millara being 8 units of 154.0729 metres, and is thus 9.6 Millara Stades to the Roman Mile, or 720 per degree latitude, accepting the above comments.
  • 3)90 Millara = 720 M Stades; but Eratosthenes stated that 700 Stades make a degree and thus if you believe that fact a degree must equal 87.5 Millara of 8 units. However, if Eratosthenes stade was 157.5m then the discrepancy is only c3.5m per stade, 154 to 157.5.
  • 4)A league equals 4 miles (of indeterminate length country by country, but generally Roman Miles) but with each league a degree of 700 units will be either 175 units or 21 7/8ths Millara (21.875).
  • 5)At 15N the Cosine is 0.9659528 or 966/1000 or 483/500, but can be resolved to 676/700. Thus 700 units at 15 degrees = 676 units or 21 .125 leagues, which if resolved to a degree of latitude = 21.87 via the Cosine i.e. 21.875 or 21 7/8ths as above.
  • 6)Actually a degree is 75RM or 22.5 leagues. Hence at 15N it produces a degree of 21.73 leagues (21.75) and the possibility that the calculated units have been mis-used as 21.73 leagues reduces to 21 leagues at 15N. These figures are all acceptable and could be originally used, only to be written in these letters to suit Jaime Ferrer.
  • 7) Jaime Ferrer states that at 15N the degree is 20.625 leagues which would give a latitudinal degree of 21.353 leagues via the Cosine (or in fact 676/700) and not the 20.625 he states but a length of 20.888, i.e. 20 8/9ths at 15N
    • But 21.625 x 4 = 86.5 and not the 87.5 of 700 units.
  • 8)There is obviously a problem with the copy text as we would expect Jaime Ferrer to coordinate his calculations, having taken so much trouble to write two letters to their Majesties of Castile. I cannot believe that Ferdinand and Isabella having received the second 14 paragraph letter did not have competent mathematicians who could confirm the whole mathematical argument as set out. Thus I consider the investigation not worth pursuing further unless evidence of the Jaime Ferrer actual distance measure was confirmed.

The Treaty; missing from all texts is the single fact required the length of a league in terms of finite measurement. The above investigation endeavoured to evaluate just what Jaime Ferrer is actually stating, but the figures given in the translation as shown are awry. Do we therefore require considering the consequences of the change from 720 to 700 stades as indicated. The league length changes via 90 to 87.5 Millara or 22.5 to 21.875 leagues which at 15N produce 21.73 or 21.13 leagues. The 370 leagues are stated as 18 degrees or 20.555 Leagues or 20 5/9ths per degree and thus there is only a 0.6 league per degree or 10.8 league overall change, just over ½ degree error or change in the overall distance.
Thus from Cape Verde Isles the 18 degrees is either 18 + 23 = 41 or 18 + 25 = 43 West plus ½ degree for this evaluation. Hardly worth the trouble to evaluate!
But look at most texts and the league length is given as 4 Roman miles, and generally stated as that used in Spain from the 15th to the 17th century. Would Jaime Ferrer have not known the difference between the Millara and Roman Mile? Therefore what possessed him to shorten the league from 90 millara to 87.5 millara or a 36:35 reduction?


The 1448 Chart of Andrea Bianco contains either an island 1500 miles long to the west or an island that is 1500 miles west of the position shown on the chart, which is approximately 2 degrees west of Cape Vert.
Cape Verde is at 17.5 west; then add 2 degrees to the drawn island and 18.25 degrees for the 375 Leagues (1500 miles) and we have a total longitude of 37.75 degrees west.
Jaime Ferrer states, from Cape Verde to the islands are 5.667 degrees; that + 18 degrees to the Line of Demarcation = 23.667 degrees west. Andrea Bianco shows the last land in the west at 20.25 degrees from Cape Vert. But as shown above the Jaime Ferrer distances are short by a ratio of 35:36, not much really but it all adds to the impossibility of accuracy.


The minimum westing from the Treaty is 41W and this can be read as an attempt to ensure that the Ixola Antarticha was solely within the sphere of Portuguese influence and exploration ( should it be proven to exist). It also provides a landfall position on which to mark the Line of Demarcation, as it cannot be established on the ocean. Thus any other land could be determined by sailing either northwards or southerly from this “Island” and all lands to the east are Portuguese and that to the west Spanish.
That is just a thought on the possible reasons for 370 leagues as it does not appear to be a logical distance measure, but it was politics of course, but the distance cannot be marked on the ocean.


Since the late 19th century it has been thought that the Millara was 5/6ths of a Roman Mile, and this was determined by observations on Portolan Charts etc. But the sixth part of 5000p is an awkward number and thus it was then deduced as being possibly derived from the digitus level of measurement. My texts have always stated that this is not an acceptable basis for the Millara derivation as it is both a mariners and geographer’s measurement. The mariner had to be able to mark off on the ships rail or on a logline, subdivisions such that the distance and speed could be calculated with a degree of accuracy and thus certainty. The Geographer had to be able to measure the distance on land such that the charts being drawn were again as accurate as possible. This means it had to be a finite measurement on land and one capable of constant reproduction in a very simple manner.
But at 5/6ths is it a quantifiable measurement, capable of easy use on board ship or even on land for that matter. But I believe it is not one that would be used because of its complexity. The basic figures are as follows:
1 Roman Mile = 1000pm = 5000p = 1.47917662Km given that the Pes = 11.64706 statute inches (nominally 11.65 ins.) and 5/6ths of the Roman Mile is therefore at 1/6th = 833.333 pedes or 246.529m. Thus, 5 x 246.529 metres is 1.232647183Km per Millara.
But 5/6ths x 5000p = 4186.667p and if divided into the standard 8 stade subdivisions, each is 523.33p or 154.8205 metres. Thus the measurement of 523.333p on board ship by a log is practically impossible as its subdivision becomes a nonsense figure. But we must consider the vagaries of Medieval mathematics and consider if they would have chosen a close alternative to the actual figure, that of 525 pedes which subdivides in either, 21 x 25p; 35 x 15p; 70 x 7.5p; 105 x 5p, all of which are capable of calculation on board ship with no error determined. But none of that is from any information available to us today.
Thus I was convinced the precise 5/6ths division of a Roman Mile was not the actual Millara as known in the 14th century, it was a close figure and sufficed for our use as a check on the charts, but it was certainly not the actual measurement used by mariners/geographers.


The land surveying techniques of the Roman Agrimensores or Geometres is well known and documented. It has as its basis a square unit measuring 120 x 120 pedes entitled the Acti Quadrati. This was an important measurement as it could be set out by a simple right angle triangle of 120 x 120 x 170 pedes with no appreciable error. That is because the Pythagorean calculation gives 120 x 120 = 28800 and 170 x 170 = 28900, and hence the hypotenuse should be 169.7056p, or 0.2944p error which in out terms is 3.5 inches in 1980 inches hypotenuse.
If we now consider the handrail on a medieval ship and the fact that it in all probability had length marks carved thereon for the log, how easy it would be for a 6p or a fathom interval, the twentieth part of the AQ to be the norm to gauge the distance travelled or speed of the ship.
But of course we now meet the usual problem of non-coordination of measurement as the Roman Mile of 5000p is actually 41.667AQ. This however is easily resolved as other rather awkward measures were by the medieval methodology of manipulation, sleight of hand and substitution of an approximate unit to resolve a difficult calculation. Hence the Roman Mile, originally 1.47911Km and having 5000p or 41.667AQ, morphed into a Roman Mile of 42AQ, 5040p or 1.491Km, a mere 11.81 metres expansion and probably not even noticed by the majority of the population as they were not really involved.
Thus 5/6ths of the new unit is so easy to evaluate and use on board ship or land as 5/6ths of 42AQ is 35AQ or 4200P reflecting the original 42AQ Mile, and the Millara was thus born. Simply, 5/6 x 42AQ = 35AQ = 4200p = 1.24251Km and it is a mere 9.86 metres/millara difference which quite frankly is irrelevant in terms of mariners measures and geographers measurements on a chart.
But I think there is also a precedent for the use of the 42AQ or 5040P. In this period and the preceding eras the Geography of Strabo and thus Eratosthenes was available with its determination of the world size as 252000 stades calculated from the 1/50th part by measurement, which is 5040 stades. As we have seen in the preceding text the 700 stades per degree was considered correct and thus I can opine that the Millara is 5/6ths of 5040p and is thus 4200p or 1.24251Km.
But of course that also means that the Millara is 700 fathoms of 6 pedes thus reflecting the 700 stades per degree of latitude given by Eratosthenes. But I now remind you that in Genoa the Miglio Italiano was thought of as 1000 passi or 7000 palmi and they developed the Genoese Marittimo Miglio of 10 stades of 185.2 metres approx. There is a complete logic within the metrological system developed in Genoa in the Middle Ages.

M J Ferrar February 2017.