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The “Boke of Idrography” is held in the British Library and is one of their digitized manuscripts which is a totally interactive web page enabling the complete atlas to be studied.

There are a large number of blank pages when it is viewed as the presentation is one side only. The first folios, f1v to f6r contain an introduction, winds, pole star and “the extraction of radices”.

Folios f7r to f30r contain the 11 charts which are all plain charts with the exception of folios f29v-f30r which are the western and eastern hemispheres. There is a separate research paper concerning those folios, dated 2019 as is discussed later.
Folios 31v-32r. There-on is noted; “made by me Johne Rotz, servant to the Kingis mooste excellent Majeste, in the yer of owr Lord Gode and of his triumphant regne the xxxiiij. Yere.”

The British Library data for this atlas is an excellent commencement point for study.


He was born of a Scottish Father and French Mother and became part of the “Dieppe School of Cartography”, being described as a hydrographer, instrument maker, student of marine astronomy and an experienced sailor.

There is a second manuscript of his written in French which is noted as “sometimes difficult to decipher” and it is entitled “Treatise on the variation of the magnetic compass and of certain notable errors of navigation hitherto unknown, very useful and necessary to all pilots and mariners. Composed by Jean Rotz, native of Dieppe, in the year 1542”.

That is the year in which Jean Rotz decamped from France to England and entered the service of Henry VIII and presented (?) or commenced (?) the “Boke of Idrography. As there are two important texts by Jean Rotz supposedly dated 1542 it is conceivable that the Atlas was already in preparation when he left France. The text noted above came with an instrument named “Differential Quadrant” which from its form was not an easy or quick manufacturing process. Hence I am wary about the dates all being 1542 and believe they commenced much earlier and were completed in England for King Henry XIII to give Jean Rotz a £20 payment, half of the total promised.


To fully describe several of the atlas charts I have chosen to illustrate I have placed them in a continuity sequence as if travelling from north to south, not as they are listed and shown on the atlas itself. I have included the full list at the end of this text.

Folio 2r Introduction; “The introduction of the thingis conteint in this booke”


After a textual description of the “conteint” there is a single sentence which is the most pertinent to the examination of the 11 charts and their construction.

“Item in the bordorres of the leffes of the booke or set the tabilles, quhar the leigges or merkit and writin with cyfres be their nombres, efter the proportion of xvij legges and one halfe for every degree of latitude, as be al saillars it is acoustumit.”

The first peculiarity to be noted is that the folio 2r shown in the Diagram ChBoI/1/D01 with the preceding folio 1v and an enlargement of the section referred to above clearly indicates a possible second hand/script from the dedication to Henry VIII which is written in French. The Introduction passage is in a much smaller script and a less florid hand. However, as there are 15 lines of script squeezed into c2/3rds of the French text spacing, it is possibly just a quirk of presentation?

The latitudinal measure of 17 ½ leagues per degree is not quantified by an actual measurement per league and at this period, c1542, there were many leagues in use and I can find only one country with 17 ½ leagues per Degree and that is Spain.

Fortunately we can show that the imposition of the league when Portolan Charts and Atlases used the Miliaria, 90 per degree and also at times the Roman/Italian Mile of 75 per degree comes from the Spanish request to the Pope to adjudicate between Spain and Portugal regarding which country had access to the new found lands both west and east. This led to the Papal Bull establishing that the world be divided by a line of demarcation set 370 leagues west of the Cape Verde Islands. My text “MsFER/1, Jaime Ferrer, 1495 and Jean Fernel, 1528”, fully explains and illustrates the complete stupidity of the distance given and its impossible positioning. Hence the arguments raged on for years between the two countries.

Folios 19v-20r: General chart of the Black Sea, the Mediterranean and the coasts of Europe with the North Sea and the Baltic.


This is a standard Portolan Chart with side latitudinal scales at 10 degrees =175 leagues. However the western scale is set c5 degrees north of the eastern scale and is the more accurate apropos the charts coasts.

When putative latitudinal and longitudinal lines are applied to the chart and the measurement is taken via the League scale along the 36E longitude, from 31N to 45N the league scale bar indicates a length of 300 leagues for the 14 degrees and thus each degree is 21.42857 (21 3/7ths) leagues and each league is either 4 1/5th Miliaria or 3 ½ Roman Miles. Each latitudinal degree is thus 1.2245 larger than the 17 ½ leagues of the text.

If we now carry out the same measurement check for longitude along the 36N latitude commencing at 6W and then to 36E, i.e. 42 degrees longitude, the league measurement is 720 and thus each degree of longitude is actually 17 1/7th leagues.

At 36N the standard ratio for latitude to longitude given by the cosine of 36 degrees is taken as 5:4 and thus we have 21 3/7ths to 17 1/7th which is a perfect 5:4 ratio used for the charts from Claudius Ptolemy forward. If the 17 1/2 degree had been used then the ratio would be 17 ½: 14 leagues.

The latitudinal scales either side of the chart are thus null and void, but why are they a feature of all charts and are some c5 degrees adrift from each other?

Folios 19v-20r, the setting out of the wind rose.


Study the setting out of the wind rose and the normal rectangular graticule lines are by ratio 35:30:20:7 for the radius measure and are very muted but the red lines are heavy.

From the centre point the 11 ¼, 33 ¾, 56 ¼ and 78 ¾ degree lines have been drawn, but they are not the normal setting out for a wind rose construct. The main feature is the 11 ¼ degree grid of red lines which give the chart its slew counter-clockwise as per a normal Portolan Chart.

Thus Jean Rotz is emphasizing the methodology which follows through to all of his charts including the hemispherical charts and this is clearly shown later.

Folios 21v-22r; chart of the western coast of Europe with the opposite coast of N America from 74 1/2N to 29 1/2N.

The latitudinal scales have a 4 degree shift from the west to the east, and the eastern scale is reasonably correct for Europe and thus we may consider that the latitudes are 17 ½ leagues on this chart. What longitude it is possible to see from 9W to 2E gives either 16 or 17 leagues per degree. However, the coast of Labrador is wrongly oriented and Newfoundland, 46 ¾ to 52N and at 52 1/2W is positioned acceptably. Iceland latitudinally is correct and if Labrador had been drawn correctly then no doubt it would be further west to give 14W-24W and 64N-66N.

Folios 23v-24r; chart of the eastern coast of America from 51 ½ to 6 ½ N, the West Indies and Gulf of Mexico.

Having drawn the putative geographical latitudes and longitudes, it is quite obvious that yet again Jean Rotz has tried to use the western latitude scale bar for the chart which commencing in the south is relatively geographical but from the Tropic northwards is completely awry. The latitudinal distances are very varied and the longitudinal distances between 60W and 80W are reasonable but from 80W to 85W there is a glitch.

Newfoundland in the north is as described on Diagram D04, with the putative 50W longitude close to the 52 ½ W actuality.


Folio 19v; the Black Sea.

The detailed record of the manuscript by B.L. has included a thumbnail chart of the Black Sea. However it lists the Mediterranean Sea Chart as folios 20v-21, which is wrong. Folio 19v-20r in fact covers this chart area and the Eastern Mediterranean, but it does not actually exist.
It is included because of the self-evident mis-placement of the Latitudes and their scales as already discussed for folios 19v-20r, diagram D02.

Folios 13v-14r; chart of the eastern coast of Africa from the Line to the Cape with Madagascar and the Ethiopian Archipelago.

The heading by B.L. is obviously wrong. Firstly the chart extends from the Cape of Good Hope, 35S/20E to the Equator and hence the islands south of the Equator are those of Tanzania and are; Pemba Island c40E and 5S; Zanzibar Island c39 1/2E and 6 1/4 S with Mafia Island , the southerly at c40E and 8S. Ethiopia is basically 5N to 17 ½ N.

From the measurements it would appear to be a square chart with degrees of 18 ½ leagues.

Folios 9v-10r; chart of the Indian Ocean, from Cape Comorin on the west to Aimoey Bay in China on the east of the chart. And from 25 1/2N to 19 1/2S, including “lytil java”.

This is a remarkably accurate latitudinal portrayal of this area from 5S to 22N with agreement of the two side scale bars to 0 ½ degree. However Jean Rotz has not included a league scale bar upon these two folios and hence the only comparison to be made is the latitudes to the putative longitudes. Each degree of latitude is the same as folios 13v-14r, but their measurement cannot be assumed.


Folios 29v-30r; The eastern and western hemispheres.

Whilst researching for this text I found that the Journal of the Australian and New Zealand Map Society had published a paper in 2019 entitled, “The Lande of Java, on the Jean Rotz Mappa Mundi”. The text published in The Globe, Number 85, has this on page 4;

“The Jean Rotz Mappa Mundi (fig 1) is a type of dual-hemisphere Equatorial Stereographic (azimuthal) Projection with one hemisphere having a specific prime meridian and the other its anti-meridian. It is the oldest surviving example of the equatorial stereographic projection being used for a amp of the world (Keuning, 1955: Wallis 1981a)”.

The preceding paragraph is as follows: “In the plate showing West Africa, it is clear that Royz used FERRO as his prime meridian for the portolans in the atlas. The Rotz Mappa Mundi clearly has longitude measured from a prime meridian at BOAVISTA in the Cape Verde Islands, and not at FERRO (Wallis, 1981a, pl.17-18). Wallis also notes that the Mappa Mundi has details not shown on the portolans which she suggests indicates two original sources, with the source for the Mapa Mundi being of a later date (Wallis, 1981a)”.

FERRO is in fact HIERRO, the western most of the Canary Islands and was thought by some ancient geographers to mark the western limit of the world and hence longitude could be reckoned from it. However, folios 17v-18r only indicates one of the wind rose graticule lines passing through it , with the centre at approximately 11W in Sierra Leone.

The prime meridian on the Western Hemisphere is indeed the Cape Verde Islands and there are actually two islands noted?

I was perplexed by this and decided to look closely at the setting out of those Hemispheres and the chart set there-in.


It is obvious that Jean Rotz has drawn the 11 ¼ degree lines on each chart and has continued to do so upon these hemispheres. They of course set out a rectangular wind rose graticule as the diagram indicates and the question must therefore be asked: “Did Jean Rotz in fact intend And draws a standard chart, that is rectangular and not a chart which would require curved latitudes and longitudes to be stereographic”?.

At the same time I set out the problems of drawing the stereographic projection lines and used a 135mm diameter circle, hence the 90 degree sub-divisions were easy to denote. From the diagram you can readily perceive that if it was full size, i.e.c300mm the radius required would be enormous with a calculated maximum of some 1080mm radius. That would require a large beam compass some 1200mm in length and the utilization of a large table to achieve it. But, some of the lines are wobbly and I do wonder if it is all a freehand drawing.


At this stage I thought the “Stereographic Plot” was a non-starter and therefore decided to evaluate the chart as drawn ignoring the fact it was in a circle.

The first, western hemisphere was set out putatively to evaluate the chart as drawn by standard latitudinal and longitudinal lines, and it was clear when they were plotted the chart was actually a standard Portolan Chart onto which it was possible to note geographical units.

Hence we have the Greenwich Meridian and the Mediterranean Sea to establish the length of a degree of Longitude. From 9W to 36E on the 36N latitude it is the equivalent of 43 degrees latitude and thus it is in all probability a square chart as so many other Planispheres of this age are.

The Eastern Hemisphere is not so easily determined with putative latitudes and longitudes. However, it does follow folios 9v-10r setting out which again clearly indicates it is not a stereographic presentation.

The scale bar on the eastern chart is a series of 50 leagues per division with 90 degrees scaling 1575 leagues. That is 10 degrees = 175 leagues. But, as each degree is 17 ½ leagues the equator is thus 4 x 1575 and 17.5 x 360 or 6300 leagues., Thus given the world circle size each league is thus;
Miliaria: 90 per degree x 360 = 32400 and the league is 5.142857 Miliaria.
Roman Mile: 75 per degree x 360 = 27000 and the league is thus 4.2857 Roman Miles.



Jean Rotz has kept the basic scale bars as 1 degree equals 17 ½ leagues, when in fact the charts themselves do not actually agree when analyzed.

We are also informed that Jean Rotz studied the work of Pierre Apian whose basic units were the German League of 15 per degree, and the Italian League with 60 per degree and thus corresponding circles of 5400 German and 21600 Italian leagues. Sebastian Munster uses the same diameter and circumference.

From this we can establish that his Italian League is 1 ¼ Roman Miles or 10 Stadia, which is actually the Miglia Marittima. In Belgium, France and Holland the marine mile also equals 60 per degree and is now called the marine league.


Jean Rotz in his “English” introduction is quite clear that he is showing all the coasts of the world (obviously as then known) and intends that each degree shall have 17 ½ leagues. That has been shown to be a complete fallacy and is no doubt just the actions of a cartographer placing the same scale bars on each chart without thinking if the chart he is drawing is actually to that scale. One must ask if in fact another person assisted and prepared the sheets with those and the beautiful tracery on the sides?

But it is obviously more than that in its presentation of coasts at a large scale, subdivided into separate sheets such that each section of a voyage can be clearly understood by coast and features and ports. It is also clearly intended to show mariners the routes to the Spice Lands, India and the Spice Islands and in the opposite direction to the very lucrative fishing grounds off Newfoundland and Labrador whilst also indicating the Spanish domains of the West Indies and how to find them. England did not care about the Papal diktat of a line of demarcation for Spain and Portugal with the resulting pillaging by privateers. Thus the Portuguese cost of South America and the southern portion of that coast supposedly Spanish was also fair game. King Henry VIII must have been overjoyed at such information for the Privateers who took full advantage continuing into the reign of Elizabeth 1st.

The maps of the complete atlas are now listed as per the British Library text as I have chosen just a few for my analysis.
FF7v-8r; Chart of the Gulf of Mexico and the Pacific ocean from 34 1/2N to 11 1/2S
FF9v-10r; Chart of the Indian Ocean, from Cape Comorin on the west to Aimoey Bay in China on the east, and from 25 1/2N to 19 ½S, including ‘Lytil Java’.
FF11v-12r; Chart of the coasts of Asia and Africa from Cape Comorin to Cape Degado, including the Persian gulf and the Red Sea.
FF13v-14r; Chart of the eastern coast of Africa, from the Line to the Cape, with Madagascar and the Ethiopian (sic) Archipelago.
FF15v-16r; Chart of South Africa from 15 1/2S on the east coast to 6 1/2N on the west coast.
FF17v-18r; Chart of the western coast of Africa from the Gulf of Guinea to the Straits with adjacent Isles and a small part of the coast of South America.
FF19v-20r; General chart of the Black Sea, the Mediterranean Sea and the coasts of Europe, with the North Sea and Baltic.
FF21v-22r; Chart of the western coast of Europe, with the opposite coast of N America from 74 1/2N to 29 1/2N
FF23v-24r;Chart of the Eastern coast of America from 51 1/2n to 6 1/2N, the West Indies and Gulf of Mexico.
FF25v-26r; Chart of the Atlantic Ocean, with the Western coast of Africa from 27 1/2N to 9 1/2n and the opposite coast of South America from 9 1/2N to 10 1/2S.
FF27V-28r; Chart of the eastern coast of South America from 6 1/2S to the Straits of Magellan.
FF29v-30r; The Easter and Western hemispheres.

From the listing of the charts as in order in the atlas it is obvious that each folio was an entity and its size enabled as much information as possible to be included.

I must return to the text, “The lande of Java on the Jean Rotz Mappa Mundi” by Brian Lees and Shawn Laffan and its conclusion that a modern coastline can be projected onto it. Their methodology is explained on page 6 of the text and I quote;

“Rotz’s method was implemented as a computer program (available on request) by one of us (SWL). A modern outline of the world’s coasts was rotated to use Rotz’s central Meridian, and then divided into eastern and western hemispheres. Each was then projected using Rotz’s method, shifted and rescaled to image units, and overlaid on the image of the Mappa Mundi (Figs 4 & 5). By projecting the vector data to fit the imagery, any re-sampling effects caused by projecting the image to a modern projection are avoided.”

As shown on my three diagrams, D09, D10 and D11, the first setting down the curved Latitudes and longitudes and then abstracting the coastal profile so that it may be considered a standard chart the above text quoted was obviated.

There are no curved Latitudes or longitudes drawn by Jean Rotz, he has merely drawn a standard chart and imposed a singular stereographic construct no doubt seeing it as the height of cartographic presentation for the 16th century.