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I have a historic map similar to Sexta Europae tabula, with a seemingly degree-based coordinate system marked along the border. Wikipedia suggests that this could be the one Ptolemy used in his maps (example in the first link). The 0-Meridian was somewhere of the coast of West Africa, Northing is based on the Equator.

Since I need to georeference this quite distorted map, and landmarks are either poorly drawn or have changed over the course of 400 years, I'd like to put my trust in the coordinates at the map border. Now the challenge is to either find the correct EPSG-code (if one exists) or a conversion formula between Ptolemy's CRS to modern day WGS 84 coordinates.

Does somebody have a clue?

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  • There are no original maps preserved from Ptolomy (2nd century AD) and it is even debated if he himself created maps or not - at least he published a detailed description of how to produce a map and a long list with lat/lon for different places. What you have is a map from Renaissance period, when Ptolomy (and other antique sources) were re-discovered. One commonly used 0-meridian before 19th century was Ferro: the island El Hierro, see: en.wikipedia.org/wiki/El_Hierro#The_%22Meridian_Island%22
    – Babel
    Dec 29, 2022 at 21:09
  • If you want to go back to the source, the reference resource - a critical scientific edition of the greek original and german translation is this 2 volume edition: Klaudios Ptolemaios: Handbuch der Geographie. Griechisch - deutsch. Ed. Alfred Stückelberger, Gerd Graßhoff. Basel: Schwabe 2006-2017. ISBN 978-3-7965-2148-5, see: explore.bl.uk/primo_library/libweb/action/…
    – Babel
    Dec 29, 2022 at 21:10
  • For background information, the standard handbook to consult is The History of Cartography. Volume Three (Part 1 and 2): Cartography in the European Renaissance, ed. by David Woodward. Chicago, London 2007. Can be downloaded (legally!) here press.uchicago.edu/books/HOC/HOC_V3_Pt1/Volume3_Part1.html
    – Babel
    Dec 29, 2022 at 21:17

1 Answer 1

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According to your Wikiepdia link, "Ptolemy's 2nd-century Geography [...] measured latitude from the Equator", and the prime meridian, as you say, somewhere off the west coast of Africa. But there doesn't seem to be a solid definition (furthest land to the west known at the time).

Looking at the linked map: Venice seems to be 45N, 34.33 E, and Rome around 41N, 35E

Venice is actually 45.43N, 12.33E, while Rome is 41.53N, and 12.28 E.So the latitude seem to fit, no transformation needed. And it would point to the prime meridian being between the Cabo Verde and Canaries islands (22E). Elbe island looks to be around 32E on the map, as compared to 10W. again a difference of 22degrees.

Which would imply:

  • the same latitudes, but a shift of 22 degrees to the east.

However, the north tip of Corsica is around 43N, but 41.5N on your linked map. So even if you can get a exact conversion formula, the maps will still not match, as a lot of the features were not so well placed...

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