Analysis of Dulcert 1339 Portolan Map

The whole map

With coordinates registered with Google Earth


See also previous blog Portolaan

Tyrrhenian Sea

Fit with Compass Charting:

In the graph of the local metric (i.e. how many kilometres there are in one pixel size) we see a large peak in the Eastern Algerian coast. That means that that the map is compressed there relative to the average.

So when we go around the Tyrrhenian Sea, mapping with a Compass Charting method, taking this place as start and endpoint would offer a good chance of giving a good fit.

Compass Charting is moving from place to place, and mapping each next place relative to the previous place according to the direction and distance of a constant compass course, without taking into account the curvature of the Earth. Note that following a constant compass course is not a straight line (or more precisely, a great circle). And when charting a large route with places around a central point, your map will not return to the same place where it started, because the circumference of a circle on a globe is less than 2π times the radius, while the length of your mapped course will be 2π times the radius, because you use the same scale everywhere.

And indeed, the Compass Charting method results in a better fit than the Mercator Projection.

With an average error of 22.8 km the quality of the mapping is very good. The Mercator projection has an average error of 26.0 km.

Going around with the cut in Algeria gives a better result than going around with the cut between Sicily and Algeria. That suggests that Tunisia was charted from Sicily and Morocco from Tarifa, with a joint in Algeria.

Atlantic Coast

Fit with Compass Charting, the average error is 25.6 km:

Fit with Mercator Projection below. Evidently the enlargement towards the North, that is typical for a Mercator Projection, doesn’t fit this map. The average error is 36.6 km.

This is the local metric of the fitted Mercator Projection:

And this is the actual local metric, calculated by fitting the real positions of a small neighbourhood of each place (note: the scale differs from the previous graph).

There is no systematic trend from North to South, just peaks caused by local distortions in the map.


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