As far as I know, the Spanish Cadastre WMS server (run by the Ministry of Economy of Spain) is one of the first to provide its data in the Spherical Mercator projection EPSG:3857 (aka EPSG:3785 and EPSG:900913), so now we can switch between the great cadastre layer and the OSM layers for example. This is the Montjuic Palace in Barcelona:
As we know, the Spherical Mercator projection is conformal (keeps angles and shapes locally) and all parallels have the same length E:
R = 6378137 m
E = 2 * pi * R = 40075016.686 m
But we know that the true length of each parallel is approximately P = E * cos(lat), so if one wishes to measure distances, perimeters or paths directly on a map in EPSG:3857, it’s necessary to multiply the result by a factor f = cos(lat). For areas, f = [cos(lat)]^2. For Barcelona, for example, we have:
lat = 41.402634
f = 0.75008067 (distance)
f = 0.56262101 (area)
Discussion
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