Pseudo Mercator (or Web Mercator) is a projection that is not suitable for distance (or area) measurements. So the discrepancies are expected. For a short explanation, see for instance https://gis.stackexchange.com/a/14532 or wikipedia.
Håvard On 07.06.2020 20:13, Stewart Holt wrote: > Working with a project in EPSG 3857 (Pseudo Mercator, Google Maps or > USGS Topo) when using the measuring tool for distance the Cartographic > distance and Ellipsoidal distance differ greatly. I assume that > cartographic distance is a simple calculation using plane coordinates > which have been projected such that they yield correct distance if the > projection preserves this quality, but I have not found a concise > definition. The ellipsoidal distance is, of course, following the path > on the surface of the project ellipsoid. I have the project geoid set to > WGS84. I would think that there should be no significant difference in > these two distances for a distance as close as 100 feet, for example. > > As an example, for the distance between 2 survey pins of 84 feet, the > Ellipsoidal distance is correct but the Cartographic distance is 102 > feet. I am confused as to why the computation type exists, though there > may be nothing in the definition of a projection which indicates it is > equidistant. If the projection is equidistant then the result should be > close to the same. If not, it is invalid, although for such a short > distance, I would not expect to see much difference. I am observing this > in 3.12, but I think I have noticed this over many versions. > > Any insight into what is going on here would be appreciated. > > Thanks, > > Stewart _______________________________________________ Qgis-user mailing list Qgis-user@lists.osgeo.org List info: https://lists.osgeo.org/mailman/listinfo/qgis-user Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user