[Qgis-user] Measuring Tool Discrepancy

[Stewart Holt]

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
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