[Qgis-user] Measuring Tool Discrepancy

[Håvard Tveite]

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