[Qgis-developer] Why is ellipsoidal calculation tied to project OTF setting?

[Greg Troxel]

"Blumentrath, Stefan" <Stefan.Blumentrath at nina.no> writes:

> I am no geodesist, so I do not have the expertise to judge which type
> of measure should be favored / the default. But I would expect the
> other packages have their reasons for using Cartesian
> measurements... Given that the code for the ellipsoidal measurement is
> actually copied from GRASS, which mostly does not use it, I – as a lay
> person in this matter – would be a bit concerned to have an
> ellipsoidal area/distance measure as the default...

I'm not an actual geodesist either, just someone who has read too many
books.

I do not understand the notion of tying ellipsoidal distance
("geodesic", presumably) to whether reproject is going on.  If the data
is in a datum, then an ellipsoid is known and the points can be reduced
to the ellipsoid (to remove the effects of longer distance from
elevation) and then the distance along the ellipsoid computed.  In
geodetic survyeing, this seems to be the standard approach.

I'm unclear on what "cartesian" means.  Presumably that's an
approximation to ellipsoidal distance assuming small distances from some
point.   That's an ok thing to show but it is an approximation.  Another
value to show is the distance computed in some projected coordinate
system, like UTM.  But that's different than an actual distance.

For computing ellipsoidal distance, the best open source code these days
seems to be from geographiclib, which is now in proj.

It seems to me that if coordinates are in some lat/long system,
ellipsoidal distnace/area is the right default.  If in UTM, then in UTM
space seems right.  But it should be clear which is which, and the user
should be able to get the other, or at least to get ellipsoidal when in
UTM or some other projected coordinates.


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