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

[Jelmer Oosthoek]

Dear all,

I've been using the geodesic functionalities of ESRI ArcGIS within my academic work: http://www.gispla.net/docs/Oosthoek_Arriazu_MarcoFiguera_GeodesicGIS_Human_LandingSite_Mars.pdf and http://www.lpi.usra.edu/meetings/explorationzone2015/pdf/1049.pdf 

But within my freelance work I use QGIS. It would be great if QGIS also had similar functionalities.

@Greg. As far as I would imagine, cartesian would mean measuring on a 2D projection while geodesic measures on the sphere/ellipse.

Cheers, Jelmer Oosthoek

-----Original Message-----
From: Qgis-developer [mailto:qgis-developer-bounces at lists.osgeo.org] On Behalf Of Greg Troxel
Sent: Saturday, February 06, 2016 01:28
To: Blumentrath, Stefan
Cc: qgis-developer
Subject: Re: [Qgis-developer] Why is ellipsoidal calculation tied to project OTF setting?


"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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