[Qgis-developer] Doesn't measure tools/calculations + ellipsoid + OTFR require a rethinking?

G. Allegri giohappy at gmail.com
Thu Jul 3 09:49:46 PDT 2014

This topic comes back often, and in various situations I find me in a tight
spot when coming to explain the intricacies of the relationships between
these concepts.

Can we try to make the point on this, trying at least to layout a clean
"intersection matrix" between the various situations?
The concepts/tools involved in measurements and length/area calculations

 - Project CRS (GRS/PRS)
 - Measure Ellispoid
 (- Unit of measure)

We could try to write the various combinations:

1) Project with GSR + OTFR off
    -> Ellispoid is Planar (and cannot be changed)
    -> Measurements are wrong

 2) Project with GSR + OTFR on
     -> Ellispoid is set, e.g. WGS84 (and can be changed). This is set to
the last selcted layer's CRS.
     -> Measurements are right (given the correct ellipsoid is set)


If you have a better way to discuss the topic, great ;)
I think it's an important point, because most users are not aware of such
complexities underlying this tools...


Giovanni Allegri
Twitter: https://twitter.com/_giohappy_
blog: http://blog.spaziogis.it
GEO+ geomatica in Italia http://bit.ly/GEOplus
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