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 are: - OTFR - 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) etc. 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 -- Giovanni Allegri http://about.me/giovanniallegri Twitter: https://twitter.com/_giohappy_ blog: http://blog.spaziogis.it GEO+ geomatica in Italia http://bit.ly/GEOplus
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