I have a doubt about dynamic CRS, ellipsoids and area calculations. I am using QGIS 2.14.0 (still waiting for IT to install 2.14.11...)

When I load a polygon table into a new, empty project, it comes in as EPSG:32723. Looking at the Project Properties > General tab, no ellipsoid is defined. When I go to the CRS tab and activate dynamic transformation, I see that the ellipsoid has now been defined in the General tab (corresponding to the ellipsoid used by EPSG:32723).

Does this mean that until I tell QGIS to activate dynamic transformation, it is ignoring ellipsoid parameters for area and distance calculations? Because when I calculate area values for these polygons using EPSG:32723 and EPSG:32723 (Dynamic), I get different results.

EDIT: I saw a question similar to this from 2015, but it (and the replies) make no mention of the ellipsoid parameters being used or not used for area calculations.

  • Open QGIS browser and check the CRS of your polygon layer. A CRS is a combination of ellipsoid parameters and projection parameters if the CRS is projected. EPSG:32723 is UTM zone 23S, hence a projected CRS with the WGS84 ellipsoid. So the first step is to check the CRS of your data. – Techie_Gus Mar 7 '17 at 8:03
  • My data is in EPSG:32723, of that I have no doubt. But what I was able to verify from the browser (as in the CRS tab) is that there is no ellipsoid parameter included: (+proj=utm +zone=23 +south +datum=WGS84 +units=m +no_defs). So I assume this means that I do have to activate dynamic transformation to include the WGS ellipsoid in area calculations. – Ronald Halliday Mar 8 '17 at 12:46
  • By definition, the EPSG:32723 is using the WGS84 ellipsoid, or datum. Check this. When you create an empty project, what CRS is it created in? – Techie_Gus Mar 8 '17 at 12:59
  • As I stated in my original post, EPSG:32723. In QGIS (unlike Proj4) there is no ellipsoid parameter included for this projection. – Ronald Halliday Mar 8 '17 at 13:20
  • I tried to replicate your problem by using a polygon layer in EPSG:32636 and loading it in a project with the same CRS. Calculating the layer with OTF transformation set to On or Off yielded the same result. Can you share a chunk of your data so I can give it a go? – Techie_Gus Mar 9 '17 at 11:43

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