Initially, my question was about strange results in area calculations in QGIS. I got bigger results for planimetric calculations - using area($geometry) - compared to ellipsoidal calculations - using $area.

Trying back and forth I made this behaviour to turn right: ellipsoidal > planimetric. BUT: I don't know why this happens. Here the setting:

I use some simple example data from the Federal Agency for Cartography and Geodesy, namely the counties of the state Brandenburg in Germany, excerpt from the digital landscape model 1:250000, data CRS EPSG:25832, ETRS89 / UTM Zone 32N. After import into an empty project, project CRS changes to 25832 as expected; consequently does the ellipsoid settings at the general tab of the project properties: WGS 84. So far, so good.

Area calculation result: planimetric > ellipsoidal :-(

Strange. So I tried the original data as downloaded from the agency's website. It contains all counties of entire Germany, data CRS EPSG:3044, ETRS89 / UTM Zone 32N (N-E). New project, import, project CRS changes to 3044, ellipsoid setting shows WGS84.

Area calculation result: planimetric < ellipsoidal :-) Yeah! Should be right! But why?

I continued with a suspicion:

Step 1: I opened a project with project CRS 25832, imported the 3044-data and exported it using the project CRS (via layer panel, context menu of the 3044-layer, export, save objects). I imported the new shapefile into an empty project: Project CRS changes to 25832, ellipsoid to WGS84.

Area calculation result: planimetric > ellipsoidal :-(

Step 2: Import of the 3044-data into an empty project, in contrast to step 1 export via Vektor menu, Data Management, reproject layer. Export with 25832. Again import of the new 25832-shape into an empty project.

Area calculation result: planimetric < ellipsoidal :-)

BUT WHY !!! What is the difference of export/reprojection via 'context menu > export > save objects' using 25832 as in step 1, and the export/reprojection via 'vector > data management > reproject layer' using 25832 as in step 2? What's the reason for the area calculation turn out with different results although apparently using identical shapefiles (both 3044 --> 25832) and identical project CRS & ellipsoid settings?

Up to now, I always did reprojections as in step 1, but I don't trust this way anymore.

Software: QGIS 3.4LTR @ Debian 9

  • Why do you think the ellipsoidal areas will be bigger than the planimetric? Depending on where the data is in the "zone", the plane of the projection may be above or below the ellipsoid surface. – mkennedy Apr 17 at 18:18

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