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
