I'm overlaying polygons on a raster to get the average value inside the polygon (using
SAGA->Shapes-Grid->Add grid values to shapes in the QGIS Processing Toolbox), using a geographic and a projected coordinate system.
When I compare the average polygon values, they are different based on the projection. That is, when I use a polygon layer and raster in WGS84 (EPSG: 4326), I get one value. If I reproject both the polygon layer and the raster to MGA56 (EPSG: 28356) and redo the average calculation, I get different values for most of the polygons. Why is this? I feel that the average should be preserved. Is it because of the algorithm used to determine which raster cells are inside the polygon? Is one value "more correct" than the other?