Trying to find the maximum of a several Shapefiles I execute a cascade of unions. Out of all the tools, so far GRASS v.overlay seems to be the one working the best. However I am running into trouble with a dataset and it seems to be a topology issue.
Consider this example: I have two Shapefiles/layers.
Input 1: Input 2: The resulting union:
This first union works as expected, even if the resulting polyong is very small. Long diagonal of the yellow polygon is about 1mm. The field I want to maximise is an Integer called "int" and the resulting table is the upper one:
However if I execute the next Union with another polygon which covers all the area displayed in the images completely with a single variable (so no more slivers, yellow polygon is identical) the table suddenly misses values of the Unions already made. E.g. a_a_int in the second table should be a_int of the first (0). All layers are in the same coordinate system. This happens only for a few polygons in the whole process and they seem to always be very small. How can I avoid this/work around?
QGIS 2.18.16 GRASS 7.4.0