I have 141 main polygons, and then a list of thousands of smaller polygons that are clipped to the main polygons. I am interested in knowing what percentage of the surface area of the main polygons is covered by the smaller polygons.
My idea was to do this as a spatial join, where I join data from the smaller polygons to the main polygons based on location. summarized as the 'sum' of smaller polygons. Then I can use the field calculator to go with area_of_smallerpolygons/area_of_mainpolygons. This all works fine.
The problem is that my spatial join is only partially completing, the resulting shapefile only has 127 polygons, instead of the 141 main polygons. I thought perhaps it would leave out main polygons that do not intersect with the smaller polygons, but upon visual inspection this is not the case.