playing with the demo at https://maptcha.crown-shy.com/ , I had problems recognizing the outlines, as they often were colored similar to their structure (low contrast). Some examples I couldn't find any outline at all.
Also, seeing a rectangular outlined thing from above, it's often impossible to tell if it qualifies as a building.
Also, seeing a rectangular outlined thing from above, it's often impossible to tell if it qualifies as a building.