@alexandre sk said:

I think it's the displacement map. in general options disable displacement to try.

yes! that did it! thank you very much

Have a look at Fredo6's Curvizard, it wont make them squares but you can certainly get some good results/clean lines over multiple shapes using simplify contour or cleanup contour, have a go and see if it helps.

I understand. So if you have to use that model in SketchUp, it seems you can have soft edges on the terrain or you can keep the vertical faces but you get to decide. Fortunately deleting the unneeded geometry is a simple two step process in SU.