It’s 7.5 millions polygons, but it’s mostly flat, so there is no need for so many triangles; but if I apply “simplify” filter, angles are not preserved, they get smoothed.
Is there any method/tool/SW to avoid this? A “flattener filter”?
So you used Absolute to 1 million?
We recommend to use 50% incremental steps, in other cases the geometry can be simplified too much.
In your case I would use even smaller incremental steps.