How to make a smooth terraforming hill

I am trying to accomplish something like the bottom picture but when I add
either a rectangle or a polygon and lift it a bit (terraforming) I get this
horrible triangular spikes (top picture). Does anyone know how can it do this?
Am I doing something wrong?

None of the terraforming tools work properly with areas and elevations that
small or narrow, or I haven’t found a way yet. It does seem possible for the
sim’s own terrain but that must be a different system we can’t access. The
only way of doing it with any precision will be a 3d model.

Try create rectangle with 256x256 map, then terraform. The result is certainly
not predictable, but in some cases it allows you to achieve a result. Or make
a DEM mesh. Perhaps this will give the best result.

You May want to consider my answer in this 3d The current mesh system Is
limited by the resolution of the grid underneath It , you can take see It by
enabling the wireframe settings in the Dev menu. Basically, the grid Is
aligned north-south, so any terraforming aligned to the girld Will give you
straight edges, any rotation Will give you a some jagged edges You can’t avoid
It, it Is the way . You can minimize the effect by adding some falloff (aka
smoothing) . Or you Can Hope in a higher definition grid, eventually they Will
manage to obtain that, dunno It Is mesh related. As per @superspud answer, if
you Need a perfect shape, go for the 3d solution, you can borrow the sat
imagery for your textures and plant some “fake” grass, most users Will not
notice it