Export and import terrain height through another program?

Unity Engine
1

The terrain editor within Unity is basically broken, especially the “smooth height” tool.
I’m thinking I should just export the heightmap into another application, smooth and fix everything there, and then import it back into Unity.

What is a decent terrain-editing program that I can trust to import Unity’s heightmap with a fair degree of accuracy, and also export it again into the proper format for Unity to read again?

2

The terrain tools are pretty shocking.

I’ve always just modelled them in Blender (sculpt, retopo, then decimate), brought into Unity, and used some of the freely available scripts out there to convert a mesh into terrain.