How to calculate arithmetic mean roughness Ra using ImageJ instead of MountiansMap software?

Question

How to calculate arithmetic mean roughness Ra using ImageJ instead of MountiansMap software? MountiansMap has a 3 days trial limition, I have the STL 3D data, how to use this data in ImageJ

enter image description here

calculate a real roughness Ra value with stl 3D data in ImageJ.