How to define surface change due to wear/abrasion

Hi,

I wonder what is the best way to describe qualitatively and quantitatively the change of a surface due to wear/abrasion. More precisely: I have a surface, which is kind of rough after the manufacturing of the surface, like sand or gravel on the beach. Due to wear and abrasion (like 100, 200, 500, 1000, 5000, 10000 grindings) , the surface gets smoother but there are long and deep grooves in the surface (depending on the grinding material).

Here are two pictures prepared with Fuji’s 3D Surface Plot plugin:

I wonder if someone who is not (like me) absolutely new to the business could give me some directions how to compare these two images and also how to define a kind of calibration, for using a picture for calculation how many grindings have already taken place…

Truly yours,
Wolf