# Create a "reverse terrain layer" to visualize the volume of a gorge?

JSL
Global Mapper UserPosts:

**403**Trusted User
Hi, I was wondering if anyone had any suggestions on how to find the volume of a gorge through a linear ridge, and then visualize it . What I'd like to create is like a reverse negative of the DEM, so where there's a valley in the DEM, I'd see a hill. I've been experimenting with compare terrain layers, where I create a polygon around the gorge, then give it the elevation of the highest height of the ridge and subtract the DEM from the polygon, but I can't seem to configure the elevations so the result is a reverse of the DEM. Below is a image of the polygon and the ridge--any help would be most appreciated!

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