Offset required if using Terrain Aware

Let's say you are mapping a heavily treed area and the trees are 120 ft tall. If you are taking off from the ground you would want to add a 120ft offset to your flight to make it so the amount of overlap you are specifying is valid at the tops of the trees and not the ground. 

If you are just mapping the ground, don't need to add an offset since that is what Terrain Awareness does for you.