Is there a way that you want to connect two different points of altitude with a constant slope of 7% or another, to draw a line in the 3D model?
As @whuber says in his comments you should be able to use PathDistance to do this.
"PathDistance computes routes that meet requirements based on location and direction. Set it up so that any route steeper than 7% has a huge cost and anything 7% or less has (almost) no cost. PathDistance should find short routes meeting these criteria. As @julien notes, though, there usually are still infinitely many solutions. (Note that if you try to make the slope a constant 7% then in many cases there will be no solutions.)"