Surface spot elevation label...

[Jerry_Barnes]

I'm looking to create a surface spot elevation label that will subtract the elevation of the existing surface from the elevation of the proposed surface, at specific spots. I've tried using expressions, but apparently I'm not doing something right. Any ideas? 

[mzjensen · Accepted solution]

What I typically do for this is create a volume surface with the existing ground as the base surface and proposed ground as the comparison surface. The theory behind this is that the base surface is the datum for the volume surface, whereas the datum for a normal surface is elevation 0. So anything above the base will have positive elevation and anything below will have negative. A spot elevation label on the volume surface will effectively show the difference between the two surfaces.