Originally Posted by
pdlamb
It's interpolation of elevation from interpolation of two-dimensional (i.e., xy) location. This may be way off topic, but it's at least tangentially related as it explains why mapping solutions are sometimes erratic.
Seems like an odd approach.
It would seem easier and faster (and less erratic; maybe, more accurate, on average) to just use the topo lines where the topo lines cross the road.