Duane, this discussion came to mind when I was measuring a 10k yesterday (believe it or not, temps were in the 50s on Jan. 1 in CT).
The first half mile of the course goes through a farm, following a dirt road and field trails, and it was warm enough yesterday that both were very squishy, to the point I had to walk the bike on one steep uphill. When I was finished, the start was about 180m farther than the race director had figured, using his GPS. I wonder if the surface had any effect.
I'm also reminded of measuring the course for the Rushmore Marathon/Crazy Horse Half (I think you did the revised course the following year). Last 6+ miles were on crushed stone rail trail, and I recall my rear wheel slipping like crazy, esp. going uphill.