I just have to add this anecdote on measuring dirt.
It was my first solo attempt at measuring a race, the Danville Half Marathon, Danville VA. The RD was running along with me as I rode to show me the course, and as we hit the unpaved section I thought it looked VERY well maintained, freshly graded with roller compaction marks and signs new gravel had been recently added. At mile seven, I came around a bend on a steep (12%) downhill and could not avoid running over a rattlesnake crossing the path. I did not stop to take data as he was still moving, and would not be a relatively permanent feature.
Saturday morning, I rode it again to verify the distance. As I hit the gravel section, I noticed that construction equipment had been there that morning, and new gravel had been placed, but not rolled. I had to use the offset method to get around some of the equipment grading the road.
Needless to say, the measurements did not agree, so I had to try again. Saturday afternoon, the equipment had made it down to the start. All in all, I measured that stretch three times in 24 hours, and never rode the same surface twice.
I told this to the RD, and he then informed me that it was in the process of being paved, and would be complete in a week or two. Hmmm.
I had to go back to remeasure that stretch.
The paved distance came out a bit longer than the unpaved, I'll have to get my notes to compare the two and post the results.