Variation of my rides of September 7:
The 12 flat rides (C-D and D-C ) had a total span of 3.15 inches, less than a count. This is pretty good riding.
The four downhill rides (B-A) had a span of 7.5 inches, just under two counts.
The four uphill rides (A-B) had a span of 11 inches. Not good, indicating erratic riding. It's hard to maintain straightness on a steep uphill.
On the more modest grade of C to B the agreement was what one would expect from flat riding, and anyone examining the data would have no reason to suspect any hill was present.