Phil Holland and I have done some work on measurements on the Stratford Greenway, which I photographed for a post earlier in this thread. The Greenway, a former railway track, has been converted to a cycle/walking path by spreading crushed limestone. The surface is quite good, but it is not a sealed road and has small loose stones.
From our report (pdf) report you can see that our gravel calibrations were in good agreement with those on a nearby road:
Phil: -0.014%
Mike: +0.006%
Our measurements over a a total of 4 km of the Greenway differed from one another by around 0.05%, probably because our two tyres responded differently to the varying roughness.
More worrying was the fact that the previous measurer of this course had in 2008 obtained much larger values. His results had been about 0.13% larger over a smooth section and 0.45% larger over a rougher section.
We think there has been no mistake in identifying the reference points so wonder whether the surface has changed over three years.
Phil prepared a paper for a recent meeting of our Course Measurement Working Party. He suggested that we might institute 3 levels of certification:
1 Road courses.
• Sealed road or pavement - less than 5-10% other surfaces.
• Issued with a Certificate of Course Accuracy showing compliance with IAAF
requirements for road races.
• Logo wording is CERTIFIED ACCURATE.
2 Gravel courses.
• Sealed road and/or good compacted gravel - less than 5-10% other surfaces.
• Issued with a Certificate of Distance confirming a full-length course, but not
claiming to be comply with IAAF limits of accuracy for road races.
• Logo wording suggested – DISTANCE CONFIRMED
3 Off-road courses.
• Over 5-10% on surfaces other than sealed road or good compacted gravel.
• Issued with a Statement of Course Measurement confirming the measurement
method used but stating that 'the distance recorded is measured only as accurately
as the definition of the course and the course surface permits'.
• Logo wording is MULTI TERRAIN.
No action has yet been agreed. Discussions continue.
I have made a webpage at http://coursemeasurement.org.uk/offroad.htm we plan that it should hold photos of surfaces as a guide to measurers, and also report further comparisons measurements as these are undertaken.