My greatest concern about the surface we are going to measure is the presence of small stones mostly up to about 5mm in size which you can see in this photo.
You can use the scale provided by the keys in the section, which I have magnified by 3 times, to judge the stone size.
It is worrying to note that one can see traces of cycle tracks on the path. I think that is another indication that calibration could be off.
This picture was taken close to the surface looking along the cycle path so you can see the poorly defined grass edges. These could be a problem if the path was twisty - how on earth would one then define the effective edge of the course for measuring the SPR? This particular path being a former rail track is straight for miles. But that could be a major problem on a twisty trail.
I don't know what stone is used in this trail, but when we measure on Sunday week there will be a former road engineer present so we should be able to get some expert opinion.