For our rides the past few years we used a GPS unit and measured the
distance. This takes an XYX position every five seconds (latitude, longitude
, elevation) then our software converts this into a map and provides slope
distance (not "map distance) this means that it takes the "ups and downs"
into consideration. Accuracy is in the range of two to five meters with the
equipment I have been using.
If on Public Lands work with your local Land Manager. They most likely will
have the equipment available and be happy to get the trails mapped.
Bob Morris
Morris endurance enterprises
Boise, ID