http://ocrs.wlu.edu/Images/whyOCRS.pdf
(Was this the article that Jay mentioned at the top of this thread?)
The 5 page article describes the problems using the familiar "Click Wheel" to measure on a variety of surfaces including grass, wood bark, wood chips, dirt, gravel, sand, paved roads and track surfaces. Here's an excerpt.
quote:In 101 tests on a variety of surfaces typically found on cross country courses the wheel showed under read (UR) errors of between 6 inched and 19 feet (yes feet) in only 100m. The greatest UR errors occurred on sand and gravel. UR errors in the 5’-8’ range were frequently observed on thick tall grass, with the greatest error in that range found on thick wet grass. 1 foot to 3 feet UR errors were consistently observed on short dry grass or grass dirt combination surfaces. Over read (OR) errors of between 6 inches to 2 feet occurred on undulating or bumpy surfaces such as lumpy field grass or on dirt roads with potholes. The greater the lumps or holes were, the greater the observed error. Understandably, deviating from a perfectly straight line is almost unavoidable for anyone walking with a device such as the click wheel.