"Slippage" of a non driven wheel? We've been hearing for years about "slippage". We've heard "slippage" is why we don't measure in the rain or on wet roads. What would make the front bicycle tire slip, except to the side (maybe while turning the handlebar)? There was a good discussion of "slippage" above (on 10/21/2018). We need to slide off the "slippage" excuse.
Mike Sandford and others have shown that rougher road surfaces result in longer measurements than smoother surfaces. This is for paved surfaces. It can reasonably be extended to hard packed (or frozen) non-paved surfaces, but not to grass (again see the 10/21/2018 post). Maybe it will hold for grass on frozen ground. This rapidly becomes a huge science experiment, well outside the "everyman is a measurer" concept.
Bottom line, our policy of not certifying cross country courses is probably a good idea.