@gordhun: This method is still used today (or rather yesterday): The traditional ultra long end-of-season race in Trondheim called Hu & Hei took place in the traditional Bymarka hills and marshes. It is nominally 30 km in two laps (18+12) with about a km of vertical climb, and quite often some snow.
The "Gjertsen Factor" compares each year's course to the one planned by Einar Gjertsen in 1965. In case of significant snow (20-80 cm?) the rule is to shorten the race by 1 km for each 10 cm of snow cover, this did not happen the first year I started, in 1980: Shortly after we hit 20 cm of wet snow on top of Bodsbergheia both my legs started cramping since I as a complete novice had started with thin nylon socks and no wool layers.http://huoghei.com/