Indeed mapping could require lot of knowledge... ..but much depends on the task. The aim of OSM clearly cuts off all problems concerning accuracy, error correction and projection issues: no one would use our data to make reliable or mission-critical calculations. This does not mean than all OSM data is rubbish: the use of cheap consumer-GPS gives a certain degree of reliability which cannot be worse than a certain level, provided that the user can correctly operate his equipment, avoiding the main threats to the quality of his tracks (thick vegetation over paths, high cliffs or buildings hiding large amount of the sky, ...)

Marco Foi