Directory enquiries cmo se toma la ivermectina de 6 mg para piojos Your GPS measures how far you have walked in three dimensions. In other words it takes into account the small increases in distance travelled bought about by changes in elevation. An OS map is a two dimensional representation of terrain and cannot show the extra legwork involved in traversing hills and dales. Accuracy of measurement is another possible factor. On a map you generally calculate distances between waypoints in terms of straight lines, which ignores all of the bends and curves in the path and minor deviations from the plotted route. Sometimes it is only a few footsteps, but they all add up.