There has been some discussion about contour lines. From the standpoint of one who does a fair amount of backpacking and mountain climbing, 10 m is much more useful. It is also pretty close to the standard 40 ft contours that appear on thousands of USGS topo maps. Doubling that gives one a map which doesn't display much useful info to a hiker. Lots of gullies and cliffs can hide within a 20 m elevation change. HARRY WYETH
