If you ever wondered why the contour lines in the USA Topo CDs are strange
number of feet not based on orders of 10, they appeaer to be derived from
the USGS topo maps of 1:100,000 which use contour intervals of 10 meters
or 20 meters. This also appears to be the reason that these maps
are more accurate than the Roads & Recreation maps, Lowrance IMS City
Streets, and Street Atlas maps which are based on the Census Bureau's Tiger
maps. A comparison is shown below.