----------------------------Original message---------------------------- Could perhaps John Shanton or some techno-type at USGS answer this question? A researcher here wants to know what is the best horizontal accuracy one can expect to achieve using a USGS 7.5' topo map. He was told by a cartographer that the position of, say, a building would come within 6 feet of a spot on one of these topos. I understand that six feet is about fifteen thousandths of an inch on these maps. Well, .015 is also the size feeler gauge I use when setting the points on my '65 Ford and would be quite a small measurement on a topo map. Which leads me to believe that there's a limit of accuracy one should expect from a topo map. I would suspect that any surveyor, for example, neededing an accuracy better than six feet should use a GPS instrument. - PML