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          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