MapServer Help:  USGS Topographic Map Symbols

Bathymetric Features



Area exposed at mean low tide, Sounding datum
Area exposed at mean low tide; Sounding datum. The sounding datum is the plane to which water depth measurements are referred: the normal water elevation of the surface of the body of water in question. A sounding datum is used in the formation of depth curves.
Channel
Channel.
Offshore oil or gas
Offshore oil or gas.
Sunken rock
Sunken rock.