John,
The Prototype Global Shoreline (PGS) was developed by EarthSat (now MDA Federal) for NGA. It will become the replacement for World Vector Shoreline (WVS) (1:250,000). It is an approximation of the High Water Line, but it is not a Mean High Water Line. Read the White Paper for further information. The base imagery was Landsat ETM+ 2000 . According to one of the primary workers on this project, over 3,000 man-years were invested in this vector shoreline. I have a number of discussions and comparisons of the NGA PGS with other shorelines on my blogsite, EVS-Islands .