GRASS 6.4.1's Visibility Vector Graph Application

  The map below was produced using GRASS's 6.4.1 network analysis Visibility Vector Graph Application.  I imported our subway and SIRT shape file and the New York City borough map.  I performed the analysis and a polygon map is produced which shows the shortest distances between extreme points in the network.  You can see that the last stop on the Staten Island Rapid Transit is the most southern point (Tottenville) while Jamaica 179 Street is the most eastern extreme point.  What good is this map?  Perhaps to show which areas of a city are really far away from rapid transit (areas outside the polygon).  This analysis can be preformed for any type of transportation form or other types of networks.  The areas outside the polygon can be calculated by another program.  A good rapid transit or tram system that is inside a similar polygon which covers the boundary of an urban area is doing a good job.  New York City does not have good Rapid Transit coverage according to this type of map.