GRASS 6.4.1 Predicted Correctly the Location of LIRR Bay Ridge Division

  In the first frame, I used GRASS 6.4.1  and a New York City Elevation shape file.  I then used the terrain analysis feature and use the DY derivative procedure.  The shadow of the LIRR Bay Ridge Divisio cut was visible. This is because raster elevation files are aerial photographs.  The LIRR Bay Ridge Division Cut is very narrow in some places.  In the second frame, I added a Rail Road shape file which shows open air rail road rights of way.  The Bay Ridge Divsion trackage as shown in the map using the RR shape file is correctly aligned by the DY land terrain analysis procedure.  This may be helpful for looking for forgotten rights of way.  By the way, the Culver Viaduct does not show up in the DY frame, I was wrong.

DY GRASS 6.4.1 land terrain analysis.  Subway shape file added after analysis.  LIRR cut is visible.

Below, same frame but RR shape file added.  The RR shape file shows outside railroads in yellow. Includes outside rapid transit trackage.