The Manhattan Municipal Building Water Shed

   Several of my last postings dealt with the ease of creating a watershed map using ARCGIS on-line vs. the difficulty of doing it several years ago in GRASS.  One blog that deals with streetlamps and New York and various forms of Transportation (Forgotten NY) had a posting on the mysterious and perhaps haunted Chamber Street subway station at Chambers  and Centre Street Manhattan, under the Municipal Building.  That station has what is called a "water condition" and the site where the Municipal Building  is located was once the site of a pond or some sort of collect pool, according to old maps.  I ran a watershed analysis in ARCGIS on-line and the results are above for a focal point of Chambers and Centre Streets.  The watershed is shown bounded by the red line..  It is interesting to note that in the bounded region, there is a park called the "Collect Pond Park" and is one source perhaps of the water.  The range was set at 1000 feet.  Larger ranges over 2000 feet creates a watershed that goes to Brooklyn, which is an impossibility because of the East River.

The Sea Beach Line (N) Watershed Area

Calculation of watershed area using ARCGIS on-line facility.  The first map is from the Avenue "U" station of the "N" Sea Beach Line in Brooklyn.

  Several years ago, a few days after Hurricane Sandy struck NYC, a picture was shown of either the 86th Street Station or the Avenue U Station in Brooklyn on the "N" Line ( Sea Beach ) showing the open cut completely covered with water almost up to the platform level.  This water came from the bay I thought because the open cut elevations at track level was probably near sea level values.  I was introduced recently to ARCGIS on line that has a nice analysis section dealing with various operations such as view shed analysis and watershed analysis.  What would happen if I would enter a point at the Avenue U or 86th Street Station on the N Line?  First, I would need to bring up a base map that shows subway stops and lines.  Surprisingly, the Watershed Analyzer here is very simple to use.  If you do not have a point shape file, you can add a point by "hand" and just select a range, let us say, 2000 feet.  To my surprise, the resultant layer was not south of the 86th Street Station but Northwest.  And now I can see why.   A watershed is an area that provides water to a selected point.  I believe that for this to work, the watershed has to be at higher elevation than the selected point.  A very interesting resultant shows that the watershed for the 86th Station (theoretically), if there were no buildings and sewers in Brooklyn, would be along the "N" train all the way to Sunset Park!

Water analysis in GRASS is more difficult to use.  The resultant "layer" can be zipped to ARCGIS maps and used in your favorite style maps, as shown below.