Your Maps are all Wet: Using the Typographic Index to find Rapid Transit Lines

  In the above map, I brought in our elevation file for New York City and then I tried something different.  I used the GRASS 6.4.1 Raster  Typographic Index.  This index is used to generate a map showing water run off.  I then added my standard subway route line and station shape file.  To my surprise, the yellow areas (lines) seem to follow the existing rapid transit structures that are either on the embankment or a open cut and show the water drainage areas.  Interestingly, the iron elevated structures, such as found on the West End (D) and Culver Lines (F) do not produce such an effect.  Although I did not bring in the RR shape file, the LIRR Bay Ridge Division is clearly visible because it is either on an embankment or an open cut.  Going from west ot east the labels show the Fourth Avenue Subway, (N and R trains) with no effect, the West End El (D), with no effect, the Sea Beach open cut (N), with an effect, the Culver Line (F) with no effect and finally the Brighton Line  (B, Q) on an embankment with a strong effect.  The bottom line is that the software is able to pick up great changes in elevation near rapid transit structures.  While a map is good to have, perhaps the software can be used for finding interesting artifacts no usually visible.

Anyone interested in the code in GRASS 6.4.1 that creates this can find it below:

r.topidx creates topographic index (wetness index), ln(a/tan(beta)), map from elevation map
where

a: the area of the hillslope per unit contour length that drains through any point,

tan(beta): the local surface topographic slope (delta vertical) / (delta horizontal).