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Saturday, December 19, 2015

Errors in NYC Elevation Raster File

  For several years, I have been posting in this blog several postings dealing with the elevation of various parts of New York City.  Some of these postings are from a raster file, which is basically a aerial photograph of a land area.  It is possible, through some mapping programs, to change a raster file to a contour picture, which shows a series of lines.  The closer the lines appear close together, the slope increases.  For some time, I noticed in one set of posts, a straight line that is exactly at 90 degrees to the bottom of the picture.  In the frame below, this line appears not at the arrow, but near the intersection of the West End (D) and Sea Beach (N) routes in  Brooklyn near 62nd Street and New Utrecht Avenue.  It does not appear exactly at that intersection, but parallels the West End line structure to where it curves on 86th Street and continues south to Coney Island.  This line stops at the water and continues at the same angle in western Coney Island.  Using QGIS, I was able to activate an interesting feature that is able to assign an elevation in a map at any particular point.  It is important to note, the this plug in is not getting the elevation from my raster or contour file but probably from the internet.  In the second map below, the area around 79th street and 18th Avenue is shown in Brooklyn.  The red vertical bar of many lines comes from my contour file and is the error.  The numbers around the bar are elevations.  The vertical red bar crosses either 18 or 19th Avenue at this spot ( I was too lazy to put in a label in QGIS to show the exact location).  Elevations in the bar show no abnormalities in elevation.  A street view inside the "bar"shows no major changes in elevation or structures such as a sunken highway or elevated tracks.   So what is it?  Probably an aerial photograph that was not joined together probably causing a "seam".  The last map shows the "seam in black and white".  Notice that it looks like a rapid transit line.

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