Coordinates Mismatch

[Green Eggs n Pelham]

I recently made a New York City georef raster map and went about the proper steps to download the proper DEMs from the National Map Viewer Server website. When scrolling over the coordinates of my map, the basic gist was N40* 55' and W73* 55'. I went onto the website and highlighted the different counties of New York City and downloaded the appropriate DEM files. When I uploaded them and overlayed the Georef map onto it, no part of my map was covered by the DEM which left me confused.

I also tried downloading the same DEMs from this website, and got the same result:

http://dds.cr.usgs.gov/srtm/version2_1/ ... h_America/

It turns out that I needed N41W74 and N41W75, but then a friend of mine confirmed that even though it covered the map area on the georef map as expected, lots of the elevations were off.

Any suggestions?

-A

[geophil]

With The National Map Viewer, the easiest way to mark the download area is "by map extent". Zoom in to the region you want, position the preview map to roughly match your project and then select the "by map extent" option.

As the chunks are always 1x1 degree (for 1 and 1/3 arc sec), you will get vast areas you don't need. Chop them off in TransDEM.

The USGS NED DEM files you will receive will be named by their NW corner, so N40* 55' and W73* 55' will be named n41w074. (Remember that western longitudes are counted as negative. As you move westward, longitude numbers increase.)


For orbital SRTM, the naming schema is different, it's the SW corner here, so the file name for N40* 55' and W73* 55' will be named n40w074.

SRTM data has been gathered from radar sensors mounted on satellites flying in outer space. We don't know how accurate the data actually was, but the version we earthlings are given leaves something to desire for structures above the terrain surface. This includes both botanical forests and all the man-made forests, i.e. built-up areas.

For the US I would never use SRTM. USGS NED is just so much better. See here for a comparison.