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I am trying to merge four GeoTiffs from the National Elevation Dataset (EPSG: 4269) into a 2x2 DEM using gdal_merge.py. For some reason, when it merges the upper two NED rasters, gdal produces a seam between the two of approx. 1 px that it fills with incorrect values (either 0 or some large negative number). It seems like this is probably an alignment issue. How can I eliminate this artifact?

Thanks for the help.

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Can you tell us which particular NED entries are causing the problem? –  BradHards Apr 14 '13 at 5:43
    
The two NED entries are for the NW and NE of San Francisco, file names 1-436482266\1-436482266.tif and 2-486081568\2-486081568.tif, which is the upper left and right quadrant from the following bounding box: (-122.635, 37.645), (-122.279, 37.843). There is no such issue with any of the other three seams. –  James Conkling Apr 15 '13 at 15:03
    
Also, does the National Map Viewer no longer allow users to download geotiffs? On April 4, I had no problem finding elevation data stored as geotiffs, but now can only manage to download .img files. –  James Conkling Apr 15 '13 at 15:40
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