[Gdal-dev] HDF control point dump for Aster DEM (and Modis)

[Brent Fraser]

Andrey (or other HDFers),

  I'm looking into control points in HDF files again, this time for Aster DEMs.  Can gdalinfo or gdal_translate dump the control points as text to the screen?


Some background...

I've downloaded an Aster DEM from the EOS Data Gateway (http://edcimswww.cr.usgs.gov/pub/imswelcome/) and translated it to GeoTIFF with gdal_translate.  For comparison, I also translated it GeoTIFF using HEG (http://hdfeos.gsfc.nasa.gov/hdfeos/details.cfm?swID=59).  The positioning of HEG's GeoTIFF was accurate to about 200m (compared to 1:250k VMAP1 vector data), while GDAL's was out by about 2 kilometers.

I expect the difference is due to HEG using the 8 x 8 array of control points in the HDF file.  I've got a program that will use the points to warp the DEM using triangulation, but I don't have an easy way to get a list of the gcps.

I used to HDFview (http://hdf.ncsa.uiuc.edu/hdf-java-html/hdfview/) to see how the gcps are stored in the HDF file, and it's not simple.  Under "Geolocation Fields", it's a single grid named as MRGFLD_Latitude, as a single three-dimensional (2 x 8 x 8) array of 64-bit floats (64 gcps with lat in the first plane and lon in the second).  This is significantly different from Modis HDF gcps which are stored as 2 two-dimensional arrays(one Latitude, the other Longitude) of 32-bit floats.

Given the popularity of Aster DEMs and Modis data it might make for a nice enhancement to gdalinfo to dump the gcps for these two types of HDF datasets...

Brent Fraser
GeoAnalytic Inc.
Tel:(403) 213-2700
bfraser at geoanalytic.com
www.geoanalytic.com
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