USGS NED in .img format
Posted: 24 Mar 2013 15:08
Last December USGS made the announcement about abandoning seamless services for DEMs and retiring data formats. They also said that the next change will be in April:
http://nationalmap.gov/data_delivery/ned.html
After some research it appeared that ".img" could mean ERDAS Imagine, another of these formats from the early 90s, when GIS was born.
Meanwhile, USGS has updated that announcement and offers some sample data for download on that page. That helped me a lot. I have now implemented support for ERDAS Imagine as a prototype and it seems to work. The big difference to ESRI Binary Grid, the format we are currently using: Img is much faster to read. Opening ESRI Binary Grid involves a lot of jumping hence and forth when accessing tiles and blocks and that it slow. ERDAS Imagine has a more efficient file structure and it also packs everything into a single file, like GeoTIFF.
However, I do not yet know the encoding for void points in this format. ESRI Binary Grid as used by USGS brought some surprise here in January. So I will wait until more data becomes available for further testing before publishing the TransDEM update.
Both ESRI Binary Grid and ERDAS Imagine are rather complex formats and documentation leaves a lot to desire. Fortunately there is a library, called GDAL (X11/MIT license) which most people seem to use (including the USGS) and TransDEM does the same.
http://nationalmap.gov/data_delivery/ned.html
I don't know about "nearly all GIS software" but unfortunately TransDEM wasn't among those.In addition, beginning with the April 2013 release of the NED, it is anticipated that all elevation products (1-, 1/3- and 1/9-arc-second DEMs) will be available as pre-packaged, downloadable files in '.img' format, a robust format easily read by nearly all GIS software.
After some research it appeared that ".img" could mean ERDAS Imagine, another of these formats from the early 90s, when GIS was born.
Meanwhile, USGS has updated that announcement and offers some sample data for download on that page. That helped me a lot. I have now implemented support for ERDAS Imagine as a prototype and it seems to work. The big difference to ESRI Binary Grid, the format we are currently using: Img is much faster to read. Opening ESRI Binary Grid involves a lot of jumping hence and forth when accessing tiles and blocks and that it slow. ERDAS Imagine has a more efficient file structure and it also packs everything into a single file, like GeoTIFF.
However, I do not yet know the encoding for void points in this format. ESRI Binary Grid as used by USGS brought some surprise here in January. So I will wait until more data becomes available for further testing before publishing the TransDEM update.
Both ESRI Binary Grid and ERDAS Imagine are rather complex formats and documentation leaves a lot to desire. Fortunately there is a library, called GDAL (X11/MIT license) which most people seem to use (including the USGS) and TransDEM does the same.