TransDEM Forum

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
I don't know about "nearly all GIS software" but unfortunately TransDEM wasn't among those.

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.

Thanks for the update! Any idea yet whether this new (old) format will be in smaller chunks like the GEOTiff used to be, or large areas like the ESRI format covers?

Andrew

The latter I'm afraid. Even worse, rumours say that for 1/9 arc sec the partitioning will be 15 min. With 1/9 arc sec having 9 times the data of 1/3 arc sec, but the chunk onle 4 times smaller, download size would increase significantly again. 1 x 1 degree in 1/3 arc sec amounts to about 325 MB / 485 MB zipped/unzipped, 0.25 x 0.25 degrees 1/9 arc sec will become more than 1 GB. Let's hope the rumours aren't true.

Will the upcoming TransDem release be able to process the .IMG formats?

It seems that the 1/9 arc sec (at least in the areas I downloaded - Central Pennsylvania and Southern New Jersey) are only available in the .IMG formats on the USGS National Map Viewer. Each chunk covers 4 "quads" and a central PA one is 180 MB/252 MB zipped/unzipped, while a southern NJ is 71 MB/93 MB. I guess the difference in sizes is central PA is fairly mountainous while southern NJ is relatively flat.

At least they aren't nightmarishly huge...

Thanks.

Andrew

Yes, it will. And it will offer an option to load only part of the file to save memory.