# USGSDEM -- USGS ASCII DEM (and CDED)

GDAL includes support for reading USGS ASCII DEM files. This is the traditional format used by USGS before being replaced by SDTS, and is the format used for CDED DEM data products from the Canada. Most popular variations on USGS DEM files should be supported, including correct recognition of coordinate system, and georerenced positioning.

The 7.5 minute (UTM grid) USGS DEM files will generally have regions of missing data around the edges, and these are properly marked with a nodata value. Elevation values in USGS DEM files may be in meters or feet, and this will be indicated by the return value of GDALRasterBand::GetUnitType() (either "m" or "ft").

Note that USGS DEM files are represented as one big tile. This may cause cache thrashing problems if the GDAL tile cache size is small. It will also result in a substantial delay when the first pixel is read as the whole file will be ingested.

Some of the code for implementing usgsdemdataset.cpp was derived from VTP code by Ben Discoe. See the Virtual Terrain project for more information on VTP.

## Creation Issues

GDAL supports export of geographic (and UTM) USGS DEM and CDED data files, including the ability to generate CDED 2.0 50K products to Canadian federal govenment specifications.

Input data must already be sampled in a geographic or UTM coordinate system. By default the entire area of the input file will be output, but for CDED50K products the output file will be sampled at the production specified resolution and on product tile boundaries.

If the input file has appropriate coordinate system information set, export to specific product formats can take input in different coordinate systems (ie. from Albers projection to NAD83 geographic for CDED50K production).

Creation Options:

- PRODUCT=CDED50K: When selected, the output file will be forced to adhere to CDED 50K product specifications. The output will always be 1201x1201 and generally a 15 minute by 15 minute tile (though wider in longitude in far north areas).

- TOPLEFT=long,lat: For CDED50K products, this is used to specify the top left corner of the tile to be generated. It should be on a 15 minute boundary and can be given in decimal degrees or degrees and minutes (eg. TOPLEFT=117d15w,52d30n).

- RESAMPLE=Nearest/Bilinear/Cubic/CubicSpline: Set the resampling kernel used for resampling the data to the target grid. Only has an effect when particular products like CDED50K are being produced. Defaults to Bilinear.

- PRODUCER=text: Up to 60 characters to be put into the producer field of the generated file.

- OriginCode=text: Up to 4 characters to be put into the origin code field of the generated file.

- ProcessCode=code: One character to be put into the process code field of the generated file.

- TEMPLATE=filename: For any output file, a template file can be specified. A number of fields (including the Data Producer) will be copied from the template file if provided, and are otherwise left blank.
- ZRESOLUTION=float: DEM's store elevation information as positive integers, and these integers are scaled using the "z resolution." By default, this resolution is written as 1.0. However, you may specify a different resolution here, if you would like your integers to be scaled into floating point numbers.

Example: The following would generate a single CDED50K tile, extracting from the larger DEM coverage yk_3arcsec for a tile with the top left corner -117w,60n. The file yk_template.dem is used to set some product fields including the Producer of Data, Process Code and Origin Code fields.

    gdal_translate -of USGSDEM -co PRODUCT=CDED50K -co TEMPLATE=yk_template.dem \
                   -co TOPLEFT=-117w,60n yk_3arcsec 031a01_e.dem

NOTE: Implemented as `gdal/frmts/usgsdem/usgsdemdataset.cpp`.

The USGS DEM reading code in GDAL was derived from the importer in the VTP software. The export capability was developed with the financial support of the Yukon Department of Environment.