ArcInfo ASCII Grid (agr, asc)
ArcInfo ASCII Grid refers to a specific interchange format developed for ArcInfo rasters in ASCII format. The format consists of a header that specifies the geographic domain and resolution, followed by the actual grid cell values. Export is supported through the ASC format with an associated PRJ file.
ArcInfo Binary Grid (adf)
dblbnd.adf, hdr.adf, log, prj.adf, sta.adf, vat.adf, w001001.adf, w001001x.adf
The ArcInfo Binary Grid is formed from a set of the files contained in a directory (listed above). When loading a raster into ArcGIS, it sees the containing directory as a single DEM, not as a directory containing other files. Due to software limitations, Geographic Imager does not see the whole directory as a single DEM and instead requires that the appropriate ADF file be selected for use (navigate to the directory containing w001001.adf file).
ArcInfo Binary Float Grid (flt)
The ArcInfo Binary Fload Grid is a binary file that stores floating point data (topological elevation values) line by line from north to south. It must be used with an accompanying header file (HDR) that contains the georeferencing information (both files must be stored in the same directory to function).
The BIL format, meaning "band-interleaved-by-line", is an uncompressed file containing the actual pixel values of an image. Pixel information is stored in separate bands within the file. It is possible to display one specific band in a multi-band BIL image. BIL files can consist of black and white, grayscale, pseudocolor, true color, and multi-spectral images. There are four image description files (ASCII text file format) that can accompany a BIL file: a header file, a statistics file, a resolution file, and a color file.
The header file (HDR) provides a description of the data through the use of keywords and values. The statistics file (STX) is an optional file that describes the image statistics for each spectral band. It records the minimum and maximum pixel values, the mean, the standard deviation, and the two linear contrast stretch parameters. The resolution file (BLW) describes the height and width of each cell and the coordinate position of the top left cell of the data. The color file (CLR) is an optional file that describes the image colormap.
ENVI DEM (hdr, dat)
The ENVI image format is a flat-binary raster file with an accompanying ASCII header file. The data are stored as a binary stream of bytes in one of the following formats, often referred to as the interleave type: BSQ (Band Sequential), BIP (Band-interleaved-by-pixel) or BIL (Band-interleaved-by-line). The header file (HDR) is an ASCII file containing the metadata associated with the binary file, and is needed to load the binary data.
ERDAS IMAGINE DEM (img)
ERDAS IMAGINE uses IMG files to store raster data. These files use the ERDAS IMAGINE Hierarchal File Format (HFA) structure. A tiled format is used to store raster layers. This allows raster layers to be displayed and resampled quickly. Each raster layer within an IMG file has its own ancillary data, including the following parameters: height and width (rows and columns), layer type (continuous or thematic), data type, compression, and lock size.
Grayscale GeoTIFF DEM (tif, tiff)
GeoTIFF DEM is unique in that it uses a floating point 32-bit data type as opposed to a more typical signed 16-bit data type for elevation data. GeoTIFF DEM must be imported using Geographic Imager Advanced Import and is converted from 32-bit to 16-bit on import. Cropping when using Advanced Import will create a new DEM schema range based on the cropped area’s elevation.
When saving a GeoTIFF DEM, the Source Data Type is always Integer. The schema range will never contain any precision (Integer value).
Military Elevation Data / DTED (dt0, dt1, dt2)
The National Geospatial-Intelligence Agency (NGA) developed a standard digital dataset called the Digital Terrain Elevation Data (DTED®). It is a dataset of terrain elevation values that provides basic quantitative data for systems and applications that require terrain elevation, slope, and/or surface roughness information. DTED datasets come in three levels: Level 0, Level 1 and Level 2.
DTED Level 0 elevation post spacing is 30-arc-second (nominally one kilometer). DTED Level 0 was derived from NGA DTED Level 1 to support a federal agency requirement. DTED Level 1 is a uniform matrix of terrain elevation values with post spacing every 3-arc-seconds (approximately 100 meters). The information content is approximately equivalent to the contour information represented on a 1:250,000 scale map. DTED Level 2 is a uniform gridded matrix of terrain elevation values with post spacing of one-arc-second (approximately 30 meters). The information content is equivalent to the contour information represented on a 1: 50,000 scale map.
USGS SDTS (ddf)
The Spatial Data Transfer Standard (SDTS) is a robust way of transferring earth-referenced spatial data between dissimilar computer systems with the potential for no information loss. It is a transfer standard that embraces the philosophy of self-contained transfers, i.e. spatial data, attribute, georeferencing, data quality report, data dictionary, and other supporting metadata all included in the transfer.
USGS DEM (dem)
This is the traditional format used by USGS before being replaced by SDTS, and is the format used for CDED DEM data products from Canada. Most popular variations on USGS DEM files should be supported, including correct recognition of coordinate systems, and georeferenced positioning.
The USGS Digital Elevation Model (DEM) data files are digital representations of cartographic information in a raster form. DEMs consist of a sampled array of elevations for a number of ground positions at regularly spaced intervals. These digital cartographic/geographic data files are produced by the U.S. Geological Survey (USGS) as part of the National Mapping Program and are available in 7.5-minute, 15-minute, also known as 30-minute and 1-degree units. The 7.5- and 15-minute DEMs are included in the large scale category while 30-minute DEMs fall within the intermediate scale category and 1-degree DEMs fall within the small scale category.
SRTM is an international project spearheaded by the National Geospatial-Intelligence Agency (NGA) and the National Aeronautics and Space Administration (NASA). Digital elevation models of the earth’s surface were created with measurements derived from the return signals received from two radar antennae on a spacecraft. Each data file covers a one-degree-of-latitude by one-degree-of-longitude block of the earth’s surface. SRTM consisted of a specially modified radar system that flew on board the Space Shuttle Endeavour during the 11-day STS-99 mission in February 2000, based on the older Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR).
DEM Data Considerations, Limitations and General Information
Geographic Imager imports DEM files into Adobe Photoshop as grayscale 16-bit files. Currently, Geographic Imager can only save to Grayscale DEM GeoTIFF, USGS DEM, ASCII Grid, and BIL.
The Raw Data DEM Schema creates a potentially low-contrast image and keeps the source elevations directly mapped to actual pixel values. Mosaicking DEM files using Advanced Import is recommended. Geographic Imager will create a DEM schema to fit the range of the images being imported.
To view DEM values, use the Geographic Imager panel Survey tab. Use the Color Sampler Tool to choose up to four points on the image to view its DEM value. Drag and drop individual points with the left mouse button pressed to change their location. Use Ctrl (Win) or Command (Mac) and click to select individual points to move or delete or right-click for additional options. Another way to view DEM values is to use the Georeference dialog box. The elevation value is displayed below the preview image.