The Raster Product Format (RPF), from NIMA, is primarily used for military purposes by defense contractors. RPF data are organized in 1536 × 1536 frames, with an internal tile size of 256 × 256 pixels. RPF data are stored in an 8-bit format, with or without a pseudocolor lookup table, on CD-ROM.
RPF Data are projected to the ARC system, based on the World Geodetic System 1984 (WGS 84). The ARC System divides the surface of the ellipsoid into 18 latitudinal bands called zones. Zones 1-9 cover the Northern hemisphere and zones A-J cover the Southern
hemisphere. Zone 9 is the North Polar region. Zone J is the South Polar region
Polar data is projected to the Azimuthal Equidistant projection. In nonpolar zones, data is in the Equirectangular projection, which is proportional to latitude and longitude. ERDAS IMAGINE includes the option to use either Equirectangular or Geographic coordinates for nonpolar RPF data. The aspect ratio of projected RPF data is nearly 1;
frames appear to be square, and measurement is possible.
Unprojected RPFs seldom have an aspect of ratio of 1, but may be easier to combine with other data in Geographic coordinates.
Two military products are currently based upon the general RPF specification:
• Controlled Image Base (CIB)
• Compressed ADRG (CADRG)
RPF employs Vector Quantization (VQ) to compress the frames. A vector is a 4 × 4 tile of 8-bit pixel values. VQ evaluates all of the vectors within the image, and reduces each vector into a single 12-bit lookup value. Since only 4096 unique vector values are possible, VQ is lossy, but the space savings are substantial. Most of the processing effort of VQ is incurred in the compression stage, permitting fast decompression by the users of the data in the field.
RPF data are stored on CD-ROM, with the following structure:
• The root of the CD-ROM contains an RPF directory. This RPF directory is often referred to as the root of the product.
• The RPF directory contains a table-of-contents file, named A.TOC, which describes the location of all of the frames in the product, and
• The RPF directory contains one or more subdirectories containing RPF frame files. RPF frame file names typically encode the map zone and location of the frame within the map series.
• Overview images may appear at various points in the directory tree.
Overview images illustrate the location of a set of frames with respect to political and geographic boundaries. Overview images typically have an .OVx file extension, such as .OVR or .OV1.
All RPF frames, overview images, and table-of-contents files are physically formatted within an NITF message. Since an RPF image is broken up into several NITF messages, ERDAS IMAGINE treats RPF and NITF as distinct formats.
Loading RPF Data
RPF frames may be imported or read directly. The direct read feature, included in ERDAS IMAGINE, is generally preferable since multiple frames with the same resolution can be read as a single image. Import may still be desirable if you wish to examine the metadata provided by a specific frame. ERDAS IMAGINE supplies four image types related to RPF:
• RPF Product—combines the entire contents of an RPF CD, excluding overview images, as a single image, provided all frames are within the same ARC map zone and resolution.The RPF directory at the root of the CD-ROM is the image to be loaded.
• RPF Frame—reads a single frame file.
• RPF Overview—reads a single overview frame file.
CIB
CIB is grayscale imagery produced from rectified imagery andphysically formatted as a compressed RPF. CIB offers a compression ratio of 8:1 over its predecessor, ADRI. CIB is often based upon SPOT panchromatic data or reformatted ADRI data, but can be produced from other sources of imagery.
CADRG
CADRG data consist of digital copies of NIMA hardcopy graphics transformed into the ARC system. The data are scanned at a nominal collection interval of 150 microns. The resulting image is 8-bitpseudocolor, which is physically formatted as a compressed RPF.
CADRG is a successor to ADRG, Compressed Aeronautical Chart (CAC), and Compressed Raster Graphics (CRG). CADRG offers a compression ratio of 55:1 over ADRG, due to the coarser collection interval, VQ compression, and the encoding as 8-bit pseudocolor, instead of 24-bit truecolor.
Topographic Data
Satellite data can also be used to create elevation, or topographic data through the use of stereoscopic pairs, as discussed above under SPOT. Radar sensor data can also be a source of topographic information, as discussed in "Terrain Analysis" on page 645.However, most available elevation data are created with stereo photography and topographic maps.
ERDAS IMAGINE software can load and use:
• USGS DEMs
• DTED Arc/second Format
Most elevation data are in arc/second format. Arc/second refers to data in the Latitude/Longitude (Lat/Lon) coordinate system. The data are not rectangular, but follow the arc of the Earth’s latitudinal and longitudinal lines.
Each degree of latitude and longitude is made up of 60 minutes. Each minute is made up of 60 seconds. Arc/second data are often referred to by the number of seconds in each pixel. For example, 3 arc/second data have pixels which are 3 × 3 seconds in size. The actual area
represented by each pixel is a function of its latitude. Figure 33 illustrates a 1° × 1° area of the Earth.
A row of data file values from a DEM or DTED file is called a profile. The profiles of DEM and DTED run south to north, that is, the first pixel of the record is the southernmost pixel.
Figure 33: Arc/second Format
In Figure 33, there are 1201 pixels in the first row and 1201 pixels in the last row, but the area represented by each pixel increases in size from the top of the file to the bottom of the file. The extracted section in the example above has been exaggerated to illustrate this point.
Arc/second data used in conjunction with other image data, such as TM or SPOT, must be rectified or projected onto a planar coordinate system such as UTM.
DEM
DEMs are digital elevation model data. DEM was originally a term reserved for elevation data provided by the USGS, but it is now used to describe any digital elevation data.DEMs can be:
• purchased from USGS (for US areas only)
• created from stereopairs (derived from satellite data or aerial photographs)
See "Terrain Analysis" on page 645 for more information on using DEMs. See Ordering Raster Data on page 127 for information on ordering DEMs.
USGS DEMs
In 2006, the USGS began offering the National Elevation Dataset (NED). NED has been developed by merging the highest-resolution elevation data available across the United States into a seamless raster format. The dataset provides seamless coverage of the United States, Hawaii, Alaska, and the island territories.
Arc/Second Format
1201
1201
1201 Latitude
Longitude
Source: United States Geological Survey, 2006.
There are two types of historic DEMs that are most commonly available from USGS:
• 1:24,000 scale, also called 7.5-minute DEM, is usually referenced to the UTM coordinate system. It has a spatial resolution of 30 × 30 m.
• 1:250,000 scale is available only in Arc/second format.
Both types have a 16-bit range of elevation values, meaning each pixel can have a possible elevation of -32,768 to 32,767.
DEM data are stored in ASCII format. The data file values in ASCII format are stored as ASCII characters rather than as zeros and ones like the data file values in binary data.
DEM data files from USGS are initially oriented so that North is on the right side of the image instead of at the top. ERDAS IMAGINE rotates the data 90° counterclockwise as part of the Import process so that coordinates read with any ERDAS IMAGINE program are correct.
DTED
DTED data are produced by the National Imagery and Mapping Agency (NIMA) and are available only to US government agencies and their contractors. DTED data are distributed on 9-track tapes and on CD-ROM.There are two types of DTED data available:
• DTED 1 — a 1° × 1° area of coverage
• DTED 2 — a 1° × 1° or less area of coverage
Both are in Arc/second format and are distributed in cells. A cell is a 1°
× 1° area of coverage. Both have a 16-bit range of elevation values.
Like DEMs, DTED data files are also oriented so that North is on the right side of the image instead of at the top. ERDAS IMAGINE rotates the data 90° counterclockwise as part of the Import process so that coordinates read with any ERDAS IMAGINE program are correct.
Using Topographic Data
Topographic data have many uses in a GIS. For example, topographic data can be used in conjunction with other data to:• calculate the shortest and most navigable path over a mountain range
• assess the visibility from various lookout points or along roads
• simulate travel through a landscape
• determine rates of snow melt
• orthocorrect satellite or airborne images
• create aspect and slope layers
• provide ancillary data from image classification
See "Terrain Analysis" on page 645 for more information about using topographic and elevation data.