Idrisi Raster (RDC)

Long name Idrisi Raster
Short name RDC
File extension(s) .img, .doc, .rst, .rdc

The GDB library supports Idrisi 2.x raster images (.img) for import and live linking. An Idrisi raster image consists of at least two files, with the header information stored in a file with the extension .doc. Normally the image data is stored in a similarly named file as that with the .img extension. Note that Idrisi 24-bit (RGB) rasters are not currently supported.

All Idrisi raster data types are supported. That is, 8-bit (byte), 16-bit (integer) and 32-bit real (real) images are supported. All Idrisi file types, such as "ascii", "binary" and "packed binary" are also supported for reading. Only "binary" is supported for writing. And, of course, the same limitations that exist in Idrisi for the combinations of data and file type exist in PCI software; for example, Idrisi does not support packed binary real.

PCI reads and applies the georeferencing information contained in the .doc file. A "lat/long" reference system will result in a Latitude/Longitude projection using the WGS 1984 datum. A "plane" reference system will result in a meter projection. The user can apply the correct and full projection definition if that information is available to them within the PCI environment.

The Idrisi raster's minimum and maximum X and Y coordinates are used to define the extents of the image and its resolution calculated based on those inputs. Its minimum and maximum values are also used to assign SCALING_MIN and SCALING_MAX values for the raster; and, if provided, flag definition values will be used for NO_DATA_VALUEs.

If the raster has an associated palette file (.pal), its red/green/blue settings will be used to construct a pseudocolor table for the raster in PCI software. These palette files should have the same basename as the .img and .doc and be located in the same directory. Idrisi's binary palette files, .smp, are not supported as the format definition for these files is not published.

If an image's reference system identifies the basename of an Idrisi Reference System Parameter File (.ref), PCI will use the specifications in that file to create the proper projection definition. These files are assumed to exist in the "georef" directory below the Idrisi system directory or in the same directory as the Idrisi raster files. To indicate, to PCI , the location of Idrisi system directory, the user must set a system environment variable, "IDRISI_SYSDIR", and point to the proper location. For example, on Windows 95/98, the user would enter the following in their autoexec.bat:

set IDRISI_SYSDIR=c:\idrisi

If c:\idrisi was the full path to their Idrisi software. PCI will then look under c:\idrisi\georef directory for the file indicated by the basename in the ref. system of the documentation file.

From the .ref, PCI supports "Mercator", "Transverse Mercator" and "Lambert Conic Conformal" projections. "none" is interpreted as Latitude/Longitude. All other projections will be treated as generic meters. If, after evaluating the entire reference system files contents, a "Transverse Mercator" entry is found to define a typical Universal Transverse Mercator (UTM) projection, PCI will switch the projection to UTM and indicate the correct zone in its software.

The datums supported are "NAD27" and "NAD83". NAD27 datums will use the Clarke 1866 (E000) ellipsoid and NAD83, the North American 1983 (D122) in PCI software. If another datum is specified, the major and minor s-axis entries are used to indicate the ellipsoid instead and a default "E999" shown in PCI.

It should be noted that on export to Idrisi format, PCI will not create a reference system file and the .doc will indicate "plane" only unless the raster is in a Latitude/Longitude projection. The user can, of course, use the information conveyed in the PCI projection windows to create a useable .ref for Idrisi and utilize it by changing the entry in the .doc.

IDRISI32 Version

1. Idrisi32 version has a ".rst" extension for Raster images, a ".rdc"
   for documentation file.
2. a new line called "file format" has been added for version checking.

   IDRISI32 requires that all raster files be in "IDRISI Raster A.1" format.
3. "Display Min" and "Display Max" entries have been added. Autoscaling now
   uses these values rather than the actual minimum and maximum as the scaling 
   points. By default, most analytical modules set these to be identical to 
   the min and max values.
4. RGB8 datatype be added.The RGB8 is simply a byte binary file. However,
   it is understood by IDRISI32 that this is specifically an 8-bit RGB 
   composite. It therefore automatically knows it should use the 
   COMPOSIT.SMP palette with this file
5. RGB24 datatype be added.The RGB24 is essentially equivalent to the data
   portion of a Windows BMP. It contains 3 bytes per pixel in blue, green, 
   red order. IDRISI32 automatically understands that it should display such 
   files as 24-bit images.

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