Processing the Landsat dataset in IDRISI32 required the following steps:

• Unzipping the file;
• Importing the data into IDRISI using the    GEOTIF option;
• Preparing a natural color composite image of    bands 1, 4, and 2.

All of this was standard proceedure, except for the decision to select the band sequence 1-4-2 and the 5% saturation option. A conventional natural color composite appears bluish, while a composite with bands 1-4-2 appears more natural. The 5% saturation option was arrived at empirically to increase the emphasis of green relative to blue.

Making the Vector Image

Preparing the image with an outline of the reserve boundary was a little more complicated, mainly because the image available was not georeferenced. Thus the image could not be co-registered with the satellite image.

Co-registering these two images means to place one image on top the other so that the ground locations represented in the first image correspond to same the ground locations represented in the second image. To do this we need to use a common frame of reference, which might be one of the images, say the first. We would then line up all the locations of the second image with the corresponding locations in the first image. This is commonly called rubber-sheeting because one image is streched to fit the other. To do this operation it is not necessary for either of the images to be geo-referenced (coded with latitude and longitude or other geographical coordinates). Fortunately, the Landsat TM image was already georeferenced to the UTM-35N (Universal Transverse Mercator 35 North). How to Read UTM.

To produce a georeferenced Boundary.RST file and its accompanying Boundary.RDC file the coordinates of four features in both images would be sufficient to run IDRISI's RESAMPLE routine. In fact, three of the six ground control points (GCPs) selected had to be reread from the satellite image because the first attempt at fitting the boundary did not fit the alignment of rivers or field patterns. With the replacement GCPs the average error reported declined dramatically and the resulting boundary aligned well with the features in the satellite image. Locating suitable gound control points and recording their locations was the difficult part of the task.

Next, the outline image had to be converted into a vector image, an image which IDRISI recognizes as forming lines that can be superimposed on the Landsat image (which is in raster format). There may be a simpler way to do this, but the following method works:

• Highlight the outline image;
• Press "Restore the original image" (13th icon from the left) to display the entire image;
• In Composer, save as a bitmap file (Boundary_OUTLINE.BMP);
• Use a graphics program to clean up the image (explained below).

A second method of georeferencing is to make two boundary images: one with no cosmetic changes and the second, a Boolean image, as described below. The first of these images is used to obtain georeferencing imformation and the second is actually georeferenced. This method might take more time than the first method because the accuracy of the georeferencing will not be known until the vector image is overlain on the satellite image, whereas with the first method visual checking can be done before proceeding further. By way of reassurance, once an image has been georeferenced and/or co-registered with another image, sending it out of IDRISI for cosmetic surgery will not undo all the work of resampling (rubber-sheeting) so long as one rule is followed:

Getting the image back into IDRISI is simple. Import the image as Boundary_BOOL.RST using the BMPIDRIS option. (The 8-bit option should be used to make the BMP file to be used for this.) The reference system will be plane, as recorded in the Boundary_BOOL.RDC file. The information needed for georeferencing is in the Boundary.RDC file, accompanying the Boundary.RST file. Close the Boundary_BOOL file. Copy and rename Boundary.RDC to Boundary_BOOL.RDC. When IDRISI next displays the Boundary_BOOL image Composer>Layer>Properties will indicate UTM-35N as the reference system.

Cleaning up the Image

Between exporting the image from IDRISI and reimporting it, the image needs to be cleaned up. As shown in the images on the right (above), the detail must be removed from the original image to produce a black and while image, called Boolean because the image has only two values. There are many ways to obtain the Boolean image. Most graphics programs have tools such as magic wand and paint bucket, a paint brush for the small details, adjustment to maximum contrast, and a speckle–removing filter. The choice of white as foreground and black as background is immaterial, since this can be changed in IDRISI.

Making a Vector Image of the Boundary

IDRISI has the facility for adding a vector layer on top of a raster layer. A line or polygon vector image of the boundary can be overlain on the Landsat raster image as in the image at the top of this page. A polygon-vector image can be produced directly using IDRIS's POLYVEC routine.

To produce a line-vector image an indirect method is needed. Use BUFFER to produce a two-pixel (60 meter) wide line outside the outline. Use RECLASS to make a negative image of the outline image, then use BUFFER to produce a two-pixel (60 meter) wide line inside the outline, reversing the target and non-buffer values. Use the maximum of the two images. This gives a four-pixel wide line straddling the outline of the image. Use RECLASS to make the value of the line equal to one and the background equal to zero. IDRISI will run a vector line down the center of this four-pixel line using LINEVEC.

Symbol Workshop can be used to make a symbol file that will display red lines in the image, otherwise the lines will be black. (If more than one line is generated, the symbol file should should have the same color copied to all line numbers.)

Displaying the Raster and Vector Images Together

After displaying the natural-color composite image the line-vector image is added using COMPOSER and the appropriate symbol file is chosen to obtain red instead of black lines.

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