The ERDAS/Lan (or Earth Resources Data Analysis System Version 7.4) file format consists of a 128-byte header containing selected map statistics as shown in Table 1. The data pack type, the number of columns, the number of rows, and the number of map classes are of particular interest to this research.
Table 1: Contents of ERDAS/Lan 128-byte file header.
Following the ERDAS/Lan header are the pixel data values that are packed (pack type) in either 4 bits, 8 bits, or 16 bits and represent the pixel's membership in a map class. A map class is a collection of one or more attributes, or pixel characteristics, and is usually assigned a unique integer value. Map class membership is mutually exclusive; each pixel may belong to one and only one map class at any given time. Map classes can be visualized by assigning a unique color or gray-scale to each class and displaying the pixel data. The maximum number of map classes that can be represented in a map is a function of the number of bits in the pack type where a pack type of b bits per pixel can represent map classes. Figure 1 illustrates a section of an ERDAS/Lan map illustrating 31 map classes that are represented by 31 different colors. The Southern Yellowstone National Park map, shown in Figure 2, illustrates 10 map classes. In this study, a pack type of 8 bits per pixel is used so that the number of map classes is limited to 256.
Figure 1: of a ERDAS/Lan map illustrating 31 map classes (FORD map).
Figure 2: Map of Southern Yellowstone National Park illustrating 10 map classes (FIRE map).