12/18/2023 0 Comments Gis raster meaning![]() ![]() Figure 3: Examples of complex logical expressions in map algebra. The expression a XOR b is true only if either a or b is true, but not both. Another connective that is commonly offered in map algebra is exclusive OR (XOR). We have already seen the connectives of AND, OR and NOT in raster overlay operators. Logical connectives are also supported in many raster calculi. A cell value in C will be true if the cell’s value in A differs from that cell’s value in B. It will store truth values-either true or false-in the output raster C. Green cells represent true values, white cells represent false values. Figure 2: Example of logical expression in map algebra. To this end, we may use the standard comparison operators (=, > and ). Map algebra also allows the comparison of rasters cell by cell. It has proven to be a good indicator of the presence of green vegetation. This expression, when carried out on AVHRR channel 1 (red) and AVHRR channel 2 (near infrared) of NOAA satellite imagery, is known as the NDVI (Normalized Difference Vegetation Index). Will create output raster C3, as the result of the subtraction (cell by cell, as usual) of B cell values from A cell values, divided by their sum. Will add the values of A and B cell by cell, and store the result as raster C2. Will add a constant factor of 10 to all cell values of raster A and store the result as output raster C1. Some simple map algebra assignments are illustrated in Figure 1 above. Other operators are goniometric: sine (sin), cosine (cos), tangent (tan) and their inverse functions asin, acos, and atan, which return radian angles as real values. Figure 1: Examples of arithmetic map algebra expressions Modulo division returns the remainder of division: for instance, 10 MOD 3 will return 1 as 10 - 3 × 3 = 1. Other arithmetic operators may include modulo division (MOD) and integer division (DIV). Obviously, these arithmetic operators should only be used on appropriate data values, and, for instance, not on classification values. The standard ones are multiplication (×), division (/), subtraction (-) and addition (+). Various arithmetic operators are supported. In map algebra, there is a wide range of operators and functions available. When the expression is evaluated, the GIS will perform the calculation on a pixel-by-pixel basis, starting from the first pixel in the first row and continuing through to the last pixel in the last row. The raster names and constants that are used in the expression are called its operands. The expression may contain references to existing rasters, operators and functions the format is made clear in each case. The expression on the right is evaluated by the GIS, and the raster in which it results is then stored under the name on the left. Output raster name := Map algebra expression. This is done in an assignment statement of the following format: When producing a new raster we must provide a name for it, and define how it is to be computed. ![]()
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