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CELL VALUES - legend • e.g the first example might have...

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CELL VALUES Types of values the type of values contained in cells in a raster depend upon both the reality being coded and the GIS different systems allow different classes of values, including: whole numbers (integers) real (decimal) values alphabetic values many systems only allow integers, others which allow different types restrict each separate raster layer to a single kind of value if systems allow several types of values, e.g. some layers numeric, some non-numeric, they should warn the user against doing unreasonable operations e.g. it is unreasonable to try to multiply the values in a numeric layer with the values in a non- numeric layer integer values often act as code numbers, which "point" to names in an associated table or
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Unformatted text preview: legend • e.g. the first example might have the following legend identifying the name of each soil class: 0 = "no class" 1 = "fine sandy loam" 2 = "coarse sand" 3 = "gravel" One value per cell • each pixel or cell is assumed to have only one value o this is often inaccurate - the boundary of two soil types may run across the middle of a pixel o in such cases the pixel is given the value of the largest fraction of the cell, or the value of the middle point in the cell • note, however, a few systems allow a pixel to have multiple values o the NARIS system developed at the University of Illinois in the 1970s allowed each pixel to have any number of values and associated percentages o e.g. 30% a, 30% b, 40% c...
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