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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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Raster-Based Geographic Data Sets—Layout in the Computer

The simplest storage of a raster band would be something like this:

66666666777777754444449999999
66666677777777777777444444999
66666777777777777744444445555

That is, the integers are stored in sequential locations in the memory of the computer or on a disk. In the preceding case, the number of columns (i.e., the length of a row) is 29 and the number of rows is 3.

The nice thing about storing a raster band is that very little addressing needs to take place to know the location of a raster cell in geographical space. If you know, for example (a) the location of the upper-left corner of the upper-left cell, (b) the cell size, and (c) the orientation of the raster, you can then easily calculate the location of any cell, given its row number and column number. To know what the value of the cell at column 14 and row 3, you need only multiply the row number minus one by 29 and then add the column number. In the example, this gives 2 × 20 + 14 or 72. If you start counting in the upper-left corner of the sequence raster values, wrapping around the end of a row to the beginning of the next row, you will find a “7” when you get to the 72nd value. Though this example is highly simplified, it gives you an idea of an addressing scheme that may be used to store raster bands.

Rasters may be very large. A raster composed of hundreds of millions of cells is not unusual. (The Kentucky-wide land use raster that you looked at has about ...

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