There are two possible ways to calculate the standard deviation of a grid.
The first would be to use all the cells in the grid. Since all cells are used, this is said to be using the population. When calculating the standard deviation for a population, the denominator is N.
The second would be to use a sample of cells in the grid. Since not all cells are used this is said to be using a sample. When calculating the standard deviation for a sample, the denominator is N-1, which yields an unbiased estimate since you are not using the entire population.
ArcInfo uses the population method to calculate the standard deviation for a grid.
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