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This formula is only used for regular latitude-longitude and Gaussian grids. For all other grid types integrate() simply returns the mathematical average of the values (just like the average() function does).
If the input fieldset contains only one field, a number is returned. If there is more than one field, a list of numbers is returned. Missing values in the input fieldset are bypassed in this calculation. For each field for which there are no valid values, nil is returned.
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