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This function computes the average of each a field in a fieldset over an area. The computations are based on the assumption that all the cells have the same extent in latitude and the individual cell areas area of the individual cells can be approximated with the following formula:
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A_{i} = 2 R^{2} cos\phi_{i} sin(\frac{\Delta\phi_{i}}{2}) \Delta\lambda_{i} |
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