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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 can be approximated with

A_{i} = 2 R^{2} cos\phi_{i} sin(\frac{\Delta\lambdaphi_{i}}{2}) cos(\Delta\phi)lambda_{i}

where:

• • R is the radius of the Earth
  • φ_i is the latitude of the i-th grid cell
  • Δφ is the size of the grid cells in latitude (constant)
  • Δλ_i is the size of the i-th grid cell in longitude.

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