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geopoints nearest_gridpoint ( fieldset,geopoints[,string] )
Generates a set of geopoints from a field. The first field of the input fieldset is used. The result is a set of geopoints whose locations are taken from the original geopoints, but whose values are those of the nearest gridpoints in the field to the geopoints given as a second parameter. Where it is not possible to generate a sensible value due to lack of valid data in the fieldset By default, when the nearest gridpoint value is a missing value or the location is out of the grid area, the internal geopoints missing value is used (this value can be checked for with the built-in variable geo_missing_value or removed with the function remove_missing_values). If an extra parameter 'valid' is added to the function call, then of the surrounding points, the nearest valid one is returned; geo_missing_value will still be returned if all the surrounding points are missing. This function will return a missing value where the geopoints have missing lat/lon.
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