Hello,
  I want to calculate the area-weighted average of the grid values in a time-series of netcdf files, summarized over a shapefile with a single polygon.  The polygon overlaps several of the gridcells in the ncdf grid, but only partially overlaps some cells, requiring an area-weighted average.  1) Do any of the spatial packages in R have an area-weighted mean option for a polygon overlaid on a grid?  2) Do I need to convert the ncdf files into SpatialPointsDataFrames first?  I know how to open and read the ncdf files (ncdf library), how to read in the shapefile (readShapePoly), but am having trouble easily converting the ncdf files into a format that can be easily overlaid by the shapefile polygon.
  Thank you for your help!
  Trent Biggs
  San Diego State University

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