> >> If your data is in a data frame of coordinates x, y, (and maybe z), > >> and variable W for all cells, and all your cells are the same size, > >> then its just the weighted average of the coordinate (assuming your > >> dataframe is the complete grid). So: > >> > >> xm = sum(d$x*d$W)/sum(d$W) > >> ym = sum(d$y*d$W)/sum(d$W) > > > > You're right, it's simple for a grid where dx and dy are constant. > Thanks. But if they vary? > > Treat the masses as points centred at each grid cell and then find > the weighted mean of X and Y as before.
Sorry, I was wrong in thinking that a regular grid is needed. The simple weighting scheme above will work for any set of points, as long as the weight represented at each point is centered there. Scott Waichler _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
