[R-sig-Geo] center of mass for variable values in a grid

[Waichler, Scott R]

> > Is there an existing function to compute the coordinates for the center
> > of mass of a variable in a 2D or 3D grid, where dx, dy, and dz are known
> > for all cells?  The mass in each cell varies by location.
> 
>  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?

Scott Waichler