Compute spatial (semi-)variogram of spatial data or residuals.

Syntax:
            variogram(krig, nint, plotit=T, ...)
Input:
krig	  trend-surface or kriging object with columns `x', `y', and `z'
nint      number of bins used
plotit    logical for plotting
....      parameters for the plot

Output:
`x' and `y' coordinates of  the  variogram  and  `cnt',  the
number  of  pairs  averaged per bin.  

Side effects:
Plots the variogram if `plotit = T' 

Details:
Divides range of data  into  `nint'  bins,  and
computes  the  average  squared  difference  for  pairs with
separation in each bin.  
Returns results for bins with 6  or
more  pairs.   

See also:
`correlogram' 
 
Examples:
topo.kr  <-  surf.ls(2,  topo)
variogram(topo.kr, 25)