Bulletin board wisdom

Question: Can I determine large scale variability by an indicator plot? When I look at a plot of my data by x coordinates, there seems to be a spatial trend.

Answer: The indicator plot suggests a large scale trend because it looks like your x coordinate drives the trend in your indicator plot, and coordinates are ipso facto large scale factors. In order to verify this, you should account for the x coordinate covariate, then do a semivariogram on the residuals (a residual semivariogram, natch) to determine whether this apparent trend was really being driven by that x coordinate or by some yet-unaccounted for variable.

You can account for any variable you feel may be driving your spatial trend and lump that variable into your large scale variability (as long as it is not a regional variable, obviously), then run a residual semivariogram to see if there is still spatial trends.