Mask Grids for Statistical Work

Initial grid to use as a mask.  Note:
  • Values in the Great Lakes.  These are flat, and including their statistics will skew results.
  • Values over the border in Canada and Mexico, which are often not valid because of a very coarse data set used in Mexico, and because Canada was set to zero and had some round-off errors for missing data.
Shape file of the states overlaid in red.  Note that in the Great Lakes, half of the lake is included in the state.

The masking should be done with the map at 1:1 or higher.  Masking is done with the screen map's resolution, and doing it at less than 1:1 could result in a "salt and pepper" look to the mask.

Using menu choice "Edit, Mark as Missing, All but Specified Color", everything outside the boundaries will be set to missing.


Shape file of the Great Lakes in red.  This shapefile of all lakes was edited to create a smaller file with just the Great Lakes.

The color for the lakes was changed to black, and then the menu choice "Edit, Mark as Missing, Specified Color", all of the Great Lakes set to missing.

The two masks.  After both masks are applied, the grid can be saved.
Final masked grid.

It can be selected on the Raster GIS menu.  In addition to selecting the grid to use, a range of values can be set that the grid value must lie between in order for the point to be used. 

The mask can only be used with grids that are compatible--same extent, size, and spacing.


Last revision 2/6/2017