Matching Criteria

To find locations for something, you can prepare a set of grids which mark acceptable locations.  These can be defined in a number of ways:

• Unacceptable locations are marked as missing data.  In GIS, missing data is marked with a special value in the grid.
• Locations can be coded as 0 (unacceptable) or 1 (acceptable).
• Locations can be coded as 0 (unacceptable), the grid for each criterion can be 1, 2, 4, 8, 16, 32, 64, or 128.  This is equivalent to setting a single bit.

To get these grids, you can use several GIS operations:

• Mark as missing
• Reclassify--assign values to new values.

The criteria can be combined in several ways:

1. Start with a complete grid, and then sequentially mask out (mark as missing) those that are missing in each of the acceptable locations grids.  The final grid then shows all locations that match all of the criteria.  The actual values in the final grid are not important (they will be whatever was in the starting grid), but the locations are.
2. Sum all of the criterion grids, using  0/1 values.  The resulting numbers will be the number of criteria met.  If you have impossible criteria, this may help to determing the regions that come closest to meeting the criteria, and how you might go about finding a revised definition of acceptable.
3. Sum all of the criterion grids, using the bits.  This shows you for each point which criteria are met--for example a value of 12 indicates that the point meets the criteria for the grids with for 4 and 8, but not 1 and 2.

Last revision 11/13/2018