Hawth's Analysis Tools for ArcGIS

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 Summary

THEMATIC RASTER SUMMARY TOOL
Input: a thematic (categorical) raster layer, and a polygon layer
Output: results are written to a new DBF file
Features:

  • for each polygon this tool summarises the frequency of occurance of cells of each category in the thematic raster layer
  • for each unique category in the raster layer, a field is added to the output table to hold the count data for each polygon
  • the FID of the polygon layer is added to the output table so that it can be joined back to the polygon layer
  • since the polygons are processed consecutively, overlapping polygons are allowed
  • the user can opt to only process selected polygons

Limitations:

  • this program will run slower if there are a) very large polgons, or b) a large number of unique values in the raster
  • this program will fail if there are more than 254 unique values in the raster due to limitations on the number of fields that can be added to a DBF table


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 Help

Getting started. A polygon and thematic raster layer must be loaded into ArcMap in order to use this tool. If the polygon layer does not contain a FID field and this program fails, it is suggested you convert it to a shapefile (as shapefiles are automatically provided with a FID field) or add a unique FID (feature ID) field to your polygon attribute table.

Naming convention for new fields. Field names are limited to 10 letters for DBF files. The new fields follow the naimg convention TRS_VALUE, where value is an integer value representing the unique values in the raster layer. If you have values greater than 999,999 in your raster layer, this program will rename them using the following convention: TRS_ABBRVALUE_NUM, where ABBRVALUE is the first four characters of the number and NUM is a value starting at 1 that is simply incremented each time a large number is encountered (1, 2, 3... etc).

Output: frequency versus area. The output is a count of the number of cells within each polygon. To convert this to area, you must multiply the count by the area of each cell. E.g. if the cell size is 30m, and the count is 5 cells for a particular category, then the area is: (30*30)*5 = 4500m^2 (4500 meters squared).

Output: frequency versus percent. The output is a count of the number of cells within each polygon. To convert this to percent, you must divide the count by the sum of all counts for that polygon and multiply by 100. (This is most easily achieved in Excel). E.g. if the count is 5 cells for a particular category, and the total number of cells covered by that polygon is 87, then the percent is: (5/87) * 100 = 5.75%

NoData cells. An additional field is added to the output table to records the frequency of NoData cells.

Processing method (OLDER VERSIONS OF HawthsTools) If the version of HawthsTools you are using has "Processing method" options, then it is STRONGLY RECOMMENDED that you upgrade to the most recent version where the problem this option addressed has been resolved in a better manner.

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