Finding and ranking contiguous areas in a raster

The final step in the analysis for Assignment 3 is:

Once your suitability raster is ready, your final step is to interpret your results by identifying the 10 most suitable sites, ranked by most contiguous area.

This breaks down into two tasks:

  1. Identify the contiguous areas in the suitability raster.
  2. Order these areas by decreasing size.

There are (at least) 2 possible ways to approach this:

Contiguous areas: raster solution

This procedure uses raster regions, like we discussed in the Raster Data lectures.

  1. Add the Region Group tool to your model.
  2. Name the output raster Suitable Regions.
  3. Run your model and add Suitable Regions to your map.

The COUNT field in the Suitable Regions attribute table gives the number of cells in each region.

Contiguous areas: vector solution

This procedure requires converting the suitability raster to polygons.

  1. Add the Raster to Polygon tool to your model.
  2. Name the output feature class Suitable Polygons.
  3. Run your model and add Suitable Polygons to your map.

The Shape_Area field in the Suitable Polygons attribute table gives the size of each polygon.

NOTE: The areas of the polygons will only be automatically calculated if they're written to a file geodatabase (not to in_memory, or to a shapefile).

Displaying your Top 10 sites

There are a number of ways you can isolate the top 10 sites for mapping purposes. One fairly flexible way to do this is to make a new attribute field to store your top 10 site rankings.

  1. In the attribute table for your region group raster or your polygon dataset, under the table menu select Add Field.
  2. Add a new field called "Rank" and set it to be a short integer.
  3. Option 1: If you don't care about distinguishing between the sites, just whether they are in the top 10 or not:
    1. Select your top 10 sites so that they are highlighted in cyan.
    2. Right-click on the "Rank" field header and choose Field Calculator. Set the value to 1. This should set the records in your selected set to Rank = 1.
    3. Now you can symbolize from the new "Rank" field, which contains a 1 for regions/polygons in the top 10 and NULL otherwise.
    Option 2: If you want to distinguish between the sites in your top 10:
    1. In this option we will manually rank our top 10 sites 1-10. You need to be in an edit session to manually type values into your new "Rank" field. To start an edit session, go to ArcMap's Customize menu, go to Toolbars, then choose to turn on the Editor toolbar.
    2. From the Editor toolbar, under the main Editor menu, select Start Editing. You will then need to tell Arc which dataset you want to edit. It may give you a warning message that you have datasets in your map that are in a different projection than your edit dataset... this is OK since we are just editing attribute values.
    3. Now in your table view, sort by the Count (raster) or Shape_Area (vector) field. Manually type in your 1-10 ranking in your new "Rank" field.
    4. Once you have finished editing values, Save Edits from the Editor menu and then Stop Editing (also from the Editor menu). Now you can symbolize, label, etc. from the new "Rank" field.

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