Project 7: Allocation Based on Straight-Line Distance

Note:

Analysis - Environments...should be set appropriately before doing any analysis (Figure 7.5).

In particular, use the centreCountyCivilDivisions layer as the Mask and also for the Processing Extent. You should also carefully consider what is an appropriate Cell Size for the analysis and set that parameter.

Screenshot of how to set analysis environment

Figure 7.5: Setting Analysis Environments.

Both the Mask and Processing Extent should be set to centreCountyCivilDivisions in the Analysis Environment window.
Credit: A. Griffin © Penn State is licensed under CC BY-NC-SA 4.0

The first analysis we will do uses the Spatial Analyst Tools - Distance - Distance Allocation tool from the Tools menu (Figure 7.6). This allocates each part of the map to the closest one of a set of points and is the raster equivalent of proximity polygons.

 

Distance Allocation data entry interface
Figure 7.6: The Distance Allocation user interface with parameters set to generate an allocation of areas to high schools.
Text description for Figure 7.6.
Screenshot of the Distance Allocation geoprocessing tool in a GIS application. The Parameters tab is open, showing settings for a distance allocation analysis. The input source is “highSchools” with “ID” selected as the source field. The output distance allocation raster is named “Euclidean Allocation_Schools.” Additional fields for barriers, surface raster, cost raster, distance accumulation raster, and back direction raster are present but left blank. The Distance Method is set to “Planar.” Several optional sections, including source characteristics and movement costs, are collapsed below the main parameters. The image shows the tool configuration interface only, with no map or analysis results displayed.
Credit: Credit: A. Griffin © Penn State is licensed under CC BY-NC-SA 4.0

Running this Euclidean Allocation (straight line distance) analysis will produce an allocation layer (Figure 7.7).

Allocation layer

Figure 7.7: Allocation layer created from the Euclidean Allocation analysis.

This image shows the output of the Euclidean Allocation analysis with the settings described in the lesson. There is a mismatch in alignment between the allocation areas generated by the tool and the actual school districts. The allocation districts are most poorly aligned in areas that contain mountain ridges.
Credit: A. Griffin © Penn State is licensed under CC BY-NC-SA 4.0

You can also request a distance layer (Figure 7.8).

Distance layer

Figure 7.8: Results of the Euclidean Distance analysis for high schools.

In the Euclidean Distance analysis output, the raster cells contain values that describe the distance to the nearest school. The image looks like concentric buffer rings whose overlaps have been dissolved.
Credit: A. Griffin © Penn State is licensed under CC BY-NC-SA 4.0

You can further analyze these layers. For example, it may be easier to read the distance analysis if you create contour lines. The results of the allocation analysis can be converted to vector polygons, which might make subsequent analysis operations easier to perform, depending on which approach you decide to take.

Deliverable

In your Project 7 write-up, describe how the distance analysis operation works. In your description, comment on how you would combine multiple distance analysis results (one for each high school) to produce an allocation analysis output. [Hint: First run the Euclidean Distance Tool for each high school individually and then look for a tool that when used can generate the same map that the Euclidean Allocation analysis provides (i.e., the map shown in Figure 7.7). There is a tool which could help with this -- take a look at the tools available in Spatial Analyst - Local (e.g., Cell statistics or Lowest).]

As part of your discussion, include the output maps generated by the Euclidean Allocation and Euclidean Distance processes.

Finally, comment on the differences in the Euclidean (straight-line) distance allocation and the actual allocation of places to school districts. (You will need to look at the roads, topography, and minor civil divisions to make sense of this.)