Project 7: Allocation Based on Road Distances, Cost Allocation

Using the roads raster layer for distance analysis

Distance analysis using a roads raster layer is straightforward. The values in the layer are regarded as 'weights' indicating how much more expensive it is to traverse that cell than if the cell were unweighted. Thus, with the roads layer you just created, traveling on major roads incurs no penalty, traveling on local roads is twice as expensive (takes twice as long), and traveling off-road is very slow indeed (100 times slower). These are not accurately determined weights, but serve to demonstrate the potential of these methods.

Select the Spatial Analyst Tools - Distance - Cost Allocation tool (Figure 7.10).

Here you are performing another distance analysis, but this time weighted by the roads raster layer you just created. Again, request an Allocation output from the analysis. (You might also be interested to request the Output distance raster to look at the distances that accumulate for each cell.)

Figure 7.10: The Cost Allocation user interface for doing cost distance analysis. Note that you will probably have named your input cost allocation raster something different. This is simply the final, weighted roads layer you produced. Also notice that there are several parameters you can specify to further tweak the cost calculations. For now, we’ll not use these options, but if you have the time, you could experiment with them and consider which might be relevant to this particular application of cost distance analysis. Hover over the information icon that appears when you mouse over the parameter name for a more detailed explanation of what the parameter manipulates.

Click for a text description of the Cost Allocation tool image.

Again, your input feature layer is the high schools layer and the source field is ID. Your input cost raster is the roads layer you created in the prior step of the exercise. Give your output a sensible name such as Road_Cost_Allocation_Schools.

Credit: Griffin, 2019