You will now combine the reclassed slope and landuse datasets to produce a dataset of the cost of building a road through each location in the landscape, in terms of steepness of slope and land-use type. In this model, each dataset has equal weight.

To do so, expand the Overlay toolset in the Spatial Analyst Tools toolbox, click the Weighted Overlay tool and drag it onto the ModelBuilder window.

Right-click the Weighted Overlay tool element and click Open. The default evaluation scale is from 1 to 9 by 1. As in exercise 2, a scale of 1 to 10 was used when reclassifying the slope dataset, so before adding input rasters to the Weighted Overlay tool, you’ll set the evaluation scale from 1 to 10 by 1. This means you will avoid having to update the scale values after adding your input slope dataset.

Type “1”, “10”, and “1” in the From, To, and By text boxes.

Click the Add Raster Row button to add the input rasters.

Click the Input raster drop-down arrow and click the Reclassed slope variable.

Accept the default value for the Input field parameter and click OK.

The raster is added to the Weighted Overlay dialog box. The Field column displays the values of the reclassified slope output. The Scale Value column mimics the Field column because the evaluation scale was set to encompass the range of values in each input raster. You’ll keep the scale values as they are, since they have already been altered according to suitability.

Click the Add Raster Row button again to add the landuse input raster.

Click the Input raster drop-down arrow and click the landuse layer.

Click the Input field drop-down arrow and click Landuse. Click OK.

You’ll weight the scale values of the landuse layer so they are comparable with your Reclassed slope derived dataset. A higher value indicates that it is costlier to build a road through a particular landuse type.

Type the following scale values:

Brush/transitional—5

Water—10

Barren land—2

Built up—9

Agriculture—4

Forest—8

Wetlands—10

Note: Be cautious when using Restricted for the scale value when creating a cost surface. Using Restricted assigns a value to the cell that is the minimum value of the evaluation scale minus 1 (zero in this exercise), so your restricted areas will appear to be assigned the lowest cost when they are actually excluded from the analysis. Cells with this value should be rendered appropriately to avoid confusion. You could, instead, assign a high cost or set the scale value to NoData for areas you want to exclude from the analysis. If NoData is used, check first that there are no areas of NoData in your input rasters. If there are, it will be difficult to tell whether an area of NoData is there because it was excluded or there was no original information at that location for one of your inputs.

You’ll now assign an equal percentage of influence to each raster, since they are both equally important in this analysis.

Click Set Equal Influence to assign an equal percentage of influence (50 percent each) for landuse and Reclassed slope.

Accept the default path for the value of the Output raster parameter, but type “cost_surf” for the name.

Click OK on the Weighted Overlay dialog box.

Click the Auto Layout button, then click the Full Extent button to apply the current diagram properties to the elements and place them within the display window.

Right-click the derived data element for the Weighted Overlay tool and click Rename.

Type “Cost surface” and click OK. Right-click the Cost surface derived data element and click Add To Display.

Right-click the Weighted Overlay tool element and click Run to run the process.

Examine the layer added to your ArcMap display. Locations with low values indicate areas that will be the least costly to build a road through.