Marissa steps us through using the Topo to Raster tool to create hydrologically correct DEM.

The Topo to Raster tool is a spatial interpolation that has been specifically designed for creating hydrologically correct digital elevation models (DEM) from point, line, and polygon data.

 Using the theory that everything moves downwards the tool imposes constraints that ensure a connected drainage structure and correct representation of ridges and streams from the input data. The imposed drainage condition produces a higher accuracy surface with fewer input data

 There are nine types of data accepted inputs, point elevation, contour, stream, sink, boundary, lake, cliff, exclusion, and coastline. The Topo to Raster tool is the only ArcGIS interpolator specifically designed to work intelligently with contour inputs.

So now we know a little about why and how we might use the tool, let’s run through it!

Here we have some contour data for Lyttleton with the Topo to Raster tool open.

To get to this tool you go: Tools > Spatial Analyst Tools > Interpolation > Topo to Raster

From here we enter our parameters. In the input feature data section we have:

• Feature layer—The input feature dataset.

• Field—The name of the field that stores the attributes, where appropriate.

• Type—The type of input feature dataset.

Here have added the LyttletonContour layer, the field has been set as elevation and the type is contour. You can add more than one input feature data set at a time but for this demonstration, we will just use one.

We name our output and set the extent to be the same as the input layer.

The rest of the parameters can stay the same as they are.

 

Drainage enforcement is best left on enforce – it can be turned off however, this means the sink clearing process is ignored. Turning off drainage enforcement can be useful if you have data that is something other than elevation for which you’d like to create a surface for e.g., temperature.

Primary type of data needs to match your input data.

Make sure you have the correct coordinate system and run the tool! (depending on the size of your data, this tool can take a while to run)

And your output should look something like this:

 

You can see that the layer has been split into 9 classes but this can be changed to suit your analysis.

When you add the contour layer back over the top you can directly compare the interpolated elevation layer with the input data to check the accuracy.

You can also compare values – the new raster layer has an elevation of 172.52 m, comparatively the contour has a value of 180m in the same area.

 

The values for both layers are similar, this tells us that our analysis is accurate!

Thank you for reading my blog! I hope you have enjoyed learning about creating hydrologically correct elevation layers with the Topo to Raster tool.