Using Elevation in Site Plans
Updated: Oct 2, 2021
Elevation is an integral piece of data in not only choosing a suitable site, but also creating a site plan. Whether you are scouting a site for an oil pad, solar development or wind farm, elevation and slope will play an important part.
The 3D Analyst tools in ESRI ArcGIS Pro make it easy to convert contour lines into a raster and the raster into a slope output. The slope output identifies the slope (in degrees or percent change) from each cell of a raster.
The National Map, a service provided by the USGS in conjunction with state and federal programs, is a collaborative effort to provide updated and improved topographic data. The data available for download ranges from 1/3 arc-second DEM's to lidar data.
Link to map: https://viewer.nationalmap.gov/advanced-viewer/
Link to data download: https://viewer.nationalmap.gov/basic/#/
The first step to download the desired data is to draw an AOI using the bounding box tool and then click Search Products. For this project, I searched for Contours (1:24,000-scale) in an area outside Bloomington, IL. The results page will show the different datasets available for download. Hover over each result to see the area footprint. Click download Zip file, and save it locally. After downloading, you will need to quickly extract the zipped folder.
Analyze in ArcGIS Pro
In ArcGIS Pro, bring the contour lines into a new Pro project. Once setting up the project, you can copy the features into the project gdb, but is not completely necessary. Taking a look at the data, you can see the elevation of each line is in the "ContourEle" column. The default symbology for a line layer will be a single color, however by using the Unique Values Symbology, you can easily symbolize each line on similar elevation properties. When looking at contour lines like this, I like to have a basemap in the background so I can easily reference my surroundings and determine causes for different elevation changes. A common reason for elevation changes in central Illinois are riverbeds. Taking a closer look at the data, you can easily tell where multiple rivers lie.
The next step is to convert the contour lines into a raster dataset using the "Topo to Raster" geoprocessing tool. The feature layer will be the contour file and the field will be the elevation field from your feature layer. In this case, the field is "ContourEle". All other inputs can stay default.
The output raster will look similar to the image above. The dark green pixels start at 633' and range to the white pixels at 931'. We can now get a pretty good picture of suitable areas for a pad or solar farm. The areas that change colors quickly will be more of a topographical challenge than the areas that are a constant color (like the green area at the top of the image). To get a better idea of suitable areas, we will now run the Slope tool.
The Slope tool, also found in the 3D Analyst toolbox, identifies the slope from each cell in a raster. The output can be given in either degrees or percent rise (or percent slope). The degrees can range from 0 to 90. A flat surface will result in a 0% change and a 45 degree surface will result in a 100% slope change.
The slope tool is very easy to run. The input raster will be the raster that was created with the topo to raster tool. In the output measurement, choose either Degree or Percent Rise. I prefer to use the percent rise output. In my opinion, the output in percentage gives a better overall view of the terrain and surrounding environment. I also feel its easier to symbolize using the percentages.
The output will be similar to the image below. Because this area in central Illinois is fairly flat, the slope output will be pretty boring. The light green cells, and a few yellow cells, show the areas with the most slope rise. The yellow cells show a 25% change in slope. The majority of this area is less than 3% slope rise...perfect for an oil pad or solar array.
If you find yourself having to a similar exercise multiple times, creating a model in ArcGIS Pro ModelBuilder is the way to go. If you have experience creating models in ArcMap, it is a very similar process. The first step is to add the contours layer and then the "Topo to Raster" tool. You can easily drag and drop geoprocessing tools by searching for them in the Geoprocessing window. Then connect that output to the "Slope" tool. Set the parameters for each tool by double clicking on the tool name.
In the ModelBuilder example below, I renamed a few of my outputs so I could have a better idea of what is happening. ModelBuilder allows for repeatability and can streamline a process like site grading.
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