Visualizing Topobathy Digital Elevation Models (DEMs)
Creating eye-catching visualizations is crucial to a memorable presentation. Here we will introduce an Esri tool that allows users to “split” a topobathy digital elevation model (DEM) into two separate rasters, allowing for better visualization for submerged and topographic elevations. This blog will build on the “Developing a Topobathy DEM” posting; if you haven’t yet downloaded or created your own topobathy DEM, please review this blog first!
Creating eye-catching, easy to interpret visualizations for presentations is important not just for the “wow factor”, but also so your data become more accessible, and easy to understand. Remember – not everyone is familiar with LiDAR, let alone interpreting a complex topobathy DEM.
Using the method described below, you will “split” your topobathy DEM by a user-specified elevation value. The result will be two separate raster datasets – one representing all submerged (bathy) elevations, and one representing all topographic elevations. This will allow you to apply different color ramps to each raster, making the different elevations really stand out.
Start by visiting this Esri blog called “Geoprocessing to split elevation into topography and bathymetry”; even though this posting dates back to 2007, the tool they developed works great, even with ArcMap version 10.3.1. Feel free to review the blog post, but make sure you download the toolbox containing the model.
Once you’ve downloaded the toolbox, please follow the steps below to split and visualize your topobathy DEM.
1) First, we need to decide what elevation value to split our raster at. The tool default is 0, however this may not be the most accurate elevation value to split by.
Since my split topobathy DEM is going to be used in ArcGIS Online, primarily for visualization, I decided I wanted to split the elevations by Mean Sea Level (MSL) – or the sea level halfway between the means levels of high and low water. Since I know that the 2014 National Oceanic and Atmospheric Administration (NOAA) National Geodetic Survey (NGS) topobathy LiDAR uses a vertical coordinate system of the North American Vertical Datum of 1988, NAVD88, I knew that I had to determine the difference between MSL and NAVD88 to find my “split” value.
To do this, I visited the NOAA Tides and Currents website. This interactive web map allows you to zoom into your area of interest (AOI) to find a tide gauge, and datum information for that gauge.
Since my data is for the Assateague Island National Seashore in Maryland/Virginia, I zoomed into that area. Once there, I saw that the Ocean City tide gauge was the only available in the area. By clicking on the gauge, I was able to see links to pages with information about this station. Select Datums to view datum information for your selected gauge.
The Datums page will open in a new tab. On the list, find the values for the NAVD88 datum, and for the MSL datum. To determine your “split” elevation value, you will need to subtract the NAVD88 value from the MSL value.
For Ocean City, this would be:
For your AOI and based on the intended use for your split raster, you may want to explore calculating your split value using some of the other available datums from the NOAA Tides and Currents site.
2) Now that you’ve determined the appropriate “split” elevation for your location, we are ready to use the Split Raster at Value tool.
- Split Value = -0.36 (for Assateague ONLY – input your calculated value here)
- Input Raster = your topobathy DEM
- Output Workspace = where all outputs will be written
- Click OK to run.
- This will take some time to run depending on the size of your input raster.
3) Once the tool has finished running, add both rasters to ArcMap. They will be named “above_fl” and “below_fl.”
Note: Each raster will need to Build Pyramids upon loading, and this can take some time.
4) In the Table of Contents, open the Layer Properties dialog box for your “below_fl” raster by double clicking; navigate to the Symbology tab.
- In the Color Ramp drop down, right click and choose Graphic View to view the names of each color ramp.
- Select Cyan-Light to Blue-Dark.
- Check the Invert box to invert the color ramp so that the light color is at the top.
- Compare your dialog box to the one below, and then click Apply, OK.
5) We will now change the color ramp for the “above_fl” raster using the same steps as above, except this time select the color ramp named Elevation #1, and DO NOT check the Invert option.
For Assateague Island National Seashore, the final product (at full extent), looks something like the first image, while the second image shows a close-up view of the southern end of the island. You can really see how splitting the elevation values makes for a stunning visualization, and helps viewers to easily identify land versus water.
One last note is that this tool exports the rasters into the Esri Grid format. To save these rasters in a different format, it is necessary to export them to another format using ArcCatalog.
In the ArcCatalog window, right click on one of the rasters and select Export > Raster to Different Format. Select the output location of your new raster dataset, and be sure to specify a file extension (unless you are writing to a file geodatabase)! Click OK to run the tool.
This blog posting was developed with the support of a competitive grant (cooperative agreement number P09AC00212; task agreement number P13AC00875) from the National Park Service in partnership with the North Atlantic Coast Cooperative Ecosystems Studies Unit.