By Gaby Carter, Undergraduate Student, Virginia Tech
One of the first things I did as part of the Global Rivers Group was use digital elevation models and relative elevation models to create a piece of artwork of the part of the New River that runs through Blacksburg. I did so by following two tutorials developed by Dan Coe, which can be found on his website: https://dancoecarto.com/tutorials/
The tutorial utilizes a GIS data point called a digital elevation model (DEM). In their 2021 review, Guth et al. describe a DEM as a data point that “provide[s] fundamental depictions of the three-dimensional shape of the Earth’s surface” by “record[ing] the interface between the atmosphere and the lithosphere using a discrete two-dimensional grid” (2021). Additionally, a more specific type of DEM is a digital terrain model (DTM). A DTM is known as a “bare-earth” representation of the surface, as it removes non-topographical features, such as buildings and trees (https://www.usgs.gov/faqs/what-difference-between-lidar-data-and-digital-elevation-model-dem). For this project, I used publicly available DTMs to create a relative elevation model (REM) of the New River, and applied a color gradient to the REM along the elevation to create a piece of art for our lab.
To create the image, I first had to create a compiled DTM of the area I was interested in: the part of the New River that runs closest to Blacksburg. I selected the desired area in the USGS’s 3DEP LidarExplorer and downloaded the DTM tiles that contributed to the area. The next step was to mosaic the downloaded tiles together to create one compiled DTM. However, I didn’t need the whole area in this compiled DTM, only the parts that contributed to my area of interest. So the next step was to clip my area of interest out from the compiled DTM. To do this, I used the open source software QGIS. Once I had downloaded and installed QGIS, I was able to easily follow the tutorial and create the DTM of my area of interest.
As a side note: I ran into some difficulties stitching the DTM tiles together because the area I was interested in crossed the Montgomery-Giles county line, meaning the tiles were collected during different surveys. I got around this problem by downloading all the DTM tiles that contributed to the desired area, including tiles from both surveys. Once I imported the tiles into QGIS, I was able to overlay the Giles county DTMs on top of the Montgomery county DTMs to create the compiled DTM of the entire area- above and below the county line.
Once I had the combined DTM, I was able to start working on making an REM. As explained by Mr. Coe, REMs are “produced by detrending the baseline elevation to follow the water surface of the stream” (Coe, 2019). This means that instead of the baseline elevation being sea level, as in DTMs, the baseline is determined by the river surface itself, making REMs useful for visualizing finer river features that are difficult to see on a DTM. For this project, I followed this tutorial, again developed by Dan Coe, in QGIS: https://dancoecarto.com/creating-rems-in-qgis-the-idw-method
Here is the final product (complete with the official Virginia Tech color gradient):
I really enjoyed working on this project. It was a great introduction to working with QGIS, and I learned a lot about DEMs and REMs. Plus, seeing something I helped create up in our lab every time I go in makes me smile.
References Cited
Coe, Dan, 2019, Dan Coe Carto: https://dancoecarto.com/creating-rems-in-qgis-the-idw-method (Accessed January 2023).
Guth, P. L., Van Niekerk, A., Grohmann, C. H., Muller, J., Hawker, L., Florinsky, I.V., Gesch, D. B., Reuter, H. I., Herrera-Cruz, V., Riazanoff, S., López-Vázquez, C., Carabajal, C. C., Albinet, C., Strobl, P., 2021, Digital elevation models: Terminology and definitions: Remote Sensing, v. 13, no. 18.
Gaby Carter is a sophomore undergraduate student at Virginia Tech getting her B.S. in Biology. She works as a research assistant in the Global Rivers Group.
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