DEM of Difference (DoD)
This assignment builds on students' point cloud and raster differencing skills practiced on their SfM and TLS data in the previous assignment. In this assignment, the students use two lidar-derived raster datasets collected before and after the 2013 floods of the Colorado Front Range on a river (South St. Vrain Creek) that experienced substantial geomorphic change. As such, students practice their raster differencing skills in the context of geomorphic change detection (GCD) and also characterize their detection limit with simple thresholding approach. This assignment complements earlier assignments by using pre-existing LiDAR datasets to conduct a Digital Elevation Model (DEM) of Difference (DoD). It also serves to introduce students to additional high-resolution topographic data (lidar), covered in more detail in the next portion of the course (Day 9 of the course). This activity could be completed as a stand-alone activity in an upper-division geomorphology course or as part of a broader unit or course on high-resolution topography, such as used here.
Day 8 Afternoon - This activity is part of the 2-week remote field course Geoscience Field Issues Using High-Resolution Topography to Understand Earth Surface Processes
In spring 2020, the world was hit by a pandemic that spread globally by March, causing universities and most of the world to move to remote means. Summer field camps, long hailed as a rite of passage in the geosciences, were canceled throughout the US. The community moved quickly, with NAGT developing remote learning tools and arranging for sharing and collaboration between instructors and institutions. As such, UNAVCO (GETSI)and University of Northern Colorado embarked on a data collection campaign for a summer field course entitled "Geoscience Field Issues Using High-Resolution Topography to Understand Earth Surface Processes" – originally slated for in-person teaching. The team collected GPS/GNSS data, drone imagery for use in structure from motion, and terrestrial laser scanning from a site near Greeley, Colorado on the Poudre River.
This exercise is intended for majors-level geoscience courses that have field or remote (online) field components.
Skills and concepts that students must have mastered
Familiarity with geospatial data, raster data and a background in geomorphology and/or physical geography concepts is helpful.
How the activity is situated in the course
This is situated after the first week of SfM activities and the TLS activity, as outlined in the course page. As such, students have been introduced to point cloud data and associated post-processing techniques, and raster data. Furthermore, students have used ArcGIS before this exercise. This exercise could be used as a stand-alone exercise after introducing lidar/raster datasets and geomorphic change detection or as used here in a course about various topographic data collection methods and products. Students need to have ArcGIS and familiarity with basic post-processing tools therein. This could be adapted to an open-source GIS. As situated in this course, students learn more about lidar in the Day 9 lectures and activities that follow, but this could certainly be used after introducing lidar, rasters, and raster differencing. Different datasets could be used covering a geographic region of more significance to the students and course.
As situated in this course, students have already been introduced to raster differencing through a lecture and activity completed in the morning. The instructor should remind or introduce students to the GIS tools used in the activity after covering the basics of rasters (and lidar, if appropriate). Students may want to cover background information on the 2013 Front Range Colorado floods, as appropriate (see references). In this case, students are already familiar with the 2013 floods and will learn more about lidar in the lectures and activities following this exercise (Day 9 of course). The students work for ~2 hours on completing the assignment. A check-in or office hours for trouble shooting is appropriate.
Content/concepts goals for this activity
Conduct a DEM of Difference analysis with rasters in the context of geomorphic change detection related to flooding
Higher order thinking skills goals for this activity
Interpret geomorphic change while including error analysis
Other skills goals for this activity
Using ArcGIS raster calculator and zonal statics; error analysis
Description and Teaching Materials
This exercise uses two DEMs created from LiDAR data surveyed over South St. Vrain Creek near Lyons, CO: one surveyed in 2011 (SV_2011) and one in 2013 after the Colorado Front Range flood (SV_2013). Students conduct a DEM-of-difference (DoD) that estimates the cell-by-cell change in elevation between these two LiDAR surveys. With it, they calculate the volume and mass of sediment movement from reach to reach for the South St. Vrain Creek that experienced substantial channel movement and floodplain erosion and deposition. In addition, they estimate the bias in the DoD as well as its uncertainty through error thresholding. Students learned the basics of cloud-to-cloud and raster differencing in the context of error analysis in Day 8 morning activities (these were conducted on datasets collected at the same location but at different times with two methods, thereby comparing the difference between the two methods) whereas this activity compares the same location at two different times with the same method (thereby a geomorphic change detection). Students were introduced to these concepts and importance of raster orthogonality and best practices in Day 8 morning lectures that could be added to this activity if done in isolation (raster differencing lecture). Students interpret their geomorphic change detection in the context of how much sediment was eroded from St. Vrain Creek.
Raster DoD Student Exercise (Microsoft Word 2007 (.docx) 4.4MB Jan4 21)
DoD Geodatabase (Zip Archive 152.1MB Jan4 21)
A computer with ESCI ArcGIS Map (Raster Calculator and Zonal Statistics used)
Teaching Notes and Tips
You will need to help students find the appropriate tools in ArcGIS and remind students who to convert from volume to mass using density. As such, synchronous remote instruction, in-person computer lab teaching, breakout rooms or frequent check-ins would be appropriate.
As the summative assessment, students answer questions about the differencing methods used. Formative assessment should be done through discussion with students as a whole group or individually.
References and Resources
Rathburn, S.L., Bennett, G.L., Wohl, E.E., Briles, C., McElroy, E., Sutfin, N. 2017. The fate of sediment, wood, and organic carbon eroded during an extreme flood, Colorado Front Range, USA. Geology. 45 (6): 499–502. doi: https://doi.org/10.1130/G38935.1
Sholtes, J.S., Yochum, S.E., Scott, J.A. and Bledsoe, B.P., 2018. Longitudinal variability of geomorphic response to floods. Earth Surface Processes and Landforms, 43(15), pp.3099-3113. https://doi.org/10.1002/esp.4472
2011 Denver, CO Lidar, FEMA Region VIII, Collected Apr 27-Oct 2, 2011.
USGS LPC CO SoPlatteRiver Lot2a 2013 LAS 2015, Collected Oct 25, 2013 to May 31, 2014.
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