Structure from Motion for Analysis of River Characteristics
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 cancelled 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 GNSS data, drone imagery for use in structure from motion, and terrestrial laser scanning from a site near Greeley, Colorado on the Poudre River. In this assignment, students conduct a Structure from Motion (SfM) survey of the Sheep Draw field site. The end product is a point cloud used in additional assignments to conduct analyses.
Day 3 Part 2 - This activity is part of the 2-week remote field course Geoscience Field Issues Using High-Resolution Topography to Understand Earth Surface Processes
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
This exercise assumes students have some familiarity with Structure from Motion and GPS/GNSS. It is helpful if students have some background in geomorphology and/or physical geography concepts.
How the activity is situated in the course
This activity is the third component of a virtual field campaign and associated activities outlined in the course Geoscience Field Issues Using High-Resolution Topography to Understand Earth Surface Processes. This exercise follows an introduction to SfM (Getting started with Structure from Motion (SfM) photogrammetry) and GPS/GNSS georeferencing (Post-processing GPS/GNSS Base Station Position and Ground Control Points for Structure from Motion).
This activity takes about four hours. The instructor gives an introduction to the activity and reminds the class of the Agisoft Metashape SfM workflow. This could be conducted synchronously remotely, such as in Zoom. Alternatively, and in-person implementation could be completed. The instructor follows the Agisoft Guide for Sheep Draw activity (Microsoft Word 2007 (.docx) 5.3MB Dec18 20) to help the students get started, which could be conducted by screen sharing or projecting the instructor's screen. Students then work independently to complete the post-processing, focusing on either Area of Interest 1 or Area of Interest 2 (introduced in previous activities of the series).
Content/concepts goals for this activity
- Creating a 3D topographic models using Agisoft Metashape to post-process overlapping photos from Structure from Motion (SfM)
Higher order thinking skills goals for this activity
- Using a 3D dataset to answer a geomorphic question or test a hypothesis
- Evaluating the accuracy and errors of a spatial dataset
- Thinking in three dimensions
Other skills goals for this activity
Using Agisoft MetaShape software
Description and Teaching Materials
This exercise uses pre-collected GPS/GNSS data (ground control points) and drone imagery of the Cache la Poudre River at Sheep Draw to conduct a Structure from Motion (SfM) three-dimensional topographic model of a geomorphic feature. This exercise substitutes students visiting the field to collect images for SfM while allowing them to produce a 3D model that is used in subsequent activities. Students estimate the resolution (scale/pixel) from drone flight parameters (camera parameters). Students use Metashape to align overlapping (>75%) photos of the filed site. Previous activities introduced the site and explored the pros and cons of the ground control network collected. Students use the pre-collected GNSS ground control data (x, y, z data) as ground control. Following Agisoft Guide for Sheep Draw activity (Microsoft Word 2007 (.docx) 5.3MB Dec18 20) students produce an orthorectified aerial image and colored (R, G, B) 3D model of the site. Students evaluate the error of the model and think back to the implemented ground control network for pros and cons of the network implemented. In subsequent exercises students use their model to take measurements and compare their 3D point cloud with a point cloud collected with a different method (ground-based lidar; terrestrial laser scanning).
- Sheep Draw Vignette (Microsoft Word 2007 (.docx) 2MB Oct29 20)
- YouTube: Video Introducing Poudre at Sheep Draw Field Site
- YouTube: Video about GNSS and Structure from Motion Methods used at Sheep Draw
- Drone Images for Area 1 and Area 2
- Structure from Motion at Sheep Draw (Microsoft Word 2007 (.docx) 3.4MB Sep16 20)- Student SfM at Sheep Draw Exercise
- Agisoft Guide for Sheep Draw activity (Microsoft Word 2007 (.docx) 5.3MB Dec18 20)
- Pix4D Calculator (Excel 2007 (.xlsx) 50kB Sep16 20)- Excel calculator used to calculate pixel resolution from drone images
- Sheep Draw Ground Control (Text File 438bytes Sep16 20) - Ground control used in Metashape SfM
Agisoft MetaShape Professional software is needed. Students should be able to install a fully functional 30-day trial license on their computers. Faculty members have also been able to request 20-day licenses for the class as a whole. Faculty members who wish to use the software for more than 30 days will need to purchase the software. At the time of writing the educational license was $550.
Teaching Notes and Tips
- Be prepared to trouble shoot license installation and troubleshooting of software; recommended to install and try before this exercise.
- If students are struggling to use all the images for their area of interest, they can choose just one flight altitude, but may get less robust results.
- Students may skip steps and get slightly different answers, which makes for a good discussion of results.
Formative assessment should be done through discussion with students as a whole group or individually.
As the summative assessment, students answer questions about the accuracy and applications of their product. A rubric is included with the assignment.
References and Resources
- Adapted from GETSI Unit 1-SfM: Introduction to SfM
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