Structure from Motion for Analysis of River Characteristics

Sharon Bywater-Reyes, University of Northern Colorado

Summary

In this assignment, students conduct a Structure from Motion (SfM) survey of the Sheep Draw field site along the Poudre River near Greely, CO. 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

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.

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Context

Audience

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).

Activity Length

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 MetaShape Guide for River Characteristics activity (Microsoft Word 2007 (.docx) 5.6MB Jun28 23) 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).

Goals

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 MetaShape Guide for River Characteristics activity (Microsoft Word 2007 (.docx) 5.6MB Jun28 23) 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).

Files:

Technology Needs

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.

Assessment

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



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