Ground Control Points for Structure from Motion

Sharon Bywater-Reyes, University of Northern Colorado


Summary

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. This assignment uses real-time kinematic GNSS methods to establish a ground control points used in Sheep Draw Structure from motion activity.

Day 3 Part 1 - This activity is part of the 2-week remote field course Geoscience Field Issues Using High-Resolution Topography to Understand Earth Surface Processes

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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 Global Navigation Satellite System (GNSS) such as the Global Positioning System (GPS), map projections, and coordinate systems. It complements, and could follow, Unit 1: GPS/GNSS Fundamentals. It also assumes students are familiar with GIS systems and are proficient in either ArcGIS Desktop or ArcGIS Pro. This assignment could be adapted to use an alternative GIS such as GRASS or R. This activity also followed Getting started with Structure from Motion (SfM) photogrammetry and an introductory lecture into SfM. The activity could be adapted as a simple x, y, z point mapping activity and the SfM introduction could be skipped.

How the activity is situated in the course

This activity is the second 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).

Activity Length

This activity has a discussion (conducted virtually in Zoom break-out groups or similar) that should take no more than an hour to complete. The actual activity could take students up to two hours, depending on the GIS skill level. A follow-up discussion could be completed (virtually/synchronously) that would take another ~30 mins. This could be conducted during a lab session or introduced and assigned as a homework assignment with a follow-up discussion.

Goals

Content/concepts goals for this activity

  • Planning a ground control network for SfM
  • Importing and mapping x, y, z RTK GNSS coordinates used for ground control

Higher order thinking skills goals for this activity

  • Evaluating the spatial distribution of a ground control network for strengths and weaknesses

Other skills goals for this activity

  • Creating a professional map in GIS

Description and Teaching Materials

This exercise substituted students having hands-on training setting up and collecting GNSS data in a field course. In this case, a Septentrio receiver was setup over a known base station (established during Post-processing GPS/GNSS Base Station Location exercise) and Real Time Kinematic positions collected over ground control targets. Since this exercise followed one that introduced SfM and GPS/GNSS methods and students were previously introduced to the site and methods with videos shot during data collection, students start with a group discussion on where ground control points at the Sheep Draw site should be placed for their assigned area of interest. Students are given a text file of the x, y, z coordinates (UTM) and must import them into ArcGIS to create a ground control map. A follow-up discussion should have the students discuss the results of their map compared to the locations they discussed for placement in the discussion. They are asked to summarize the strengths and weaknesses of the implemented ground control plan at Sheep Draw.

Files:

Technology Needs

A computer with ArcGIS Desktop or ArcGIS Pro (needs to be installed, not web version)

Teaching Notes and Tips

  • Be prepared to troubleshoot/review ArcGIS setup and file structure and basic operations such as changing symbology

Assessment

Formative assessment should be done through discussion with students as a whole group or individually.

As the summative assessment, students write a discussing the strengths and weaknesses of their proposed ground control and the actual ground control implemented and also turn in their map.

References and Resources



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