Getting started with Structure from Motion (SfM) photogrammetry

Beth Pratt-Sitaula, UNAVCO

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Summary

Structure from Motion (SfM) photogrammetry method uses overlapping images to create a 3D point cloud of an object or landscape. It can be applied to everything from fault scarps to landslides to topography. This exercise is intended as an introduction to the method so students can later proceed on to other more complicated data sets situated in the context of a geological research question. In this simple starting exercise students take photos in or around their own home area. If this version of the exercise has been adapted for a remote field experience, but certainly can (and has) been done as part of an in-person field course too.

The software used in Agisoft MetaShape Pro. Other software can accomplish the same thing but MetaShape is the one most commonly used by geoscientists. Fully functional 30-day trials area available and can be loaded on both Windows and Mac student computers.

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

Used this activity? Share your experiences and modifications

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

Students should be ready to learn a new software (Agisoft MetaShape Pro) and follow straightforward instructions. Students should know enough trigonometry that they can apply it to determining x-y-z position. The exercise helps them determine the procedure but they should have learned about sin-cos-tan in the past. They should be able to use a physical compass and/or compass/inclinometer apps to measure azimuth and inclination.

How the activity is situated in the course

The exercise can come just about anywhere in a course from very early to near the end; but should come before more advanced work with complicated SfM photo sets.

Activity Length

This takes half a day to one day, depending on student experience and in-person vs remote learning. It is best to assign the prep exercise as homework the night before so that they arrive with the software installed and a bit of background trigonometry completed.

Goals

Content/concepts goals for this activity

Use Agisoft MetaShape Pro to make a structure from motion 3D point cloud from photos they take themselves.

Higher order thinking skills goals for this activity

Analyze resulting 3D model for the pros and cons of their model in particular and the method in general.

Other skills goals for this activity

Trouble shoot challenges in taking photos and measuring ground control points with simple tools and limited instructor oversight.

Description and Teaching Materials

This exercise has a preparation exercise which gets the students set up with the software, compass or phone apps, and a few other supplies so that they ideally arrive to class ready to go.

The main part of the exercise is intended as a combination of synchronous and asynchronous activities remote activities. But it would certainly be feasible to do it in person as well. The instructor starts out with a synchronous lecture about SfM and overview of the steps involved in the activity. Then students then spend time on their own setting up ground control points (GCP), measuring them from an arbitrary 0-0-0, and taking the photographs in their own location -- could be a nearby hill, shed, sofa, car, bed or whatever they have easy access to. The group then reconvenes synchronously to walk the students through using Agisoft MetaShape to create a 3D point cloud model from their photos. Most likely they will complete the project and brief write-up after the synchronous session ends.

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 Materials

  • Prep exercise: SfM preparation exercise (Microsoft Word 2007 (.docx) 1.4MB Aug27 20)
  • Presentation: Introduction to SfM presentation (PowerPoint 2007 (.pptx) 78.1MB Aug27 20)
  • Main student exercise
  • Instructor supporting files
    • sample photo set for 3D (sofas) (Zip Archive 36.5MB Aug27 20) - if any of the students are unable to collect a photo set with ground control points OR if their photos fail to align, you can give them this set of pictures to practice on
    • - this Excel template provides a quick way to do the calculations needed to convert the distance, azimuth, and inclination measurements into x-y-z coordinates. I prefer to make the students sweat through the trigonometry themselves but if that is not practical with your students or time constraints, this can speed up the process.

Teaching Notes and Tips

  • Be prepared for time spent trouble shooting the software. It is ideal to have more than one instructor/TA with experience with MetaShape so that students can more quickly be helped with trouble shooting.
  • When you start the MetaShape session, it can be helpful to just have the students spend a little time looking around the software. For instance, draw their attention to the different window panes, the tabs that toggle between views, and the icons and tools. Students can get frustrated pretty quickly if their views do not seem to match yours. Often they are simply looking at "Workspace" tab instead of the "Reference" one.

Assessment

Formative assessment should be done through discussion with students as a whole group or individually.
The summative assessment is the main student exercise. Students are asked to provide an annotated screen shot of their final 3D model and answer some discussion questions. Overall this exercise is most likely to be a precursor of a more advanced SfM project that applies the method to a real geoscience research question. To that end it does not make sense to have a very involved assessment at this point.

References and Resources

  • This activity is adapted from the GETSI Field module Analyzing High Resolution Topography with TLS and SfM. In particular, Unit 1-SfM has additional resources related to teaching and learning SfM.
  • Sources for high resolution topography data sets
    • USGS National Map - many kinds of map data sets
    • OpenTopography - mostly lidar but also some SfM data sets - has a link in the upper right box to "US Interagency Elevation Inventory" - this map shows all the other data sets they know about even if they do not have the holdings themselves.


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