Analyzing High Resolution Topography with TLS and SfM > Module Overview

Instructor Materials: Overview of Analyzing High Resolution Topography Module

Module Goals

  1. Students will be able to design and conduct a complex TLS and/or SfM survey to address a geologic research question.
  2. Students will be able to articulate the societal impetus for answering a given research question.
  3. Students will be able to justify why TLS and/or SfM is the appropriate method in some circumstances (if both methods are used).
    Supports Earth Science Big Ideas ESBI-1: Earth scientists use repeatable observations and testable ideas to understand and explain our planet; ESBI-4: Earth is continuously changing; ESBI-7: Humans depend on Earth for resources; and ESBI-8: Natural hazards pose risks to humans and Climate Literacy Principles CLP-4: Climate varies over space and time through both natural and man-made processes. (links open in new window)

Summative Assessment

The Summative Assessment, Unit 5, is the final unit of the module. The exercise evaluates students' skills in survey design, survey execution, and simple data exploration. As this is not designed to test any specific geologic context, we include a number of potential study topics with associated questions and prepared data sets for courses not able to collect data in the field. Learn more about assessing student learning in this module.

Outline

The module covers material sequentially, but the units can also often be taught as stand-alone lessons. For instructors who do not wish to use the module in its entirety, suggested pairings are included in the "Context for Use" section on each unit's page. Instructors can request support for some types of technical assistance from UNAVCO, which runs NSF's Geodetic Facility.

Unit 1-TLSIntroduction to TLS

This unit introduces students to the technical aspects of terrestrial laser scanning (TLS) survey design and execution. TLS requires a range of equipment, careful planning, and many hours of scanning in order to complete a successful survey, but it yields a high resolution topographic model valuable for addressing a range of geologic research questions. This unit is designed to be used as an alternative to or concurrently with Unit 1-SfM.

Unit 1-SfMIntroduction to SfM Photogrammetry

This unit introduces students to the technical aspects of Structure from Motion (SfM) photogrammetry survey design and execution. SfM requires less expensive equipment and less field time but more processing time than TLS. In low-vegetation field areas, it can yield a similarly valuable high resolution topographic model applicable to a variety of geologic research questions. Software for SfM varies; the unit was written to work with Agisoft Photoscan Pro but suggestions for open-source alternatives are given in the instructor overview. This unit is designed to be used as an alternative to or concurrently with Unit 1-TLS.

Unit 2Geodetic survey of an outcrop for stratigraphic analysis

Geodetic surveying techniques have many applications in sedimentology research, including lithological identification and analysis, sediment surface topography, and sequence stratigraphy. In this unit, students will design a survey of a geologic outcrop to conduct a sequence stratigraphy analysis. The goal is to calculate deposition duration and sedimentation rate based on thicknesses extracted from the data. Students tie these analyses back to societally important issues such as climatic change and energy extraction.

Unit 3Geodetic Survey of a Fault Scarp

Fault scarps are the topographic evidence of earthquakes large and shallow enough to break the ground surface, and are evidence of Quaternary fault activity. In this unit, students will design a survey of a fault scarp. The goal is to create a brief report summarizing the methods used and Quaternary history of displacements on the fault and thus its potential earthquake hazard. This unit also includes an additional optional exercise in data processing. Students will transform a point cloud into a DEM. Then students will be able to extract profiles of the scarp using ArcGIS and import these profiles into MATLAB to conduct hillslope diffusion analysis.

Unit 4Geodetic Survey for Geomorphic Change Detection

One major application of geodetic surveying techniques in geoscience research is quantifying change in geomorphological settings, such as a fluvial system, forest fire, landslide, or any other erosional features. This is done by finding the difference between georeferenced repeat data sets. Students will learn to clean up the data, remove vegetation, transform the point cloud to a DEM, and then compare that DEM to a previously collected data set to quantify change.

Unit 5Analyzing High Resolution Topography Summative Assessment

Unit 5 is a final exercise and evaluates students' skills in survey design, survey execution, and simple data exploration and analysis. Unit 5 is the summative assessment for the module. As this is not designed to test any specific geologic context, we include a number of potential study topics with associated questions and prepared data sets for courses not able to collect data in the field.

Making the Module Work

To adapt all or part of the Analyzing High Resolution Topography module for your course you will also want to read through

The module authors provided additional information about the teaching this module. If interested, click on the blue text below to display these notes.

Requesting technical support: UNAVCO, which runs NSF's Geodetic Facility, can offer some forms of technical support for the educational use of TLS and SfM. Learn more about the support available from UNAVCO and make a support request at UNAVCO's Field Geodesy Learning page.

Keeping students occupied: One of the challenges of integrating these techniques into a course with more than a few students is making sure that students stay engaged and mentally challenged even while they are waiting for their instrument time. While they wait to work with geodetic equipment, there are a series of possible tasks that instructors can assign to help students better understand the components of survey design and parameter calculation AND keep them occupied and engaged. These include:
  • Concurrently using the SfM and TLS methods: When students are not actively working on the TLS survey, they can design and implement an SfM survey of the same feature. Note: care must be taken to avoid obstructing the scanner.
  • On the first day, students should be encouraged to review their geodetic method field manual(s).
  • (TLS only) Completing the TLS Scan Resolution Parameter Worksheet from their assignment packet for all scans, including those they did not run themselves.
  • (TLS only) Compiling the equipment list in their TLS Field Manual (if not done as a part of a previous unit).
  • (SfM only) Completing the SfM Photogrammetry Calculations Worksheet
  • Traditional observations and measurements: strike/dip, rock type, sketch map of whole feature (not just the scan and target positions).
  • Students may start on portions of their final write-up for the unit.
  • Observations that would inform analysis later such as:
    • Presence of water (as the scanner cannot scan water and photographs will only represent the surface of the water)
    • Surface texture, color, and condition (for TLS intensity measurements or for reference when examining the SfM point cloud): for example, students can use the Riegl spec sheet on reflectance from their TLS Field Manual to sketch the outcrop with reflectance considered.

More about data exploration:

  • One limitation of this unit for field courses is the reliance on computers for interpretation and analysis. If using individual student laptops, the necessary programs (software for scanner or photogrammetric analysis, point cloud manipulation, and vector drawing) should be loaded prior to departure for camp, as internet connections may be slow to nonexistent and programs may not be able to be shared due to licensing issues.
  • Allow time in the schedule to transfer the field data to the lab or student computers; it is not uncommon to collect several gigabytes of data during a day of surveying, so having external hard drives to pass the data around is essential.
  • Also allow time to prepare the data for student use. If a UNAVCO field engineer is present, she will likely need a few hours to georeference the data if students will not be asked to do it themselves. Take this into account when planning the schedule for the module.

Additional tips:

  • TLS requires a large amount of equipment. Choose small sites with relatively easy access.
  • SfM does not work in areas of significant vegetation. Choose open sites with little to no vegetation. SfM equipment is easier to transport than TLS, but large field areas will result in considerable processing time, so small areas are still desirable, particularly for initial learning.
  • Anticipate the survey process moving slowly initially as students become familiar with instrumentation setup and the data collection workflow (particularly for TLS).
  • Visit the site prior to taking students there to survey-–-to assess access, site size, features and vegetation that may obstruct or complicate the survey, and to obtain landowner permission (if required).
  • It is helpful to have several range finders for the TLS method (at a cost of about $250 for one that does 1000 m; UNAVCO TLS kits include one or two). That way more teams can be filling out Scan Resolution Parameter worksheets at the same time.

Metacognition (reflection) is built into all student exercises in this module. Metacognition, or "thinking about thinking," encourages students to examine what and how they learned, to help them monitor and then alter their learning techniques to ensure best learning practices. Each unit's final write-up includes a reflection question for students to answer about their learning experience. In addition, ask students questions such as: What was rewarding about this exercise? What was challenging? How have your ideas about fieldwork changed because of your experience with geodetic techniques? What other applications would you suggest applying geodetic techniques to in future research you might do? More information on metacognition is at InTeGrate Project's Metacognition page and Teaching Metacognition by the Cutting Edge Project.

Societal importance: For each of the units students are also asked to articulate the societal importance of different types of surveys. Research shows that students are more engaged in subject matter they see as relevant and important. By tying what might appear to be dry geophysics methods to the underlying reasons we want to conduct such research, students see the relevance and importance of geoscience in everyday life. Geodetic surveys can be used to answer research questions on disparate topics, from energy and paleoclimate studies (basin analysis) to earthquake hazard (fault scarps) and landslide hazards (hillslope change), and more.

Adapting module to non-field courses: This module may be adapted to non-field courses. We have provided data sets from the full range of "high resolution topography" applications as featured throughout the module. TLS data are provided in point cloud and DEM formats with associated metadata; SfM data are photo sets and DEMs with associated metadata. In addition, all the same data analyses could be done with TLS data that have been collected by others. Check the UNAVCO TLS archive, OpenTopography, or other lidar sharing websites for community-contributed TLS or ALS (airborne lidar scanning) data sets. Research has been done on using aerial photography for SfM applications as well; some of these data sets may be on the OpenTopography website as well.

Using both TLS and SfM: If using both techniques, students can compare and contrast the challenges as well as the benefits of the two methodologies in their write-ups and create a guide for when TLS or SfM is preferable.

The main difference in using SfM and TLS is the allocation in time. SfM generally takes less field time but considerably more processing time. SfM topographic models take hours or even days to be generated, depending on processing power and field area size. Ideally, you could have students collect photographs in the morning and then process them during the day while doing other fieldwork or collecting TLS data. The other difference is the software: both TLS and SfM require specific programs for data processing and exploration.