Accuracy, Precision, and Topographic Data

The overall objective of this lab is to help students develop a critical mind about gathering and evaluating data. This sort of approach is vital to doing well in our class, and in any science. More specifically, the objectives are to learn:

1. how to use three different tools (methods) to collect quantitative position data about a landscape: a level and tape; a GPS (global positioning system); and a total station.
2. how to compile these data on a spreadsheet, and then plot them as a topographic map or surface using computer software.
3. how to evaluate the quality (accuracy and precision) of those data by comparing the two maps to what you know is actually out there.
4. finally, give an assessment of the benefits and limitations of each method for collecting data.
   

Methods of Geoscience

We believe that students must understand the sources and limitations of the spatial measurements used in studying Earth's surface. This exercise challenges students to think critically about precision and accuracy of x,y,z data. They are typically stunned to learn that 50 site-level points makes a better map than 500 GPS points. Thus, this exercise addresses misconceptions common among novice geoscientists ("technology always provides the best data" and "more data is always better").

We use this exercise as the first lab in a sophomore level geomorphology course. This may be the first course after Physical and Historical Geology for new (or undecided) Geology Majors. Thus, we expect they have knowledge of what a topographic map is, but little else. We typically have ~30 students in the course and divide them into 10 working teams for this exercise. We conduct the data collection and map production part of the exercise in one 4 hour lab period. The exercise requires substantial equipment: 1 handheld GPS unit per group; 1 site level + 1 survey rod + 50 m tape per group; 1-3 total stations (reflector or reflectorless) per class. We also show them our terrestrial laser scanner (ground-based LiDAR), but do not require its use. The total station sub-groups requires at least one experienced user to direct data collection. The data analysis requires student access to a spreadsheet program (e.g. MS Excel) and a 3D plotting program (e.g. Surfer).

• Student Handout for Geomorphology Exercise: Accuracy, Precision and Topographic Data (Microsoft Word 87kB May7 12)
• Grading Rubric for Topo Accuracy Exercise (Microsoft Word 27kB May7 12)

This is a jigsaw exercise, where student start in a MAP team of 3, then are broken up to become 'experts' on one method of topographic surveying (now part of an expert group of ten), then reconvene into their MAP team for map making and data analysis. We use a small (~40m x 40m; probably engineered) swale on campus, within a 5 minute walk of our building. The total relief on the feature is approximately 5 meters, but the shape is clearly discernible as a valley. We delineate the boundaries of the landform with flagging. The total

After division into MAP teams and re-organizing into expert groups, I start the exercise by marching all of the students to the site and ask them to make a sketch of what the topographic lines for this feature should look like. Then they break into expert groups, which is each accompanied by an instructor (GPS) or TA (site level and tape) or tech (Total Station).

The sequence typically plays out like this:

1. after a brief introduction to saving waypoints, the GPS group members each take 200-400 position measurements and are triumphant as the head to the computer lab to download their data.
2. the site level group breaks into teams and discusses how to use these tools (site level, stadia rod and tape) to locate x,y,z coordinates in a local framework. The TA must check their proposed method to ensure that they are recording sufficient data to reconstruct their positions. They typically can college ~50-60 positions.
3. the total station group finds set up excruciatingly slow (they are often still setting up when the GPS people are headed back with data). Once set up, they typically collect 25-35 points.

Students are assessed on the basis of their team's short paper and the three accompanying maps (presented as figures in the report).