Comparing GLOBE Trees to ICESat-2: A New Scientific Publication

Can your tree measurements give scientists assessing tree heights with satellite data a useful ground-based reference point? According to a new analysis of GLOBE Observer tree data, it can in certain circumstances. There are four things you can do to make sure your observations are useful.

The recommendations come in the conclusion of the first scientific analysis of GLOBE Observer Trees data, which was published in Environmental Research Letters on 21 June 2022. Called “The potential of citizen science data to complement satellite and airborne lidar tree height measurements: lessons from The GLOBE Program,” the paper compares how GLOBE volunteers collect tree height estimates with the GLOBE Observer app to the methods used to measure tree heights using airborne lidar and a satellite-based laser altimeter. It then compares the errors in the two methods to see how they stack up next to each other and when they might be used together.

GLOBE Observer tree height estimates are shown in red and ICESat-2 height measurements are in blue. The solid blue cluster at the bottom represents the ground and/or the tree canopy. The higher elevation blue circles are individual trees. It’s difficult to align the GLOBE Observer measurements (red triangles) with individual ICESat-2 measurements, but the two generally match. Figure from "The potential of citizen science data to complement satellite and airborne lidar tree height measurements: lessons from The GLOBE Program.”

GLOBE observations have the best chance of matching up to ICESat-2 data when GLOBE volunteers take care to maximize the accuracy of location and tree height following these four recommendations.

 

Recommendation 1: Take Observations Beneath ICESat-2 Tracks

Take tree observations beneath ICESat-2 tracks. You can see where ICESat-2 has measured trees near you by visiting the Open Altimetry online tool at http://openaltimetry.org. Here you can see exactly where on the Earth ICESat-2 has measured heights of Earth objects, including trees.

You can also check out the blog, “Comparing Existing GLOBE Tree Height Data and ICESat-2 Data," for more information, including a video tutorial how to locate ICESat-2 tree height data on the Open Altimetry online tool.

 

Recommendation 2: Maximize GLOBE Observer Tree Height Measurement Accuracy

Maximize height accuracy by following the instructions in the GLOBE Observer Trees tool closely and, if possible, measure the distance to the tree with a tape measure.

A controlled experiment which tested each step of the GLOBE Observer tree tool found that the biggest uncertainty or error in tree height estimates were when estimating the distance to the tree. In the GLOBE Observer app, the distance to the tree is calculated based on your height and pace length. You report how many steps it took to get to the tree and your height, and the app uses the average stride length for a person of your height to estimate the distance. The best way to reduce uncertainty is to use a measuring tape to measure the distance to the tree. If you don’t have a measuring tape with you for every tree height observation you take, you can improve your app-based measurement by updating the app with your actual stride length.

 

Repeat measurements of two trees by 76 high school students in Queens, New York, showed that the error in volunteer measurements were slightly greater (14-15% of the tree’s height vs 10%) than errors seen with GLOBE Observer tree height estimates in the controlled test. These errors probably come from mistakes in doing the protocol, such as moving the phone slightly between measuring the angle to the top of the tree and the angle to the bottom of the tree. You can reduce this error by following the instructions in the app closely. You can also help by taking multiple measurements of the same tree.

 

Recommendation 3: Measure Prominent Trees and Maximize Location Accuracy

Measure prominent or easily identifiable trees such as trees that stand above surrounding trees and refresh the location screen to reduce the location uncertainty to 2.5 meters or less. This will help scientists match your height measurement to a single point in the satellite data.

You can maximize the accuracy of your GLOBE Observer Tree observation two ways. First, click the reset arrow to the right of the accuracy bar beneath the latitude and longitude until the estimated accuracy is within 2.5 meters or less. Second, you can move the pin in the location map to the tree you are measuring. 

 

Recommendation 4: Measure Multiple Trees of Similar Heights in the Same Area

If you can’t measure the location to within 2.5 meters, then measure multiple trees of similar height in the same area so that all of the trees together can be compared to the satellite data.

 

To read the full paper, please see:

Enterkine, J., Campbell, B.A., Kohl, H., Glenn, N.F., Weaver, K., Overoye, D., and Danke, D. (2022). The potential of citizen science data to complement satellite and airborne lidar tree height measurements: lessons from The GLOBE Program. Environmental Research Letters, 17 (7). https://doi.org/10.1088/1748-9326/ac77a2

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