Measure surface reflectance and albedo with smartphone apps
Summary
Using smartphones, students can measure the reflectance and albedo of water and other earth surfaces in the field. This is a great activity to reinforce the central concepts used in remote sensing. The smartphone app takes measurements in the red, green, and blue channels of the camera, and orients the photographer to the correct angles using the internal GPS, clock, gyroscope, and compass inside the smartphone. For an advanced activity, conduct the smartphone measurements within three hours of a satellite overpass and compare the results. This activity can be done remotely for an online course if students can each access a suitable field location to take the measurements.
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Context
Audience
Undergraduate course in ocean remote sensing.
Could be adapted for: any undergraduate course that involves remote sensing of land, ice, or water.
Skills and concepts that students must have mastered
Students need to have very a basic understanding of Remote Sensing Reflectance (Rrs) or Surface Reflectance as measured by satellites, drones, aircraft, or other remote sensing platforms.
How the activity is situated in the course
This activity fits well into a single lesson or week-long unit on how satellite measure earth. It helps reinforce the geometry of measuring an earth surface from a separate platform - in this case, the smartphone is the platform.
Goals
Content/concepts goals for this activity
Students will use a smartphone to measure the reflectance of earth surfaces in the field.
Students will use the correct zenith and azimuth angles to collect reflectance measurements with the smartphone as a model remote sensing platform (in the same manner as aircraft, satellite, or drone).
Higher order thinking skills goals for this activity
Hands-on data collection, geometry.
Other skills goals for this activity
Description and Teaching Materials
Have you ever wondered how to cheaply and easily measure the reflectance of a water body or the surface of the earth? Do you want to compare the albedo of snow, grass, sand, water, and pavement? Using two smartphone apps, Hydrocolor and Albedo, students can measure surface reflectance and albedo with any smartphone device. Please see the attached documents for background information and instructions. This activity works well in any course that involves remote sensing.
Directions for using smartphone apps Hydrocolor and Albedo (Acrobat (PDF) 1.5MB Nov23 20)
Background Information on the Hydrocolor App (Acrobat (PDF) 2.7MB Nov23 20)
Teaching Notes and Tips
REQUIRED MATERIALS: photographer's 18% grey cards are required for using the smartphone apps. They serve as light calibration standards so that students can compare measurements amongst themselves. Photographer's grey cards can be purchased online at low cost.
Assessment
In the past, assessment consisted of a homework assignment to write one paragraph about the field site, including its general characteristics (metadata) and a summary of the measurement results (reflectance values in the red, green, and blue channels).
Another form of assessment would be to ask students to teach one another how to use the apps, including how the viewing angles are designed to collect the best possible measurement.
References and Resources
References:
Leeuw, T., Boss, E., 2018. The HydroColor app: Above water measurements of remote sensing reflectance and turbidity using a smartphone camera. Sensors 18. https://doi.org/10.3390/s18010256
Yang, Y., Cowen, L.L.E., Costa, M., 2018. Is ocean reflectance acquired by citizen scientists robust for science applications? Remote Sens. 10, 1–18. https://doi.org/10.3390/rs10060835
MISC Lab, University of Maine, 2014. "HydroColor" Webpage. Maine In-situ Sound and Color Lab, School of Marine Sciences. http://misclab.umeoce.maine.edu/research/HydroColor.php