Measuring Distance and Area in Satellite Images (College Level)
and is replicated here as part of the SERC Pedagogic Service.
This example is derived from the chapter Measuring Distance and Area in Satellite Images in the Earth Exploration Toolbook. Satellite imagery is used to map changes to the Earth's surface or atmosphere over time using free image analysis software, ImageJ. The exercise was originally written for secondary school standards, but can be adapted easily for introductory college level geoscience courses by letting the students do more of the work outside the classroom or as a classroom exercise/demonstration to stimulate discussion about Earth system processes.
- download, install, and use ImageJ image analysis software to quantify change over time in satellite images
- access and download satellite images for specific locations and times
- set a scale (spatial calibration) for an image in ImageJ
- select and measure distances and areas on images in ImageJ
These are GIS/remote sensing skills useful in many aspects of geoscience research.
Context for Use
People often have trouble interpreting features in satellite images because they are unfamiliar with the scale at which they are viewing the features. The ability to measure these features and see the results in real-world units gives users a way to check their actual size, assisting them in the task of interpreting what those features are.
This technique can also be used to quantify the study of satellite images. For images that show change over time, the amount of area that changed between successive satellite images can be measured, graphed, and analyzed. Beacuse the dates of the satellite images are known, rates of change can also be calculated.
Satellite images are composed of a rectangular array of pixels, or picture elements. Each pixel represents a single measurement made by a satellite instrument, and each measurement corresponds to a specific amount of area on Earth's surface or in the atmosphere. Though we are limited to measuring across pixels on a satellite image, knowing exactly how much area is represented by each pixel allows the image analysis program to convert from pixels to real-world units. To set the spatial calibration or scale of an image, users enter the desired units of measure and a conversion factor that indicates the distance on the ground represented by each pixel in the image.
Description and Teaching Materials
Earth Exploration Toolbook: Measuring Distance and Area in Satellite Images - this is the original comprehensive description of this exercise with links to all software and data resources.
Description of ImageJ, freeware image analysis software, and image data sources provided in the EET chapter.
What is Excel? - a tutorial on how to use Excel to analyze and plot data
Teaching Notes and Tips
The EET chapter is aimed at middle and high school levels, but the techniques and concepts of the exercise are certainly appropriate for introductory geoscience courses. The instructions are comprehensive and adequate for students to follow on their own outside of the classroom (e.g. a homework or project assignment).
ImageJ is freeware and available for nearly all operating systems (written in Java). Students need to know how to download materials from the web and have permission to install the software on their computer (in lab, classroom, or personal computer).
Students may be intimidated by the computer requirements of the exercise and may require hands-on time in a classroom/lab before they will be comfortable working outside the classroom. Students may also have concerns about the precision (repeatability) of the exercise since they are likely to get slightly different results depending on how they trace the features of interest. It is up to the instructor to decide how to classify the edges of the features to obtain the desired analysis results.
The recommended image data site is quite simple compared to other data sites. While the EET exercise is written for specific images of the Aral Sea, other changes in the Earth surface could be analyzed:
- Track the movement of the large Antarctic iceberg C-16 (search for "C-16" on the MODIS website).
- Calculate the area of the iceberg for a number of images and create a plot in Excel showing the change in area over time.
- Find the coordinates for the center of the iceberg in each image. Calculate the distance traveled between each pair of images and create a plot in Excel. What is the average velocity?
- The "Image of the Day" examples often show changes over time of a variety of phenomena (volcanos, dust storms, hurricanes, etc.) that can be tracked through time. Use the terminology of the example pages to search for multiple images of the same feature through time. Specify which aspects of the images should be analyzed by the students and what parameters should be measured.
- Have the students choose an atmospheric or Earth surface feature to analyze through a time-sequence of images. This could be a short project.
- The "Going Further" page of the EET chapter has more suggestions and additional image data sources
Report of the analysis including imagery, traced areas of interest, a table of observations/measurements/calculations. Assessment can be based on the accuracy of the results and how well the students have mastered the image analysis methods.