Measuring the Depth of Meteor Crater and the Height of its Rim
A classic image analysis technique is to use shadow lengths to determine vertical dimensions. In planetary science, shadow lengths are often used to determine the depths of impact craters (on the Moon, Mars, and elsewhere) and the heights of their rims.
The principles of the method are:
The same method can be applied to Barringer Meteorite Crater (aka Meteor Crater) using an image the astronauts captured from the International Space Station on March 3, 2014 from an altitude of 221 nautical miles. To conduct this exercise, we will use ImageJ, which is a public domain Java image processing program that was inspired by NIH Image.
The instructions that follow are for the PC version of ImageJ. The program works on MAC platforms, too, although there may be minor variations in how the menu bar looks and how the mouse works.
1. Download and install ImageJ on your computer.
2. Right click on Barringer_iss038e067508.jpg and save it to your computer.
3. Run ImageJ. From the ImageJ menu bar, select "File" from the pull-down menu and select "Open." Select "Barringer_iss038e067508.jpg" from the path where you save the image. The image should open in a new window.
The field of view – getting oriented
Barringer Meteorite Crater (aka Meteor Crater) appears in the right center of the image. It is approximately 1.25 km in diameter. The Canyon Diablo cuts across the landscape along the bottom of the image. The canyon is west of the crater. Interstate 40 cuts across the far left side of the image and runs roughly east-west.
Set scale of image
There is a gravel road a few hundred meters west of the crater. If you zoom in (Ctrl +), you will see smaller dirt roads intersecting the gravel road. Two intersections are marked “A” and “B.”
From the menu bar, select the line tool icon. If it is not already preselected for “Straight Line,” select “Straight Line.”
Place the cursor at the intersection marked “A,” click on that point, and then move the cursor along the road to the intersection marked “B” and click again. The pixel-length will appear along the base of the menu bar.
From the menu bar, select “Analyze” and then “Set Scale” from the pull-down menu. The distance in pixels measured from A to B should automatically appear in that box. If it does not, you can manually enter the number you measured from A to B. In the “Known distance” box, insert the value “1.322.” In the “Unit of length” box, insert “km.” The scale in units of pixels/km should then appear above the button “OK.” Select the button “OK.”
As illustrated above, the method depends on knowledge of the sun angle. In this case, the sun angle relative to the surface (or sun elevation) was 17 degrees.
The azimuth of the sun is also important, because that indicates the direction along which shadows are cast on the ground. In this case, the azimuth of the sun was 249 degrees as measured clockwise from the north.
To determine the azimuth of the sun on the image, select the angle tool icon. This tool works in 180 degree swaths (rather than a full 360 degree circle). Thus, one needs to first subtract 180 degrees from 249 degrees to obtain the angle (69 degrees) needed for the tool.
We will use the road between “A” and “B” as a reference line. That road is oriented 3.5 degrees east (clockwise) of north. Thus, we need to subtract 3.5 degrees from 69 degrees to obtain the final angle (65.5 degrees) we will use with the angle tool.
Using the angle tool, click on the intersection at “B.” Then move the cursor to the intersection at “A” and click again. Finally, move the cursor towards the west (towards the sun). Adjust the position of the cursor (left or right) until the angle along the base of the menu bar reads 65.5 degrees. Click again. You now have a line that is oriented along the azimuth of the sun. Shadows will fall parallel to that line towards the east. If you like, you can swing the angle tool 180 degrees to create a shadow fall line in the direction of the crater.
Use the pencil tool to draw one or more lines parallel to that shadow fall line. In the next step of the exercise, the angle tool will disappear, so these lines will be needed to help guide shadow measurements.
Measuring for crater depth
Select the line tool icon from the menu bar. Begin a line on the west rim of the crater where a shadow begins and draw the tool opposite the sun’s azimuth (i.e., parallel to the lines established in the preceding step) until you reach the edge of the shadow on the crater floor. The length of that shadow will appear along the base of the menu bar. Because the scale was set in units of km/pixel, the length of the shadow will be in units of km. Record that number.
Repeat the measurement at several locations from the west rim of the crater and tabulate the data.
Using the trigonometric expression in the illustration box above, use those shadow lengths and a sun elevation angle of 17 degrees to calculate crater depth.
Discuss the results. Is the calculated depth the same at each location? If not, why not? Crater depth/diameter ratios for simple bowl-shaped craters are often about 0.20. Is that the value you obtained? If not — and assuming your measurements and calculations are correct — what might that imply?
Measuring for crater rim height
Select the pencil tool from the menu bar. Begin a line on the east rim of the crater where a shadow begins and then draw the tool opposite the sun’s azimuth (i.e., parallel to the lines established previously) until you reach the edge of the shadow on the slope around the crater. The length of the shadow will appear in units of km along the base of the menu bar.
Repeat the measurement at several locations from the east rim of the crater and tabulate the data.
Using the trigonometric expression in the illustration box above, plug in those shadow lengths and a sun elevation angle of 17 degrees to calculate crater rim height.
Discuss the results. Is the calculated rim height the same at each location? If not, what does that imply?
Finally, use both the crater depth measurements from the previous section and the rim height data in this section to determine the depth of the crater beneath the level of the landscape that surrounds the crater.
For those who are interested:
A complete inventory of ISS and STS images can be found at The Gateway to Astronaut Photography of Earth.
Finally, we thank the crew of Expedition 38 and the NASA Johnson Space Center’s Earth Science and Remote Sensing Unit for capturing the image used in this laboratory exercise.
The LPI-JSC Center for Lunar Science and Exploration is a member of
NASA’s Solar System Exploration Research Virtual Institute.