Part 3—Highlight and Measure the Ozone Hole
Step 1 Highlight and Outline the Ozone Hole
Scientists who study the atmosphere consider areas with less than 220 Dobson Units (DU) of ozone to be part of the ozone hole. The TOMS images show ozone in increments of 25 DU, so we'll count pixels (picture elements) with 225 DU or fewer to estimate the area of the hole. In this step, you'll highlight and automatically outline the pixels with these values to measure the ozone hole in each yearly image.
- If you made any changes to the images during your exploration, you may need to close the stack (don't save any changes) and reopen it. Within ImageJ, choose File > Open... and navigate to the location where you stored the stack.
- To remove the continent outlines that can interfere with the highlighting, select Process > Noise > Despeckle. When queried to process all the slices, choose Yes.
- Select Image > Adjust > Threshold... Thresholding allows you to specify a range of pixel values (the ones that represent the ozone hole, for instance) and highlight them in the image.
- In the Threshold dialog box, below the two horizontal sliders, set the drop down menu on the right to "Red."
- Drag the top and bottom sliders (or use the arrow buttons to move them) so that pixels representing ozone measurements of 225 and lower are red and pixels of higher ozone levels remain unchanged. Close the threshold window.
- Choose Edit > Options > Wand tool... in the dialog box that opens check the checkbox to Enable Thresholding and close the window.
- From ImageJ's toolbar, select the wand (tracing) tool (it looks like a small magnifying glass) and click it just to the left of the group of red pixels on the image that represent the ozone hole. This will automatically trace a yellow selection line around the red pixels. It may take several tries to get the line to draw.
Step 2 Measure Area of Ozone Hole
- Select Analyze > Measure.
- The area (in pixels) you selected and measured is displayed in the Results window. To bring the Results window forward, select Windows > Results.
- Use the slider button along the bottom of the stack window to advance to the next image. Or click the greater than (>) or less than (<) keys on your keyboard.
- On the next image, click the wand tool to the left of the ozone hole again to trace the new outline.
- Choose Analyze > Measure to record each measurement to the Results window.
- Repeat the previous 3 steps until you have measured the area of the ozone hole for all nine years.
- When you're done, the Results window should have 10 measurements corresponding to the years 1996 through 2005.
- Save the Results file by choosing Windows > Results, then choose File > Save As... and go with the default name of Results.xls. Store the Results file in the same place as your TOMS images.
NOTE: If you record an incorrect measurement, try selecting it in the Results window then choose Edit > Delete. If you cannot delete single measurements, you can choose Analyze > Clear Results to start over, or you can delete bad measurements once you bring the results in to your spreadsheet application.