for high school biology and environmental science
Part 2: Examining Orthophotos
An orthophoto is a photograph of the earth's surface in which geographic distortion (due to the curve of the earth, camera tilt and other factors) has been removed. The photographs you will examine here were taken on the Centennial Campus of North Carolina State University in Raleigh, North Carolina. Many counties provide orthophotos online through their web sites if you would like to investigate areas where you live.
First, let's look carefully at the orthophotos of the kudzu infested area.
Look carefully at the orthophoto taken in 1999 [as a JPG or PNG file, opens in a new window]. What manmade objects can you identify? What about natural landmarks?
Now look at the orthophoto taken in 2002 [as a JPG or PNG file, opens in a new window]. Can you locate the same manmade and natural landmarks? What has changed in the 3 years since the first set of photos was taken?
Can you locate the kudzu in both photos?
You can use the ImageJ program to better visualize the area of kudzu in the photo and estimate the area the kudzu covers.
In this program, each pixel of the image is assigned a value from 0 to 256. As the image is now, each of those values corresponds to a different color gray. You can use the magnifying glass tool to increase the size of the photo until you see the individual pixels, or squares of color. (To reduce the size of the photo, hold down the Ctrl button while selecting the magnifying glass.) Now you need to manipulate the colors that correspond to each pixel value as well as range of pixel values you can see so that the kudzu is better visualized.
Open the ImageJ program and then open the 2002.png file. This is the black and white orthophoto of the area in question taken in 2002.
First you need to set the scale of the photo so the program will know how to make a measurement. Notice the scale in the upper left hand corner of the photo. The length of the line corresponds to 116 feet. Choose the line draw tool and draw a line over the scale of exactly the same length.
On the menu at the top of the ImageJ window choose Analyze and Set Scale. Enter 116 in the known distance field. Enter feet as the unit of length and check the Global box so that you do not need to set the scale for other graphics while the program is open. However, you will need to reset the scale each time you open the ImageJ program.
Now you can adjust the pixel values to better visualize the kudzu. On the menu at the top of the ImageJ window choose Image, Type and 8 bit.
You can now change the Look Up Table (LUT) which assigns colors to each of the pixel values. The current look up table is Gray Scale. Choose Look Up Tables under the Image menu and select Fire.
What color do the darker grays now correspond to? How about the lighter grays?
What color does the kudzu area now appear?
You can see all of the colors in the LUT by going to the Image menu and selecting Color and Edit LUT or Show LUT.
To better visualize the kudzu in the photo, you can adjust the minimum and maximum pixel values that are shown in the photo. In the Image menu select Adjust and then Brightness & Contrast.
At the top of the window that opens is a graph that shows how many pixels in the photo correspond to each pixel value. The taller the line lines denote more pixels in that color. You may notice that the right side of the graph has almost no pixels.
Adjust the Maximum slider to the left so that the end marker of the graphs excludes these values. How does your graph change? Adjust the Minimum slider similarly so that the kudzu is best visualized in the photo.
Feel free to play around with the LUT, Brightness and Contrast to create a version of the orthophoto that best visualizes the kudzu. Be sure to save your image.
Finally, you need to measure the area that the kudzu covers using your modified orthophoto.
Select the Freehand Selections tool in ImageJ and carefully outline the area covered with kudzu.
If you make a mistake go to Edit in the ImageJ window and choose Selections, Select None to undo you outline.
Once the area is outlined, go to Analyze and Measure. Record the area covered by kudzu in 1999. Think carefully about how man significant figures you should record!
Repeat the process for the 2002 photo.
Be sure to save your modified photos and record the area covered by kudzu in 1999 and 2002.
How did the infested area change between 1999 and 2002?
Continue to Part 3
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