Calibrated DEMs
You may print these instructions before starting this activity.

The DEMs like the one shown on the DEM page can be found at different government web sites. They are in a GIF or JPEG format. There are others in a different format but are unusable at this time for our purposes.

These GIF/JPEG images come uncalibrated, so you will have to do the calibration. If you read the DEM page, you learned that in DEM images pixel values are related to elevation. Download the Mobile Area (mobil2.tif.sit) DEM into your folder and open it in NIH Image. The following procedure will allow you to calibrate any DEM of this type. There is one caveat. To be completely accurate in the elevations, you must be able to match exact points on your DEM with the corresponding point on a topographic map (a near impossibility), but there are programs and USGS data sets that will provide this information. However the procedure which follows is the method to use, whether you guess at high/low elevations or you know for sure:

1. The method will allow you to determine the highest and lowest pixel value in the image. Then calibrate the pixel values to the lowest (high pixel value) and highest (low pixel value) elevation.
2. Click on Analyze/Show Histogram. The histogram window appears which shows the range of pixel values in the image and the quantity of pixels with that value. When you move your mouse pointer across the window the level and count changes in the Info window. This Info window shows that the histogram of Mobile, Alabama has pixels with a value of "114". That number represents a high point in the image, it has a low DN value. Since Mobile is on the coast of Alabama, then the highest pixel value must be sea level, and therefore must represent an elevation of zero (0) feet. The pixel value in this case is 255. However the Scion software will not read 255 so we will use 254. There are many pixels having this value.
3. Looking at a topographic map of the area I learned that the high points with a DN of 114 is at 363 feet and the lowest is 0 feet. I can now calibrate the image.
4. Click on Analyze/Reset to clear an measurements left from a previous activity.
5. You will have to do a density slice with the upper and lower range being equal in order to find the point/points on the image with a pixel value of 114. You will also have to magnify the image in order to see them. Use the "hand" in the tool box to move the image around. Do the same for the sea level (255 pixel value).
6. Use the straight line tool to measure the pixel values at these points. You must move the cross hairs around the image to find these two points (do not drag the tool). Once found, click once and then click on Analyze/Measure. Repeat the process for each point.
7. Click on Analyze/Calibrate when finished measuring different points on the image.
8. A pop-up window will be produced showing the measured points. If zeros (0) are shown in this window, you may have to close the image and repeat this procedure.
9. Once all of the measurements have been taken (you decide how many to take), type in the Known quantity for each measured point. Also place a check mark in the radio button of straight line and for the Unit of Measure, type in feet - click OK.
10. A new pop-up window occurs showing the equation and straight line relationship of the points.

You are now ready to analyze the Mobile Area DEM.

1. Click and drag the profile density tool across the image. Release at the end point.
2. The plot window appears giving you the elevation of each pixel along the line.
3. You can also drag the plot window to underneath the plot line to show the relationships between the density profile line on the image and the plot line.

Continue to experiment with different functions to learn and appreciate the amount of information you can obtain from manipulating and analyzing this image. Try surface plots of different areas to learn about the drainage basin of the Mobile area. You may also want to produce your own contour maps of the area (procedures for an advance class).

For practice, and to make certain that you understand how to accomplish an elevation calibration, calculated elevations for the points listed below. Separate your answer sets by a slash, i.e. 444,137 -5.5 meters / 263,320 - 1.2 meters, etc.