Hurricane Opal Project Activity

These Links Are To Tutorial Activities

Remote Sensing / Digital Numbers / Contrast / Brightness / Histograms / Measuring / Density Slicing / Particle Analysis / Density Calibration / DEMs / Elevation Calibration / Animation  / Tutorial Site Map

Email: proof@proofofconcepts.com

Measuring Distances & Areas
You may print these instructions before starting this activity.

This activity will explain how to measure a straight line distance on a satellite image or any other image. Each pixel on the image is 16 miles square (4 miles x 4 miles). You will use the straight line measuring tool (highlighted in the image-clicking and holding the mouse button will display the freehand and segmented line measuring tools) to measure the distance from the southern tip of Lake Okeechobee to the tip of the land just to the west of the Appalachicola River. Create a download folder for this workshop and download and save the satellite image of Florida (image.tif.sit) in that folder. Once the image has been downloaded, open it in the NIH image application program

In order to do any effective measuring of digital images you need to learn how to set the scale for the images. Before determining the scale for any image choose Analyze/Reset from the main menu bar to clear any previous measurements. Click on the straight line selection/measuring tool to activate it. Move the cursor to the image and click, hold, and drag the mouse cross hair from the southern tip of Lake Okeechobee (large lake in South Florida POINT B) to the west of the Appalachicola River ( large tip of land in Northwest Florida, POINT A). When the mouse is released, the line should turn into what is commonly known as the "marching ants."

Now choose the Analyze/Measure menu., then the Analyze/Set scale menu. Since I stated that one pixel is equal to 4 miles, enough information is available to assign a scale to the image. Enter "1" for Measure Distance and "4" for Known Distance. Choose miles from the units pop-up menu and type in the number of miles for Known Distance. Click OK. Now you can determine the distance from any two points on this image.

If the image you have has a set scale already embedded within it, then

  1. drag the straight line tool along the known distance
  2. click on the analyze menu and set scale
  3. click the units from the Units pop-up menu
  4. enter the known distance (from the scale or the known distance)

However, if you know that "one pixel equals a set distance" (4 miles in this satellite image) you don't need to do the measurement. In the Set Scale dialog box just type in the number of pixels in the Measure Distance box (1), the Known Distance (4), and scroll to miles in the Units box.

Use the straight line tool to select and redraw the line from the southern tip of Lake Okeechobee (pt B) to the tip of Appalachicola, just to the west of the river (pt A) Choose Analyze/Measure then Analyze/Show Results. Make sure the results window shows numbers in the area, mean, and length columns. If there are zeros, then you must either re-measure the distance or reset the scale. You can also see your latest measurements in the Info window of the NIH Program

You have been working with a straight line tool. However if you click and hold on the straight line tool another menu will pop up. This new menu will show two new draw lines, freehand and segmented. Select one of the new draw lines (preferably freehand) and once again open the Florida image. Click and drag this new line around the image until you have an enclosed area ("march ants") will appear. Once again follow the measure procedure and click on show results. Area calculation now shows the area measurement within the enclosed area. You can do this with any of the selection tools (square, circle, polygon, and heart). To make perfect circles or squares, hold down the shift key while you drag and release those selection tools.

Calculate the perimeter and area of Lake Okeechobee. You can check your calculations by referencing several different geography books. You can even try your Internet search prowess and find the answer there. For practice, measure the lengths (works best with the freehand or segmented line measuring tool) of a few of the rivers seen in the image, the Florida coast line, the area of Tampa Bay, etc. You will need these skills for future work.