Using the Freehand Selection Tool

The amount of ice surrounding Antarctica varies with the seasons as ice melts in the spring and forms in the fall, but it also experiences annual fluctuations analogous to variations in snowfall. The extent and duration of ice cover affects marine and animal life, navigation, and weather. Satellite imagery from the radar range of the electromagnetic spectrum measures the extent of ice cover.

This activity uses the Area Selection Tools, which allow the user to select parts of an image by drawing a rectangle, box, oval, or freehand shape. The Freehand Selection Tool will be used for this activity, since an irregularly shaped area can be selected. For more geometric shapes, the Rectangle, Oval or Trapezoid Selection Tools are available.

The satellite images used for this activity depict the changes in sea ice cover surrounding Antarctica on an annual basis. The images are available from the United States Geological Survey WWW site.

Image Software

NIH Image is a public domain image processing and analysis program for the Macintosh. A free PC version of Image, called Scion Image for Windows, is available from Scion Corporation. There is also Image/J, a Java program inspired by Image that "runs anywhere".

For more information and ideas on using Image in the classroom, see Classroom Imagery.

  1. Select the image from October, 1979. Set the scale for the image by using theLine Selection Tool as follows:
  • Point the cursor at one point on the image, click and hold the mouse button down. Move to the second point and release the mouse button. Using an atlas, determine the distance between the points on the image. ChooseAnalyze/Set Scale, set the units, and enter the known distance in the box labeled Known Distance.
  1. Use the LUT tool to measure the area of continental Antarctica. SelectOptions/Density Slice or double-click on the LUT tool to start. A red bar will appear over part of the LUT that represents the range of values selected. Click and drag the edges of the red bar with the LUT tool to expand or shrink the range selected so only continental Antarctica is selected.

To help determine which particles are measured, check off Label Particles in theAnalyze/Analyze Particles... dialog box. To see the results, chooseAnalyze/Show Results and write down the number given for the area of Antarctica.

If too many small particles are chosen that are not important, you can adjust theMinimum Particle Size in the Analyze/Analyze Particles dialog box. By increasing the minimum particle size, the southern tip of South America and the smaller islands in the vicinity are eliminated.

  1. Select the Freehand Selection Tool and trace the outer boundaries of ice cover surrounding Antarctica. Choose Analyze/Measure and the extent of the selected area will be displayed in the Results box. Subtracting the area of Antarctica from the area of the surrounding ice cover will give the extent of ice cover in 1979.
  2. Repeat the above steps for images after 1979. Plot the result to see how the amount of ice cover surrounding Antarctica changes on an annual basis.