Ratioing and Principal Components

  1. For ratios, let us first try the pair: B4 and B2. To see the ratio image, start with View, then place the cursor on Display Image Control Window and click, which brings up the Thumbnail Image Control window. Some band is entered in the Expression box. You cannot use the black triangle efficiently to enter the ratio, so delete with the cursor the band present (unless it happens to be B4) and in the blank type in B4/B2 (remember, ratioing is division of one band pixel DNs by those of another band, and / is the division operator). Press Apply. The result is a black image. Your first thought is to adjust C and B. Move C to its rightmost position (*8.00) and B to +100. A faint image appears but is hard to work with. Conclusion: not enough range for C. For ratio images, this range needs expansion. Go to the Settings button and move the cursor onto Contrast Range, which becomes highlighted and causes a window to appear to its right. There are a number of different values; the default one is *8.00. Change this to *256.0. This will cause the image control window to automatically close. Reopen it as described above. Now, redo the ratio process, with these new settings. Move the C button to *70 and set the B button to + 50, and Apply. A much improved image is the result. Now, try the ration B4/B1 and adjust the C and B buttons until you like what you see. *100 and + 40 is OK; you might find better values. Try different ratios and strive to optimize the gray tone expressions. How well can you render B6/B1? Rather weird, eh! But with some meaning. Note the relative "warmth" of the sea versus the sand dunes; the vegetation versus the town.
  2. Making color composites just requires hitting the RGB button on the Thumbnail Image Control button. The new window has the red, green, and blue subwindows as is customary. But the last single ratio value you looked at fills all three Expression boxes. You will need to type in the specific ratios you seek into each box. Then, press Apply. The resulting color image may be garish or gorgeous, depending largely on the C and B values set at entry. Try different values for each ratio pair, each time hitting Apply. Generally, you will find some narrow sets of values that give a pleasing result. Some combos will reveal more than others. Certain end results will probably call attention to particular image features whereas others won't. Experiment. Try different ratios or inverting the bands in a given ratio; also switch ratios and colors. As an example, put B4/B2 in the red box; B7/B5 in the green box, and B1/B3 in the blue. Check with the Ratio discussion in Section 1 for hints as to interpretation.
  3. Making PCI images is easy. Assuming all bands are entered and active, and you are in PIT (Thumbnail), press on the Image button and then click on the PCA option in the window that drops down. A new window appears that has Create PCI as a command. When you click on it, a large window like the one that appeared as you entered the 7 TM bands, except B1 through B7 is replaced by PC1 through PC7, is displayed. Move your mouse cursor to the Box PC1 and click on Browse. A window that has many directories listed on the right probably doesn't include "PITimages". Click on Parent Directory button and scroll down the listing until PITimages appears. When you click on it, in the box near the bottom labeled Directory, this will appear "//PITimages". Below it is a blank box marked File. Type in a file name with extension ".pc1". Here, we will choose "Israel1.pc1". Pressing Enter returns you to the 7 PC boxes. Place the cursor in PC1, hit Browse, and repeat the process, this time entering "Israel2.pc2" in the File Box. Do this again for as many Principal Components as you wish, in this case we will proceed through "Israel6.pc6. Then, at the bottom of the 7 list, hit Enter. At the lower left you will see a rapidly moving PC statement giving the percentage of processing at the moment. When this is done, you have formed the PCIs.
  4. To see the various PCI images, from the PIT window, click on View and then on Display Image Control Window, which raises the Thumbnail Image Control window. Click on the black triangle button. A list showing the 7 Bands appears, but also there is a list below with PC1 through PC6. Click on any one and it will be entered into the Expression box, and displayed by clicking on Apply. Change C and B as you please, to get a well-balanced set of gray tones. To see any other PCI, simply hit the triangle and choose the new PCI image. For our Israel case, PC1 is most like the band images, and is closest to Band 3 but initially darker (can adjust with C and B), PC2 emphasizes vegetation (bright), PC3 is largely medium to bright gray (note the interesting ellipse around the airport - probably a road associated with a fence if this is a military field), PC4 and PC6 contain little contrast and are probably low on information, whereas PC5 does have meaningful tonal patterns that seek interpretation.
  5. PCI color composites can present images that have additional information over the single PCI image. While in the Thumbnail Image Control window, click on the RGB button, and the familiar three color outline boxes appear. Using the black triangle by each box, enter the particular PCI image to associate with that color. To display, press Apply. Start with R = PC1, G = PC2, and B = PC3, display through Apply. If the color balance is not satisfactory, repeat, moving the C and B buttons (intuitively) to new values. Keep trying until you find a proper balance (not too much of one color). To acquaint you with the possibilities, try various combinations of PC1, 2, 3, and 5, discarding PC4 and PC6 (or try them if you are curious.)
  6. As mentioned in Section 1, interpreting PCA products is almost an art. You may learn little more than from natural or false color displays. But, at times certain features that have been missed in those displays may be revealed. Principal Components images have the advantage over band images in that the process of calculating the PCs leads to decorrelation between bands. Recall that the first PC contains most of the spectral information inherent to the scene; proceeding through higher numbered PCs produces images representing progressively less such information. Note that PCI images can be substituted for spectral band images in the classification of imagery.


Primary Author: Nicholas M. Short, Sr. email:

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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