Ratioing and Principal Components
- 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 Contrast (C)
and/or Brightness (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.
- 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.
- The resulting ratio color composite may or may not be pleasing to the eye and may not have optimum information content. As you no doubt already know from earlier experience in making a color image, the settings of B and C may be critical. We suggest you experiment with different settings of either or both of B and C until you find one that is satisfying. Keep a record of the B and C values, and then try several more combinations.
- 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 three more times until the list headed by AUTOEXEC is shown and scroll down the listings
until "PITimages" appears. When you click on it, in the box near the bottom
labeled Directory, this will appear "./../../../PITimages" (possibly a variant will occur in your system). Below it is a blank box
marked File. Type in a file name with extension ".pc1". Here, we will choose
"Israel1.pc1". Pressing the keyboard 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 "Create". 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.
- 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 YM 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
- 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.)
- As mentioned in Section 1 (page 1-14), 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. For information on the theory of Principal Components Analysis, consult Appendix C.
- You may wish to leave these images and perform another task. To do so, just go to the main menu (top buttons) after closing out any windows relevant to the PC creation. The default image B1 (in this case ISRAEL.1) should appear (if not, then recover in the usual way of clicking on the Image button on the main menu, then on Open. Be advised that the .pcx files you have made will remain in the PITimages folder as additions to the basic files included in that folder. Even after leaving PIT, the next time you start PIT and proceed through the image opening process, you will see the .pcx files listed in addition to PITimages. You might choose to keep them, but many users prefer to eliminate those produced in a training session. There is no Delete command in PIT and your keyboard Delete button doesn't eliminate a highlighted .pcx file. To remove these or any unwanted files you create, you must return to the PITimages folder that is included in the folders for each Section accessed from the CD-ROM (or over the Internet); highlight each folder to be discarded and hit the Delete button. On the other hand, you might desire to retain the PC files. To find them, from the top menu, choose Image-->PCA-->OpenPCI-->Window headed by Enter PCI paths, click on first Browse button and through Parent Directory until AUTOEXEC list comes up. Scroll to PITimages, highlight, double click and the .pcx images will be displayed along with PITimage (the entry to the basic image files). Click on PC1 or the lowest value of x. This enters that file into the first space; repeat sequentially for others you choose to load.
Nicholas M. Short, Sr.
Jeff Love, PIT Developer (email@example.com