3.3   A stretch that enhances land and sea

This section shows how you may enhance both land and sea areas using a manual saw-tooth stretch. From the previous section you should have two virtually identical copies open of the btemp_nadir_1100 band of the image ATS_TOA_1COLRA20040201~.N1. One was loaded with a linear Redisplay stretch with a range of 28400 - 31900 and further enhanced with a manual stretch with a knee at [69,255], the other with a linear Redisplay stretch range of 28400 - 29350, and no further enhancement.

Because it was loaded with a range that includes land temperatures, the first copy is the one you will work with for this exercise. At present the land appears white, but this is because of the manual stretch applied after loading the data, and the application of a different stretch can reveal thermal structure over land. You can identify this image by the colour bar underneath the display, which shows all the grey-tones compressed into a range from 0 to 69. Minimise the other copy for the time being.

Clearing a stretch

Before continuing, it is worth take a histogram that represents the frequency distribution of this image as it was after loading but before the application of the manual stretch. To do this you must first clear the stretch. As usual there is more than one way of doing this.

From the menu bar:
  - open the Stretch menu and select Clear from the drop down.
Keyboard shortcuts:
  - type [ALT + S] together, followed by [C] for clear.

Having cleared the stretch you can now take the histogram in the usual way, but remember to check the Apply Stretches check box. You will refer to this histogram from time to time during the following activities.

Note: To use the histogram for preparing a manual stretch of 16-bit data you need the display values used when loading the image rather than the data values in the image file. The reason for this was explained in section 3.2.

A saw-tooth stretch

Because the thermal range of land and ocean pictures are so clearly separated in this instance, you can find a stretch that makes it possible to display both the land pixel and the ocean pixels optimally at the same time. The way to do this is through a saw tooth stretch

1. Activate the image from which you have just obtained the histogram.
2. Open an new stretch document ( [ALT+S] > [M] ), double-click to create a knee, and drag it to the [x,y] position [69,255].
3. Double-click on the stretch line to add a second knee to the right of the first.
4. Drag the second knee to the position [70,0] and watch how the display of the land changes.