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3. Stretches
Images used:
Description of mer_20040201 _algal1.dat
Download mer_20040201 _algal1.dat
(83 K)
Description of ATS_TOA_1C_ 20040201~.N1
Download ATS_TOA_1C_ 20040201~.N1
(7.7 MB)
Useful information:
Numerical data formats
Null values and missing data
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Sometimes you will want to save an image as you see it displayed on the computer screen, so that you can use it in reports
or presentations prepared in software packages other than Bilko. This is easy to do, but does require that you first
create a new image with the stretch applied. If you simply save the image as it is, you may get
a few surprises when you next open the saved file. The activities below should help you understand the reasons for this.
Saving the display of 8-bit data
Activity / Question 1
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Activate the MERIS image and apply a manual stretch with knees at [60,0] and [190,255]. Save the image as
image1.gif, remembering to choose CompuServe.gif as the image type.
Now select the whole image ( [CTRL+A] ) and use the selection to create a new image
( [CTRL+N] > [I] ). Save this as image2.gif
Close both images, re-open
mer_rr_20040201_082314_algal1.dat
and apply re-apply the manual stretch (you can do this by simply clicking on one of the knees in the strech document).
Finally re-open the two GIF images you just saved.
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| a) |
What are the main differences between image1.gif and image2.gif? Which of these two is most like the stretched
display of mer_rr_20040201_082314_algal1.dat?
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| b) |
Take a histogram of each image with the Apply Stretches box unchecked.
How do these histograms differ?
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| c) |
Which method of saving would you use if you wanted to preserve the geophysical data values?
Which method would you use for saving the image to use as a figure in a report? Explain your reasons for these
choices.
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You will not need the two images you just saved (image1.gif and image2.gif), so please delete them before continuing.
Saving displays of 16/32-bit data
The second activity demonstrates the care that must be taken when saving 16-bit, 32-bit and floating point data.
As with the 8-bit MERIS image there are two methods of saving an image, one of which preserves the data, and the
other one that preserves the display after the application of contrast stretches.
Activity / Question 2
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Redisplay (or load) the btemp_nadir_1100 band of the AATSR image,
with a linear Redisplay Stretch ranging from 28400 to 31900. Apply the manual saw-tooth stretch used in section 3.3
(two knees, at [69,255] and [70,0].
Save the image in the following three ways:
- as image1.dat, choosing Bilko.dat as the file type,
- as image2.gif, changing the file tyop to CompuServe.gif,
- by creating a new image with stretches applied and then saved as image3.gif
Close the images, and re-open the
btemp_nadir_1100 band of the AATSR image, with the same Redisplay stretch
and apply the saw-tooth stretch again. Finally re-open the three saved versions of the image.
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| a) |
How would you explain the differences between these three images?
Take a histogram of each (without applying stretches)if you think this may help
you understand what is going on.
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Answers:
(Resizable
pop-ups)
Answer 1
Answer 2
Back up to:
Q1
Q2
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| b) |
Which of these three ways of saving the image is the one to use for preserving the geophysical information it contains?
Which method is the one to use for providing illustrations for reports and presentations?
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You should now know how to save images the way they are displayed in Bilko, and understand why the method for doing this
differs from the method used to save the data values representing the actual geophysical data.
In the next section you look at
the automatic stretches provided by Bilko, and how these may be used to display different types of image data in the
clearest possible way. Before moving on close all windows, and delete the three images you just saved as part of Activity 2.
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