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Answer to question 12

Can you read a 2-D histogram? Test your ability!

Vancouver BC - Landsat TM

Vancouver BC - Landsat TM


The graphical plots represent two-dimensional histograms or the frequency of occurrence of brightness levels in TM bands 3 and 5 for various sites in the accompanying image.

Match the histograms below (blue letters) to the marked sites on the image.


Did You Know ? A spectral signature can have as many dimensions as there are individual spectral bands recorded. The more and narrower the bands, the better a feature's "signature" is identified. With modern sensors, it is quite common to have hundreds of spectral bands, each very narrow and contiguous with neighbouring bands. These are "hyperspectral" sensors.

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Site 1: There is a large variation in the green tones and very little in the red, so the histogram should be quite spread out in the vertical axis but not much in the horizontal.

Site 2: Compared to site 1, this site shows a much more limited range of green tones, so the histogram spread will be noticeably less in the vertical direction. The average red values appear to be the same as for site 1.

Site 3: This site shows a very narrow range of dark tones, especially in the green, and therefore one would expect to see a histogram with very limited spread, close to the origin.

Site 4: At this location, the range of brightnesses is large, with a high mean value in both red and green. One would expect to see a large spread in the histogram, with the mean point well away from the origin.

Site 5: The range of brightness for this feature is very narrow, so the histogram will show a very small plot. The average green value is very low, while the red is very high.

Site 6: The green brightness range is very large here, but the red range is not. The mean green value is very high, much more than for the red. As a result, the histogram should be much wider in the vertical than in the horizontal dimension and the central point in the histogram well offset in the vertical direction.

Site 7: There is a good range of green and red tones at this site but the mean value of the red is noticeably higher than for the green. Thus the histogram should be located much closer to the horizontal axis than the vertical.

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