Question 12

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

Vancouver BC - Landsat TM

Vancouver BC - Landsat TM

Text Version - Figure 1

This illustration shows a Landsat-TM image of the Vancouver area in British Columbia. We notice that the water is black except in the Fraser River delta, and the mountains are green except on the peaks where they are red. The Fraser Valley appears in various shades of pale green. We see small blue polygons identified by the numbers 1-7.

  • Polygon 1: green area in the mountains
  • Polygon 2: green area in the mountains
  • Polygon 3: black polygon area
  • Polygon 4: polygon area rose in the valley
  • Polygon 5: red polygon area in the Delta
  • Polygon 6: green area in the valley
  • Polygon 7: red zone in the sea

The image is colour-coded according to the RGB bands 3, 5, 7. The two-dimensional histograms are presented with the intensity of band 3 along the horizontal axis and the band 5 along the vertical axis.


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.


Histogram A
Text Version - Histogram A

ITEM A All points are grouped and low values in horizontal and vertical.

Histogram B
Text Version - Histogram B

ITEM B The points have low values in horizontal and extend in vertical ranges.

Histogram C
Text Version - Histogram C

ITEM C The points have very extensive values high in the horizontal and vertical range.

Histogram D
Text Version - Histogram D

ITEM D The points have rather low values in horizontal and vertical range.

Histogram E
Text Version - Histogram E

ITEM E The points are grouped and have strong values in the low horizontal and vertical range

Histogram F
Text Version - Histogram F

ITEM F The points have stable values along the horizontal and highly variable vertical values.

Histogram G
Text Version - Histogram G

ITEM G The points have stable values in horizontal and vertical highly variable and are grouped more than in Item F.

Note: The image is colour-coded as: Thematic Mapper bands 3 5 7 = R G B and the two dimensional histograms are shown as intensity of band 3 along the horizontal axis and of band 5 along the vertical axis.

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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Image Information

Vancouver, British Columbia, Landsat TM