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Users of remotely sensed imagery frequently extract brightness values between points on an image. A transect is a straight line between any two user-specified points within an image. Transects are used in many digital image processing applications. The pixels that lie on the transect can be meausured and displayed to compare spectral or spatial differences. Consider the example in Figure 7-2.1. The Spatial Profile Tool in Imagine 8.2 was used in this example to measure variations in pixel values that occur between points A and B. The graphs below the image contain the associated values that stretch between points A and B for bands 1, 2, 3, and 4. The x-axis contains the distance in pixels and the y-axis, the raw pixel value. As we can see, each graph contains a unique distribution. A study such as this may be important in determining optimal bands for further analysis.
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| Transect at Band 1 | Transect at Band 2 |
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| Transect at Band 3 | Transect at Band 4 |
Figure 7-2.2 demonstrates a similar type of study for imagery of Par Pond, South Carolina. The graph in this figure shows brightness values in band 3 that occur between points A and B. The spectral differences that occur along the transect in this band were much greater than in other bands.
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Image analysts may use several transects in a single image to determine
environmental trends or patterns. When several transects are used, the
analyst must make sure that the end points of each transect fall on the
same scan line to ensure equal transect distance. This often requires rotating
the image to accomodate geometric integrity. If the analyst extracts transects
where the end points do not fall on the same scan line ( or column), the
hypotenuse of stair-stepped pixels must be considered instead of the simple
horizontal pixel distance (Jensen, 1996).
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