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Figure 11: Histogram for Shade and Pasture
There are two factors for inclusion in the automated search algorithm that can be
taken from this; firstly a reading of high levels of standard deviation from the
mean value for the red and green colour bands for a given polygon are a strong
indication that the analysis might be distorted by shade. Secondly if a histogram of
the pixel count for the red and green colour bands shows a peak at the mean for
shade, and at the mean for pasture then the area is pasture (given that the other
probable cause, water, has already been identified through vector mapping).
The algorithm could account for this by taking a sample of neighbouring polygons
–if these contain areas of pasture then the standard deviation is flagged. If the
standard deviation is above 30 values on the greyscale then the polygon is flagged
and a histogram appended for further processing. The resulting set of these types
of polygons would be returned with the result set from the analysis so as the user
could accept or reject the anomaly as the result of shade (in terms of large
amounts of standard deviation for the red and green colour bands in a small area
polygon).
The second test sample for shade took an area of pasture with a smaller percentage
of shade present for comparison with the areas of shade. The values for the sample
matched those expected for pasture with the exception of a higher level of
standard deviation in the red and green colour bands. These could again be
flagged once a level of standard deviation above that expected for pasture, and the
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