View/Open - ARAN

currently available for all areas and has not been included in the study. It should
be noted that its addition has the potential to improve the accuracy of this type of
study.
A couple of sample sections of the image were selected with varying degrees of
shade present in the image. This was to determine if it is possible to introduce a
step into the algorithm which would correct for areas of shade in otherwise
uniform closed polygons. The first sample involved selecting an area of pasture
which contained a large amount of shaded ground from some high trees along one
of the border vector boundaries i.e., along the hedge. The area of shade
corresponded to roughly half the area of the test polygon. It should be noted that a
degree of shade will be present in all of the orthophotos to which this study
applies; this is because aerial survey takes place in bright sunshine by necessity.
The sample was analyzed for mean pixel values along the colour bands, and also
the standard deviation displayed.
Shade test sample 1
(pasture with app. 50% shade)
Mean pixel value Standard deviation
Red 88.457 37.387
Green 135.151 40.01
Blue 100.616 20.386
Table 16: Shade test sample value 1
The results for this were as might be expected; with an increase in the standard
deviation for the first red and green colour bands reflecting the variety of tone
present in the sample. This, however, does not give a complete representation of
the data and a histogram (see below), reveals peaks for the shade value, and a peak
for the pasture value in both the red and green colour bands. These two peaks
indicate that the area is pasture, and that the high pixel count for the lower values
in the red and green colour bands is a result of shade.
74