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The second sample took an area of mixed forestry and a dwelling adjacent to it –
the dwelling had multiple pitches due to dormer windows. As with the first the
values of the area sampled adjacent to the roof was consistent with values
calculated from samples of this land type (mixed forestry) elsewhere in this thesis.
The roof sample itself had a high level of standard deviation (revealing a high
level of reflection and shade) and a variation in mean colour band values from
other roof samples consistent with the initial roof survey. From this it can be
concluded that roof values are not a reliable indicator of forestry values and
cannot be reliably used as a reference.
Roof test sample 3 Mean pixel value Standard deviation
Red 213.815 41.112
Green 223.148 34.272
Blue 208.148 42.432
Roof test adjacent sample 3
(cut pasture)
Mean pixel value Standard deviation
Red 205.074 7.903
Green 206.539 8.182
Blue 134.537 9.784
Table 22: Roof test sample value 3
The third set of samples for determining the potential for using roof spectral
values as a tool for validating adjacent area values used a section of cut pasture
and a flat roofed shed. The values for the shed were high (with a correspondingly
high level of standard deviation), indicating the level of reflection on the roof
surface. By comparison the standard deviation across all three colour bands was
low in the cut pasture, which is consistent with values identified elsewhere in this
thesis. The compared mean values between the two samples were similar and this,
together with the fact that buildings have been returning inconsistent spectral
values in the study, suggests there is little to be gained by examining the
relationship between the spectral values in roofs and adjacent polygons. This is
something of a surprise, as at the outset of the research I had expected the
relatively uniform nature of the shades and texture of roofing used in Ireland to
provide an ideal reference for spectral analysis of aerial imagery.
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