Background to aerial thermal studies
To help to see which buildings in Peterborough have
high heat losses through their roofs an Aerial Thermal Survey was
done in early March 2009 by Blue Sky International on behalf of
Peterborough City Council.
The aeroplane typically flies at a height of 2000 feet and makes
a series of overlapping passes over the area to be surveyed. The
aircraft is fitted with satellite navigation equipment to prevent
drifting, which would lead to 'holes' appearing in the map where
areas have been missed. Aerial thermography can only be undertaken
on a cold, clear, cloudless night during the winter months and at
least two hours after sunset to minimise the effects of the daytime
sunshine and to ensure that most properties are heated to a
A special digital camera is used that is sensitive to the
different levels of infra-red light that is given off by objects.
Different temperatures give off different frequencies of Infra-red
light, the frequencies are converted to 255 different shades from
black through to white. The digital images were taken as strips
when the plane flew over Peterborough, Blue Sky then joined the
strips together and manipulated them to coincide with an Ordnance
Survey Master Map file of Peterborough buildings.
The average pixel value for each polygon was therefore
automatically calculated to represent the average temperature of
each building roof. The pixel values for each roof were then sorted
into 8 classes, with an equal number of buildings in each class.
The hottest heat class was designated as being red and the coldest
heat class as blue, with intermediate classes being given colours
of red through blue.
It should be noted that the heat value colours do not represent
absolute temperatures; they are only intended to show the relative
temperatures, at the time of the survey, of the building roofs.
There will be false negatives, that is, buildings showing blue
or (colder) colours suggesting good levels of insulation, when in
fact the insulation is not good. This is most likely to occur where
a building is unoccupied or the occupiers are on holiday and there
is no heating on. There may also be false positives, where, for
example, a loft hatch has been left off and hot air has got into
the roof void and warmed it up.
Frequently asked questions
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The chronological series of manipulation of the thermal imagery
data down to house level.