Geography Reference
In-Depth Information
Figure 40.3 Data capture
and data analysis: using GIS
and remote sensing to
understand environmental
change.
satellites, namely LandsatThematic Mapper (TM),
SPOT, and AVHRR.
When the Sun's energy reaches the Earth's
surface, it is partly reflected back into space. This
reflected radiation may be detected using an
optical sensor and used to characterise the nature
of the reflecting surface. The TM sensor, for
example, can detect this reflected radiation in
seven different parts of the electromagnetic
spectrum. These are called the spectral bands or
channels on which the sensor produces data; three
are in the visible, two are in the near infrared and
one is in the thermal part of the spectrum (Figure
40.4). SPOT-3, by contrast, records information
for only three parts of the spectrum, while the
most recent satellite in the SPOT series records
four. As the sensors pass across an area, they record
the reflected radiation in each band for a small
patch, or pixel, on the ground. The size of the
pixel depends on both the design of the sensor
and its altitude. For TM, the pixel size is
equivalent to an area of about 30×30 m on the
ground. SPOT, by contrast, has a spatial
resolution of between 10 and 20 m, depending
on the channel. A full image is thus made up of a
large number of pixels, and for each of them we
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