In the same way we
use our eyes to judge the distance to an object, two satellite images viewing
the same area of ground from different orbits, can be used to compute
a Digital Elevation Model (DEM) of the earth's surface. Once the data has been
received and calibrated, a computer can compare the images and calculate the
ground surface
elevation to a vertical precision of up to 8 - 15m and do this for thousands
of square kilometres in just a few minutes. Overlaying a colour satellite
image onto a DEM
produced from two other images generates the picture of the Italian Alps shown
here. The clouds in this picture were not present when the image was recorded,
they have
been added afterwards for visual effect.
Image processing software can use the differing spectral properties of land
cover to ‘classify’ a multispectral satellite image very rapidly.
Land cover maps for large areas, which would take many man-years to survey
from the ground, can be produced in just a few hours. Classification accuracy
can be improved using a small ground verified sample to train the software
to recognise each cover type of interest. This ‘training data’ gives
a measure of the variability each land cover type, such that a likelihood of
each image pixel belonging to each land cover ‘class’ can be determined
and thus the image pixel can be assigned to the most probable ‘class’.
Image classification can be used for a great variety of applications. In addition
to producing a more understandable map, the digital data readily provides area
statistics and by using images from different dates, a means to detect small
changes in land use. This classification of the area around Seattle, USA was
used together with elevation data to model the transmission of mobile phone
signals. The propagation of microwave signals for communication is greatly
effected by the absorption, scattering and reflection of the signal from vegetation
and urban structures in addition to topography. Using such data the signal
strength from a transmitter network can be modeled before it is actually established.
