In recent years, geographical coordinates of reference data used for
image classification are typically acquired using GPS receivers. GPSs
are based on the concept of triangulation whereby the coordinates of a
point in three dimensions can be estimated from its distances to three
points whose coordinates are known. For a GPS, the point whose
coordinates are to be determined is the location of the GPS receiver,
and the known points are satellites orbiting the earth. Distances are
estimated from the elapsed time a signal emitted by a satellite and
traveling at the speed of light takes to travel to the GPS receiver. The
accuracy of the estimated coordinates of the receiver location depends
on the method used to estimate the elapsed time