Geography Reference
In-Depth Information
examples of good and bad satellite geometry in
relation to receiver position.
accurate method is necessary. This is offered by the
use of differential GPS (DGPS).
DGPS works on the principles of differential
positioning, which assumes that two receivers
tracking the same satellites will be subject to
similar errors. Therefore, if the errors for one
receiver can be determined, these can be used to
correct the errors recorded by the second receiver.
In DGPS, one receiver is placed at a known
location. The position of the location is
determined over time and any deviations from the
true position monitored. A second receiver,
tracking the same satellites, is then used to record
the position of an unknown location. The position
recorded by this receiver is then adjusted, either in
real time or by post-processing, to cancel out any
errors in the position obtained by the receiver at
the known location. Differential correction of
GPS data can be used to counter SA and some of
the other errors that may affect the position. Using
DGPS techniques, accuracy is usually between 2
and 15 metres; however, it is possible to obtain
millimetre accuracy with high quality equipment
and an appropriate technique.
It is not necessary to own two receivers to
perform differential correction. A number of 'base
station' networks have been established around the
globe. These transmit correction information,
often in real time. This permits those with an
appropriate receiver and decoder to correct their
GPS positions in the field. This real-time
correction of positional information is of
particular use to the navigator.
HOW TO USE GPS TO RECORD
POSITION
There are two main techniques for determining
the position of a GPS receiver: kinematic (mobile)
GPS and static (stationary) GPS. With kinematic
GPS, one or more GPS receivers is in motion
during the recording of positional information
(van Sickle, 1996). Kinematic techniques are used
for the collection of positional information while
on the move, for example in the survey of a
footpath or in onboard car, boat or aeroplane
navigation systems. With static GPS, the receiver
(or receivers) remains at a fixed point and the
position recorded averaged from a large number
of readings taken over a specified time period.
Static GPS is the technique used by the surveying
community to survey sample points or the precise
location of buildings. Both kinematic and static
techniques have found practical application in
geographical research, as the case studies described
below show. Kinematic and static techniques need
subdividing into two further approaches: absolute
positioning and differential positioning.
Absolute techniques require only one receiver
and are frequently used for navigation, where
accuracy of tens of metres is acceptable. In
kinematic GPS, absolute techniques could be used
to record the position of a forest or landscape
feature by walking around its perimeter. In static
GPS, absolute techniques might be used to survey
the location of a sampling site. The inherent
inaccuracies in this method (±100 m), caused by
SA, can be reduced by averaging the positions
recorded over a given time period. Despite the
lack of positional accuracy, absolute kinematic and
static techniques have found widespread use. The
advantage of absolute positioning is the ease with
which a position can be recorded and the speed of
measurement. In addition, a single, low-cost
receiver is required. When only an approximate
position is required, absolute techniques may be
appropriate. However, for most users a more
GPS IN PRACTICE: RESEARCH IN
MOUNTAIN ENVIRONMENTS
In order to appreciate the potential of GPS as a
field tool, it is useful to examine how it has been
used in a variety of research situations. In this
chapter, to provide context, we focus on the
application of GPS in mountain environments.
However, there is a growing literature on the use
of GPS in other areas of geographical research (see
the guided reading at the end of this chapter). The
applications described in Boxes 43.1, 43.2 and
