Geography Reference
In-Depth Information
where traditional surveying techniques would be
impractical due to the weight of the equipment
and problems with line of sight.
Clearly DGPS can be a highly valuable mapping
tool. However, the user must be aware of several
operational issues and potential problems if this
technology is to be used in appropriate ways with
maximum effectiveness. Kinematic GPS allows
rapid data collection while on the move. However,
collection of more accurate carrier phase
observations is time-consuming, taking up to ten
minutes at each individual point. Additionally, high
volumes of data stored on the receiver may be
problematic. One day's fieldwork can produce tens
of megabytes of data, which must then be
downloaded from both the mobile GPS and the
base station receivers to a PC for differential
correction. The time and operational problems that
this may cause can be avoided by the use of data
logging PCs connected to the receivers or real-time
correction of the GPS data using a transmitter to
send the base station data to the rover. However,
both these options can be prohibitively expensive
and add to the weight of field equipment. Also, real-
time correction may not be possible due to
obstruction of the broadcast correction signal
caused by the nature of the terrain, for example
mountains, high buildings or extensive tree cover.
Mountainous terrain, buildings and tree cover
can also impede post-processing correction of
GPS data. Both mobile and base station receivers
must be tracking the same set of satellites for
differential correction to succeed. This will not
always happen when large areas of the sky are
obstructed. However, the chances of success
increase the closer the base station receiver is to
the mobile receiver. Using GPS to determine the
position of an unknown base station location right
in the area of study is useful (Carlisle and
Heywood 1997). However, in Snowdonia more
than 25 per cent of data could not be corrected at
first attempt, despite the mobile GPS being no
more than 2 km from the base station.
In forested and built-up areas, satellites rapidly
disappear from and then return to view as they pass
behind tree branches, trunks and buildings. To
minimise the time spent by the receivers searching
for lost satellites or locating new ones, rather than
recording position data, receivers with ten or more
channels are beneficial. Ten-channel receivers allow
the tracking of more satellites at one time.
Therefore, when the signal from one satellite is lost
the receiver uses the next best satellite, which is
already being tracked on another channel.
There can be significant variation in
performance between different makes of receiver,
in different types of terrain and at different points
in time. This uncertainty over the exact accuracy
of the collected GPS data can be off-putting to
the novice. It is vital to verify the performance of
the equipment and the data collection method,
particularly when using GPS for the collection of
data with sub-metre accuracy. Testing equipment
by surveying known locations, such as the
Ordnance Survey's triangulation points in the UK,
to assess the effectiveness of the methodology and
define constant inaccuracies such as datum
transformation errors can be an invaluable
precursor to fieldwork and also allow the user to
develop a better understanding of GPS accuracy
issues. It is equally important to look at signal
strength, GDOP and the number of points
processed during differential correction as
indicators of the accuracy of the data.
Overall, GPS technology is feasible and
effective in a variety of research areas where other
surveying and mapping techniques would be
impractical. The summary of potential problems
given above is not meant to deter use of GPS but
to encourage sensible application of the
technology by users with an awareness of the
limitations of the technology. With such an
understanding, users will find themselves able to
map new types of feature efficiently and use the
data in new ways to discover new areas of
geographical research.
GUIDE TO FURTHER READING
There is no GPS text specifically aimed at
geographers. The best introductory text is by
Kennedy (1996). This text also provides an excellent
practical overview of how GPS data can be used
