Surveying, GPS, Digitization - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 146

quality of the receiver also contributes to accuracy. Higher quality receivers
have more or better procedures for reducing errors.
Greater accuracy can also be achieved through a variety of techniques,
regardless of the cost of the receiver. The most common of these (and the
most widespread) is called the differential global positioning system (DGPS;
see Plate 4). DGPS relies on a fixed GPS receiver that can calculate GPS
changes in its position against the accurate surveyed position. The difference
indicates the current amount of error in the GPS system for a particular
area. The fixed location DGPS receiver broadcasts a correction signal used
by GPS receivers to adjust the GPS signals and get highly accurate (down to
centimeter or inch accuracy) positions. The most common DGPS is called
the wide area augmentation system (WAAS) and is available in North Amer-
ica.
APPLICATIONS
Most people who have used a GPS probably can't imagine any limit to its
applications. Even if its shortcomings are grievous (it can't be used indoors,
nor very well in a forest, or where there are many tall buildings or cliffs),
solutions have been developed to these problems. Usually these solutions
involve broadcasting radio signals or pseudo-GPS signals that are highly
accurate. The configuration of these systems is very complicated and
requires large institutional investments. Most are made by governments. For
instance, the European Union is developing a high-accuracy network (along
the lines of the U.S. WAAS) for navigation purposes. Even if a GPS receiver
lacks the ability to use these extra networks, GPS can still be used in a num-
ber of applications, some of which are described here.
Vehicle Navigation Systems. More and more people have used GPS-based
systems in cars; many more have benefited from the use of GPS in cars,
buses, trains, and trucks. The GPS receiver may be hidden in the dashboard,
but may be critical for the taxi company to find out which taxi is closest to
you when you call for a pickup. A GPS receiver can help a trucking company
better organize deliveries to minimize the fuel used. A bus may have a GPS
installed to help the bus company indicate to passengers how long they need
to wait for the next one.
Navigation systems are used for more than vehicles on land. They are
also widely used for nautical and aeronautical navigation. They have become
for many sailors irreplaceable because they work regardless of the weather
and can easily be combined with computerized chart information. Almost all
planes use, or will use, GPS. Together with high-precision positional trans-
mission, planes can use GPS-based systems to land in any weather with centi-
meter precision.
Hiking. More and more hikers turn to GPS to help them find out more
exactly where they are and to help them to plan a route before they go. GPS
may not be reliable in canyons or along steep cliffs, but in most situations