Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 112

5.2.2. Sources of Error
Global positioning system units are capable of operation in overcast and other less than
ideal weather conditions. There are, however, some situations in which errors can occur.
These might cause only a slight error in location or they may cause errors of 1 or more
km. For instance, using the wrong datum leads to a wrong position being shown by the
GPS unit. Also, in coordinating the position shown on a GPS unit with a map be certain
that the coordinate system being used in both cases is the same.
Reflected multipath errors occur when the signal from one satellite is received both
directly and indirectly after it has bounced off an obstruction. Such errors occur where
there are large mountains or buildings near the field in which the GPS is being used. If
both the direct and reflected signals are used, the calculated position will be wrong. Some
GPS units can automatically correct for multipath signals, but others cannot. Two
precautions thus need to be taken. First, observe the surroundings for features that might
reflect a signal, and second, determine if the GPS receiving unit being used can correct
for multipath signals.
In actual use, handheld GPS units can be of great value. For instance, inexperienced
personnel can sample large fields two ways. First, the handheld unit can be
preprogrammed with waypoints. When the person doing the sampling reaches the
preselected site, the unit will beep or otherwise notify him or her. The sample can be
taken and the unit will indicate how to get to the next waypoint. A second way is to
provide the person with a map of the area showing the location of the sites to be sampled.
Even if the sites are not precisely located, the exact location can be identified if the
longitude and latitude indicated on the GPS unit are recorded at the time of sampling. At
a later time the same position can be found easily if the site location is clearly recorded.
If data are needed from between sampled sites, they can be either physically sampled
or the concentration of the desired constituent estimated using techniques such as kriging,
which is discussed further in Chapter 6.
5.2.3. Limitations in Field Sampling
Global positioning systems are extremely useful in field sampling, and no field sampling
should be undertaken without them. Sample sites should not be located at distances
shorter that the accuracy of the GPS unit, however; that is, a GPS should not be used to
locate sample sites 3 meters apart if its accuracy is ±10 meters. If this is done, two sites
that are 4 meters apart may have the same longitude and latitude even though they are
obviously not at the same location. If such longitude and latitude data are taken,
erroneous data will be obtained, and when used these data will produce inaccurate and
confusing results, particularly if used in modeling.
Availability and use of a top-quality GPS unit does not make all other navigational and
site location tools obsolete. Maps, compasses, altimeters, pedometers, watches, and
project notebooks are all still essential and will find many uses [5]; for example, a
handheld GPS unit may not have a builtin compass. Figure 5.4 shows a handheld GPS
unit and watch with compass and altimeter. The GPS unit will show the direction in
which you are going once moving, but will not show the direction in which you should