Presampling - Field Sampling: Principles and Practices in Environmental Analysis - page 76

inexpensive (less than apiece), CD-R (not CD-RW) CDs are recommended. These are
record-only, so they make a permanent record of the work done and presentations made.
3.15.1. GPS
The use of GPS for locating sampling sites in the field is highly recommended, and in
many situations will be essential. Sample sites can readily be located even if other
markers are lost or destroyed. A GPS receiver can be set up to locate preselected sample
sites in a field, and will not only tell the person sampling when he or she is at the correct
location but how to get from that site to the next. In this way relatively unskilled
personnel can be used in sampling. The movement of water and other components of
interest can be followed by noting their change in position. The rate of movement can
also be obtained from the data, either directly, allowing the receiving instrument to do the
calculation, or indirectly, by noting the change of distance and the time. More discussion
about this use will be given in Chapter 11.
Decisions as to the type of receiving units needed are made at this point. Such
questions as the sophistication needed and whether a handheld or vehicle-mounted unit
are to be used must be answered. The equipment, including any software needed, should
be bought, and operators or persons doing the sampling should be trained in their use
[12].
Knowing the position of an airplane or boat far from landing or shore, or a sampling
site in fields that are hundreds or thousands of hectares in size to a precision of ±15
meters is sufficient. When sampling a field of 1 ha, however, knowing a sampling site to
±15 meters is not good enough. There are two ways to improve the accuracy. The first is
to use position averaging. A receiver capable of position averaging takes many readings
while stationary and averages them. In this way the position to ±3 meters can be
obtained. This may be sufficient for many sampling situations. Two other ways to obtain
more precise positions are to use either a wide area augmentation system (WAAS) or a
differential GPS (DGPS)-capable receiver.
The WAAS methodology uses ground stations to correct signals received from
satellites, and a corrected signal is then broadcast by satellites. When a receiver capable
of accessing the WAAS system uses this information the variability in the accuracy of a
site position is less than ±3 meters. The Federal Aviation Administration (FAA) and the
Department of Transportation (DOT) are developing this system, which is available
without charge to any GPS unit capable of receiving the signal and making use of it. The
WAAS is limited to North America and to areas that do not have obstructions that
interfere with the signal.
The second way is to use DGPS. It is similar to WAAS in that it uses corrected signals.
A receiver capable of DGPS is necessary. The use of DPGS can increase the accuracy to
±2 cm. Obtaining DGPS accuracy using civilian GPS receivers however, requires that a
DGPS site be within range. The DGPS site is an accurately known location that
broadcasts position correction information to GPS units. This information can be used in
real time or after the fact to find the exact location. Real time would be preferable when
the exact place in the field needs to be sampled several times. Postprocessing, finding the
location after taking the sample, and returning to the field office, can also be done and is