Surveying Instruments - Practical Boundary Surveying - page 76

A number of factors affect GPS accuracy. Atmospheric effects such as interference
in the ionosphere, and the amount of water vapor have an effect. Multipath can have
a substantial effect on accuracy. Multipath is caused by GPS signals reflecting off of
large structures such as flat sided buildings, signs and trees. Multipath causes multi-
ple signals to be received—those from the satellites and those which bounce off the
reflectors. These signals can confuse the software and cause errors in position. Even
parking a survey vehicle next to a GPS receiver can cause multipath problems.
The number of satellites visible and the geometry of the satellite constella-
tion is an important factor to consider when using GPS. For high accuracy, it is
recommended that a minimum of 5 satellites be available. It is best if satellites are
spread out across the sky rather than grouped into a small cluster in one section of
the sky. Mission planning software is available which will tell the surveyor what
time of the day or night will provide acceptable satellite geometry. As the satellite
geometry is constantly changing, the time required for the observations will need
to be factored in to ensure that a favorable geometry will be continuously available
throughout the observation period. The Positional Dilution of Precision ( PDOP )
value is an indication of the impact of satellite geometry on GPS precision. The
smaller the PDOP value, the better the satellite distribution, and the better the
precision. For survey grade GPS, many consider that a PDOP of 4.0 or less is
adequate.
For boundary surveys, multipath effects and attenuated signals caused by
tree branches and leaves (canopy) can reduce GPS accuracy below that which is
acceptable. Canopy may also reduce the number of available satellites which can
be used. These factors will limit the usefulness of RTK GPS when working in or
next to wooded areas. In such cases, the system may not deliver the accuracy nec-
essary for boundary surveys. Static GPS will also be affected in the same way;
however, because of the long occupation times, it is likely that static GPS will be
used primarily for setting up control networks, not for setting lot corners in real
time. This provides greater flexibility because sites in open areas can often be
selected for the control points. This will help to avoid multipath problems.
Where obstructions will unacceptably reduce GPS accuracy, traditional survey-
ing techniques using a total station will still need to be employed. However, as we
will discuss further on, it is possible to combine GPS with conventional traverse
techniques. GPS receivers are now so economical that they should be included in
every boundary surveyor's kit. GPS will allow small or large boundary surveys to
easily and economically be tied into the state plane coordinate system. This will
provide additional redundancy should it become necessary to reestablish boundary
corners at some later date.
5.11 Data Collection
In the old days of boundary surveying, it was necessary to record all observa-
tions into a Field Book by hand. Modern surveyors now use Electronic Data