Civil Engineering Reference
In-Depth Information
applications. Greater accuracies may be claimed by GPS manufacturers or sellers,
as the technology is constantly improving. Prior to acquiring a system a surveyor
would probably want to get some hands-on experience with a particular system to
see if it lived up to his expectations for the type of work in which the system would
be used. As we discussed earlier, GPS accuracy is usually measured statistically, in
terms of probabilities, not linearly as with conventional traversing. Comparison of
the two surveying methods is possible when the distance between GPS points is
taken into consideration. When GPS accuracy is equated with linear accuracy, it is
common for longer baselines to achieve accuracies in excess of closure standards
commonly accepted for boundary surveys. Recall that for conventional boundary
surveys; commonly accepted closures are in the 1 in 10,000 to 1 in 15,000 range.
RTK GPS consists of two receivers. One receiver is set at a known point, for
example a National Geodetic Control (
NGS
) point. This receiver is referred to as
the
Reference Station
. The other receiver is referred to as the
Rover
. The rover
is usually an antenna and receiver mounted on a prism pole. The reference station
remains fixed over the reference point during the survey while the rover is moved
around to locate evidence or physical features or to set points. Occupation times
for the rover vary from 180 seconds to as short as 15 seconds. Expected accuracy
(see Footnote 1) is 1 to 4 cm horizontal and 2 to 6 cm vertical.
Survey grade GPS receivers work by
Differential Positioning
. Differential GPS
requires the use of multiple receivers. Atmospheric and other effects cause a deg-
radation of the GPS signal which affects GPS positioning accuracy. However, if a
number of receivers are located in relatively close proximity to each other (a few
miles), each receiver will be subject to the same accuracy degradation. If one of the
receivers is located at a known reference point, such as an NGS control point, which
has known state plane coordinates and a known elevation, the GPS coordinates of
the receiver on the control point can be compared with the known coordinates of
the control point and correction factors can be calculated. The correction factors can
then be transmitted to the rover so the GPS coordinates of the rover can be adjusted.
When using RTK, the correction factors must be continuously transmitted
to the rover in order for the corrections to be applied in real time. This requires
a radio link between the reference station and the rover. For static GPS receiv-
ers, the correction factors are applied during post-processing so no radio link is
required. As GPS has become more commonly used, continuously operating refer-
ence stations (
CORS
) have become available in some locations. These stations are
maintained by the National Geodetic Survey. If a GPS survey is made in the prox-
imity of a CORS station, the data can be downloaded from the NGS web site for
post-processing. In some locations, services are available so that the CORS cor-
rection data or privately owned correction data can be transmitted to the rover in
real time using a cell phone or other cellular device with a data plan. Some newer
GPS receivers include an integrated cellular connection so that an external cellular
device is not required. In addition to the NGS CORS stations, private, university
and state CORS stations may be available in certain locations. This eliminates or
reduces the need for the surveyor to set up a receiver at a reference station when
using RTK. RTK receivers can also work in static mode.
