Environmental Engineering Reference
In-Depth Information
Fig. 2.13 An example of real-time, kinetic GPS (RTK GPS) equipment. The base (
right
) and
rover (
left
) can communicate with each other up to several kilometers with radio signal booster.
RTK GPS is efficient and requires only one operator
there for up to several hours to record GPS signals. This long record will average
out most of the error associated with instantaneous GPS positions. The base station
position can be further refined by differentially correcting the data to long-term
GPS stations in the region. Additionally, NOAA maintains a free web service
(OPUS) for correcting base station data (
http://www.ngs.noaa.gov/OPUS/
)
. The
differentially corrected positions commonly have less than 1 cm of error in hori-
zontal position and less than 2 cm error in vertical position.
The rover is a GPS antenna that is mounted on a hand-held staff of known length.
The rover staff can instantly record a position throughout the wetland. The point
position is automatically “corrected” in real time by radio communications with the
base station. The same satellite errors affect the base station and rover, but the base
station knows its position. Thus, it also calculates the time-specific errors and can
correct the rover positions. At a rate of one point per 15 s (including moving from
one position to another), it is possible to collect several hundred precise survey
points in a survey session. Wilcox and Los Huertos (
2005
) describe a simple and
rapid method for bathymetric mapping using a total station and GPS.
The great value of RTK GPS is that each point is independently georeferenced,
and will plot precisely on existing regional map data. The limitations of this
technology include potentially poor satellite positions, and the inability to operate
when a tree canopy or valley walls block satellite reception.
