Civil Engineering Reference
In-Depth Information
Differential GPS requires one of the receivers to be located at a known point.
This point is normally a NGS point for which state plane coordinates are avail-
able. Connecting a survey to a state plane coordinate system will provide a num-
ber of benefits. There will be additional redundancy because a lost point can
always be replaced using its state plane coordinates. For most boundary survey-
ors, who usually work in a specific geographic area, connecting each survey to
the state plane coordinate system will place all of the surveys on a common ref-
erence system. As time goes by and physical evidence disappears, having state
plane coordinates for this evidence may prove to be invaluable in replacing the
lost points. In the past, ties were sometimes relied upon to help recover points
should they become destroyed. Unfortunately, ties are also destroyed over time.
When traversing, it is possible for a systematic error such as a scale factor, to
occur which would not be picked up during the closure and adjustment. If GPS
is used to set a number of control points which the traverse then ties into, such
errors would be discovered.
GPS can also benefit from conventional surveying techniques. If a survey is to
make use of a conventional traverse, the total station can occupy the GPS points
as a part of a closed traverse thereby confirming their accuracy. In some cases,
the coordinates for a GPS point may not be accurate, particularly if there are
obstructions nearby which cause multipath. It is possible that errors in the GPS
locations might not be discovered without linear measurements to other known
points. There are a number of methods that can be used to confirm GPS locations.
One method is to take GPS shots at a location next to a control point. A tape can
then be used to measure between the points to confirm their accuracy. Shots can be
taken on the control point at different times of the day or on different days. This
will allow the collection of data from different satellite geometries. These methods
will require additional time and expense. If a traverse is to be run anyway, it makes
sense to use total station measurements to confirm GPS locations. In practice, the
hybrid surveying technique just described has proven to be both efficient and accu-
rate for boundary surveys.
Connecting surveys to a state plane system raises certain issues relating to
the differences in distances between points located by GPS on the state plane
coordinate system and those measured on the ground using a total station. In
most cases, the distances will not agree. This is not an indication that either the
GPS or traverse locations are wrong. The problem is caused by inherent differ-
ences between making measurements on the surface of the earth and making
measurements on a state plane coordinate system. It is important to understand
why this occurs and what steps need to be taken so that boundary measure-
ments are correct. Before GPS became widely used, these issues rarely came
up because most boundary surveys were made using total stations for ground
measurements and the surveys were not tied into state plane systems. For some
surveyors, the use of GPS has required them to work with an unfamiliar state
plane system for the first time. While it is undoubtedly true that all survey-
ors have heard of state plane coordinate systems, not all have actually worked
extensively with them.
