Section 4: Surveying with GPS

Survey Background Information

All GPS surveying techniques are based upon radio signals
from a network of orbiting satellites. These signals are processed
to compute station positions by trilateration: the positions of
the satellites and computed ranges are used to determine the antenna
position.

These positions are computed in an Earth-Centered Earth-Fixed
(ECEF) Cartesian coordinate
(x, y, z) system, which can be converted to geodetic curvilinear
coordinates (latitude, longitude, and ellipsoidal height). With
the addition of a geoid height model, orthometric heights can be computed.

Planning the Survey

Because surveys involving GPS include Geoid models, plane
coordinates, projections on the surface and other obstacles matching
to older surveys, a plan of action should be set out in the planning
stages of new or continued projects.

Project control points will be referenced to the National
Spatial Reference System (NSRS) through CORS stations or FBN stations.
The static GPS survey is usually the best choice for establishing these
points. Project control points should not be more than approximately
three (3) miles apart.

Geometry plays an important role in the accuracy of the adjustment
in a static survey scenario. TxDOT Levels of Accuracy 1 and 2 apply
to the primary project control points (see
Table
3.2 TxDOT Level of Survey Accuracy). Boundary work should be done from these stations
whenever possible. Additionally, the primary project control points
should also include elevation.

From these stations, the next generation of points (secondary
control) can be set with slightly less stringent procedures. Secondary
control stations are closer together and can be done by traverse
or by using FastStatic or RTK procedures if GPS is chosen. TxDOT
Level of Accuracy 3 applies here (see Table 3.2 TxDOT Level of Survey
Accuracy). Construction work usually dictates that these points
be about 1500 feet apart.

Finally, topo work (TxDOT Level of Accuracy 4) is performed
from the secondary stations. Secondary stations will be available
for occupation with total stations or RTK base receivers.

In planning a survey, if the older coordinate positions
must be adhered to, a calibration can be done to the existing control.
The software on the data collector usually has a provision for this
to be accomplished in the field.

It is imperative that when adherence to older coordinate positions
is needed, control points surround the project and work not be done
outside the perimeter, otherwise, the cantilever effect of the calibration
becomes an objectionable factor. During calibration, if a station
exhibits high residuals (does not fit in relation to the others),
it should be excluded from the calibration.

Using state plane coordinates throughout the above stages
is the best way to maintain integrity between all points. Coordinates
of marks surveyed conventionally can be put on the state plane grid by
use of a Combined Adjustment Factor (CAF).

Furthermore, by working in plane coordinates, long corridors
can be divided into segments of different CAF’s to prevent such
a growing difference between surface measurements and geodetic positions
of NGS stations.

If total station work is to be mixed with GPS work in the
same area, it should be considered whether total station traverses
would be reduced to the state plane grid or the GPS work be calibrated
to the surface values obtained by the total station.