Section 4: Surveying with GPS

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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.

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Accuracy of a GPS Survey

The accuracy of a GPS survey is dependent upon many complex, interactive factors, including:

  • observation technique used, e.g., static vs. kinematic, code vs. phase, etc.
  • amount and quality of data acquired
  • GPS signal strength and continuity
  • ionospheric and tropospheric conditions
  • station site stability, obstructions, and multipath
  • satellite orbit used, e.g., predicted vs. precise orbits
  • satellite geometry, described by the dilution of precision (DOP)
  • network design, e.g., baseline length and orientation
  • processing methods used, e.g., double vs. triple differencing, etc.
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Error Sources in a GPS Survey

Error sources in a GPS survey include the following:

  • reference position errors - coordinate, monument stability, crustal motion
  • antenna position errors - equipment setup, phase center variation and offsets
  • satellite position errors - orbit ephemeris errors
  • timing errors - satellite or receiver clock errors
  • signal path errors - atmospheric delay and refraction, multipath
  • signal recording errors - receiver noise, cycle-slips
  • human errors - field or office blunders
  • computing errors - processing and statistical modeling errors.
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Operational Procedures

Identify and minimized all errors by redundancy, analysis, and careful operational procedures including:

  • the repetition of measurements under independent conditions
  • make redundant ties to multiple, high-accuracy control stations
  • ensure geodetic-grade instrumentation, field procedures, and office procedures are used
  • ensure processing with the most accurate station coordinates, satellite ephemerides, and atmospheric and antenna models available.

    CAUTION: Be aware that these procedures cannot disclose all problems.

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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.

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