known control and how well the GPS measurements conform to the local network. If the control
stations are not of equal precision, the external closures will usually reflect the lower-order station. If
the internal closure meets the requirements of the job, but the external closure is poor, the known control
is probably deficient and an additional known control point should be included in the system.
5-44. Data Archival. The raw data is the data recorded during the observation period. Raw data
should be stored on an appropriate medium (such as a compact disk, a portable hard drive, or magnetic
tape). The raw data and the hard copy of the baseline reduction (resultant baseline formulations) should
be stored at the discretion of each unit's command.
PART G - ADJUSTING PRECISE-POSITIONING SURVEYS
5-45. General. Differential carrier phase GPS-S observations are adjusted the same as conventional
survey observations. Each 3D GPS baseline vector is treated as a separate distance observation and
adjusted as part of a network. A variety of techniques may be used to adjust the observed GPS baselines
to fit existing control. Since GPS-S networks often contain redundant observations, they are usually
adjusted by some type of rigorous least squares minimization technique. This section describes some of
the methods used to perform horizontal GPS-S adjustments and provides guidance in evaluating the
adequacy and accuracy of the adjustment results.
5-46. Global Positioning System Error Measurement Statistical Terms. To understand the
adjustment results of a GPS-S, it is necessary to understand the following statistical terms:
Accuracy. Accuracy is how close a measurement or a group of measurements is in relation
to a true or known value.
Precision. Precision is how close a group or sample of measurements is to other groups or
samples. For example, a low standard deviation indicates high precision. A survey or group
of measurements can have a high precision but a low accuracy (for example, measurements
are close together but not close to the known or true value).
Standard Deviation. Standard deviation is a range of how close the measured values are from
the arithmetic average. A low standard deviation indicates that the observations or
measurements are close together.
5-47. Adjustment Considerations. This section deals primarily with the adjustments of horizontal
control established using GPS observations. Although vertical elevations are carried through the
baseline reduction and adjustment process, the relative accuracy of these elevations is normally
inadequate for engineering and construction purposes. Special techniques and constraints are
5-41
EN0593
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