9 Measuring with GPS
The software notifies you after three points have been measured whether the calibration is in or out
of tolerance with respect to the calibration tolerances. After each point you have the following
options:
l
measure additional points
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retake a point flagged as potentially in error
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save the partial calibration and resume later
The danger of using residuals as the only means of controlling a calibration is that the best
precisions can be achieved using the wrong combination of points in the calibration solution. When
an inclined plane is in use, this manipulation of residuals can result in a steeper tilt of the plane to
best fit the data, resulting in better precisions and an in-tolerance calibration. Monitor the tilt of the
plane closely, especially when the geometry of the control points is not strong. Widespread control
that covers the entire site is good; narrow-based control around a corridor is not as good. An
incorrect tilt of the plane can result in increasing errors in height as you move away from the center
of the controlled area.
Performing a two-point calibration
Trimble recommends that wherever possible you use a multi-point calibration. Use the two-point
calibration method in situations where a baseline of only two control points is available. In the two-
point calibration, the first point establishes the position and elevation for the project; the second
point establishes the project orientation.
In a two-point calibration, the heights for the project are computed using a simple block shift
method that ties all heights to the first measured control point. A two-point calibration is carried out
in the same way as a multi-point calibration.
Troubleshooting a site calibration
If a site calibration fails repeatedly, try the following solutions:
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Try a different combination of control points. The software cannot always identify the bad
point.
l
Start the calibration process again. You may have incorrectly measured a point or points.
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Check the equipment. The source of the error may be as simple as the adjustment of the rod
bubble, or a bent rod.
Once the system is set up there are limited sources of error when using RTK GPS systems. The most
common sources of error include:
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A poor site calibration
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Incorrect base antenna height
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Incorrect rover antenna height
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Incorrect selection of the correct antenna type at the base or rover, which causes height errors
Trimble SCS900 Site Controller Software User Guide 88
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