Once an Uncertainty Field Analysis is performed for a network this network solution can then in turn be used to determine the result- ing uncertainty parameters in the Instruments that measured those points based upon the solved network points. Press the Instrument Uncertainty Analysis button to access the results.
This uncertainty is then reported per instrument for each of 3 Uncertainty Variable in the instruments properties (or XYZ for instruments with many variables).
The default instrument uncertainty values are generic values which have proven over time to provide an accurate estimate of the uncertainty of an instrument in an unknown environment under unknown conditions. However, if you wish to determine the best possible accuracy values for a particular instrument under your conditions you can use USMN to do so, using the following procedure:
Establish network of 8 or more fixed monuments to serve as control points.
Measure all the control points from at least 4 different physical locations using the same instrument, adding new stations to the job file for each instrument plant.
Perform a USMN on the network, compute the Uncertainty Field Analysis and check the Instrument Uncertainty Analysis results.
Copy the resulting values into your instruments properties, updating the instruments Uncertainty Variables.
If desired, rerun the USMN Analysis with the new Instrument Uncertainty Variables and compare the resulting Instrument Uncertainties.
Here is an example graph of the change in accuracy (Vertical & Horizontal Angle and Distance) as a result of 10 iterations of this process with an inhouse instrument. As you can see the values converge to a stable condition with repeated iterations.
If you look at the change over the ten iterations, it quite small—less than the resolution of the encoders (Horizontal Angle Change ~0.06, Vertical Angle Change ~0.001 arcseconds and Distance Change ~0.00012”). The initial “guesses” for this solution (which are the de- fault values in SA) turn out to be good enough to get close on the very first iteration.
This provides a means of evaluating the performance of your particular instrument under particular real-world conditions. Those values can then be used for the instrument uncertainties when you perform your true measurements and provide an even greater confidence that the uncertainty in your points reflect the results you will see with your instrument.
The Instrument Transform Uncertainty Fields are also computed and reported in the event as part of this process. A graphical representation of the position and orientation of this transform uncertainty can be displayed using Coordinate Uncertainty>Show Instrument Transform Uncertainty Fields (USMN). This will hide the instruments and display only the uncertainty graphics while enabled.-