Network Measurement Uncertainties
Instrument measurements (observations) can have uncertainties calculated that indicate the uncertainty of a given measurement relative to the instrument. But when instruments are networked together, this simple uncertainty no longer applies, because the common point position in a networked measurement is really determined by a com- bination of instruments that measured the common point. In fact, the computed common point in the real world never lies on top of any of the component observations (because the world is not perfect--un- fortunately!). This highlights the second major advantage to USMN: the ability to calculate “net uncertainties” for common points.
A total station and laser tracker have measured a common target in a nest. Due to noise, operator error, and a number of other factors reminding us that the world is not perfect, each instrument “sees” the target in a slightly different position in space. As a result, the measured points end up in slightly different positions.
Not only can USMN calculate the most likely position of the real target based on instrument characteristics (referred to as a USMN Composite Point), but it can also calculate the uncertainty of that composite point. The composite point is closer to the total station’s line-of-sight position (due to lower angular uncertainties of the total station), but is also much closer to the laser tracker’s distance measurement (due to less uncertainty in the laser tracker’s distance measurement). The uncertainty of the composite point itself has less uncertainty than either of the instrument observations, because much of the total station’s uncertainty along the line of sight is rendered extremely unlikely based on the laser tracker’s measurement.