External Reliability¶
The Norwegian market requires specific external reliability standards as described in this section.
What is external reliability?¶
From the "Dictionary for Mapping and Surveying" we find the definition: "External reliability is understood as the effect on the unknowns (for example, coordinates) of a (gross) error in the observations."
This definition can be applied to Gemini Terrain's measures for external reliability. In gross error detection, we can estimate (gross) errors with standard deviations in all observations. These values form the basis for external reliability:
First, the confidence limit for the largest remaining gross error is calculated based on the specified significance level. What effect do these possible (gross) errors have on coordinate unknowns in the adjustment? For each observation, the largest remaining gross error is used to find the effect on the coordinates.
Gemini Terrain goes further and asks the question: What effect do possible remaining gross errors have on angles and scale differences between neighboring points in the control network? The deformations are tested against the requirements in the selected standard for control.
In height calculation, the question becomes what effect possible gross errors have on the height difference between neighboring points.
The ratio between calculated deformation and the standard's requirement for maximum deformation is calculated. The largest impact factors are recorded as they are calculated, along with information about which observation a gross error with such an effect can occur in. If the largest factor is greater than 1, the standard is not satisfied.
Measures for external reliability¶
Gemini Terrain has a total of 4 measures for external reliability, where the first 3 are defined for control networks and the last for property surveys:
- Largest angle change to two of the nearest neighboring points
- Largest scale variation to two of the nearest neighboring points, in ppm
- Largest change in height difference to the nearest neighboring points
- Largest displacement at the point (point deformation)
Note
We can also include known points without observations in the analysis. The point is included in the analysis if it is known in the calculation dimension for which the analysis is run. It will therefore be possible to calculate deformations for angle, height, and scale also for the known point.