Positional Accuracy

Horizontal uncertainty, Vertical uncertainty, Tidal and Total Vertical Uncertainty (TPU)

Definition

Positional accuracy applies to the degree of uncertainty that can be introduced at the point of measurement, or during processing and visualization.

This includes uncertainties introduced by human intervention environment, or instrument.

Total horizontal uncertainty (THU) actually refers to the uncertainty of the position of sounding on the seafloor, the accuracy of the vessel’s GPS and the vessel speed.

Total Vertical Uncertainty (TVU) is the uncertainty of the reduced depths.

To determine the vertical uncertainty the sources of individual uncertainty need to be quantified. All uncertainties then combined to obtain a total vertical uncertainty (TVU).

Factors affecting the vertical uncertainty:

Vertical datum uncertainties, and vertical positioning system uncertainties.
Tidal measurement uncertainties.
Instruments uncertainties and Sound Velocity uncertainties.
Ellipsoid/ vertical datum separation model uncertainties.
Vessel motion uncertainties (roll, pitch and heave).
Vessel draught, vessel squat and sea floor slope.

Total Horizontal and Vertical uncertainty are provided at the 95% confidence level.

Total propagated Uncertainty (TPU) is the sum of the THU and TVU.

Checks

Total Horizontal Uncertainty
Total Vertical Uncertainty (via QAX - Quality Assurance Tool)
Tide
Total Propagated Uncertainty

Pass Requirements

TVU: Run the 32-bit Geotiff Total Vertical Uncertainty layer through QAX and ensure that the 95% confidence level (order2) has been met for the dataset.
THU: Testing currently unavailable

For the time being there is no tool in Geoscience Australia to test the THU confidence level.
During acquisition, ensure that the maximum allowable Total Horizontal Uncertainty was 95% confidence level (order 2) in the mobilization report (only for mountable sensors, we usually don’t receive mobilization reports for the permanent mounted sensors).
Tide: Ensure that the GPS tide or conventional tide (for generic surveys) has been applied to the data or submitted with the dataset.
Combined Uncertainty Bathymetry Estimator (CUBE): Ensure TPU follows appendix E of the Australian multibeam guidelines.

Examples

Pass example

Description

2000001S Austrea2 (GA-0223)

Test results

TVU: Check completed/pass.
THU: N/A
Tide applied to the data.
CUBE grid created.

Pass

Yes

Explanation

This dataset meets the requirements outlined above and passes checks 1, 3 and 4.

Total Vertical Uncertainty check test (TVU) with 100% confidence (which is even higher then maximum 95% confidence).

Fail example