|Chesapeake Times, Vol 6 | July 2021|
When real-time kinematic (RTK) corrections are available in the survey area, it is possible to achieve sub-decimeter vertical positioning of the sonar system without the need to apply tides or vessel squat measurements. Bathymetry collected in this way is referenced to the satellite ellipsoid rather than to the more conventional orthometric or tidal datum, so it is usually required to apply a vertical transformation to move the bathymetry into the desired output datum. SonarWiz has all the tools needed to perform these calculations in a straightforward workflow.
Navigation and Attitude Editor
The first step is to examine the navigation in the SonarWiz Navigation and Attitude Editor. Change the displayed charts to show both the RawAttitude.Heave and the RawPosition.Altitude as shown in Figure 1.
Figure 1: Display of a few seconds of heave from the motion sensor and antenna altitude from the positioning system.
If your data has been processed through a high-quality Kalman filter that integrates the inertial motion into the antenna positions, you will see the vertical motion of the position altitude is highly correlated with the inertial heave. This is the case in Figure 1. In this case, when you merge the data, you will want to turn OFF the heave sensor and rely on the antenna heights alone to correct for heave motion (more on this below).
Figure 2: Setting the merge dialog to ignore heave and apply antenna altitude (heights)
If for some reason your antenna altitude is not reliably measuring the wave motion seen in your heave sensor, you can still use the antenna altitude information as a “GPS tide”. What you do is apply a low-pass filter to the Altitude data to remove as much of the wave motion as possible while maintaining the longer period elevation of the vessel as shown in Figure 3. During the merge process, we will use the heave sensor to correct for wave motion and the antenna altitude to set the vertical datum (“tide”) as shown in Figure 4.
Figure 3: Display of a few seconds of heave from the motion sensor and antenna altitude from the positioning system after applying a 60-second low-pass filter to the altitude
Figure 4: Setting the merge dialog to combine heave and antenna altitude (heights)
Converting from Ellipsoid Heights to Orthometric Heights
GNSS measure elevations relative to an idealized ellipsoid model of the earth’s surface. If you examine the y-axis values of the Altitude graph in Figure 2, you will notice that the antenna height is about -20.315 m. That is, the antenna is about 20.315 m below the GNSS reference ellipsoid in this location. It is more common to reference bathymetry to orthometric heights (approximately, mean sea level) or to a chart datum based on tide measurements. So, we need to apply a vertical offset between our output vertical datum and the ellipsoid.
If your survey area is relatively small, the easiest way to offset the elevation is to enter the difference between the ellipsoid and your desired datum. In my example, I was able to use NOAA’s VDatum utility (Figure 5) to find that the offset from ITRF2014 to NAVD88 is 23.172 m +/- 0.06 m
Figure 5: VDatum indicates that the offset from ITRF2014 to NAVD88 is 23.172 m
We can enter this offset in the merge dialog of SonarWiz as a User Entered Height Offset:
Figure 6: Setting a static datum offset as a User Entered Height offset
If the offset between the ellipsoid and your output vertical datum change throughout your survey area, a single value offset may not be accurate enough to meet specifications. Instead, you will need to load a grid of offsets into the program and have SonarWiz interpolate the offset value for each ping. SonarWiz supports a large number of grid formats, including the GTX format used by VDatum. For example, Geoid18 tile g2018u1 covers the survey area of our example. You can load this grid directly into SonarWiz to examine it:
Figure 7: Geoid18 tile g2018u1 from the VDatum distribution loaded into SonarWiz.
If we want to use this grid to convert our ellipsoid heights to orthometric heights we will need to change the sign of the grid so the conversion goes in the correct direction. Right-click the grid and select the Invert Grid command. This changes the sign of the grid we have imported into our project. We can then use this new grid in our merge dialog. In the merge dialog, set Apply vertical datum offset to: YES and in the Vertical Datum Grid File box below, click the “…” button to open the file picker. Navigate to the copy of the grid in the Grids folder of your project and select it as shown in Figure 8.
Figure 8: A vertical datum offset grid loaded into the merge dialog. Each ping in the project will interpolate the grid for a position-specific offset value
When you merge the data, SonarWiz will use the platform position to interpolate the vertical offset from the datum grid. Using accurate antenna heights in surveying eliminates a lot of problems that typically occur when using tide tables leading to ugly busts in the bathymetry. Directly and accurately measuring the elevation of the platform is much better than trying to estimate it from tide tables and squat calculations. SonarWiz gives you the tools you need to apply antenna heights and transform the resulting bathymetry into the output datum your clients require.
– David Finlayson, Chief Scientist