File Import Datum Transformations in SonarWiz

Chesapeake Times, Vol 13 | April 2023

One of the signature features of the SonarWiz 7.10 series is the upgraded geodetic library. The new geodetic engine adds support for hundreds of datum and datum transformations that previous versions of SonarWiz could not support. However, a major limitation of the current SonarWiz design, and a vestige of our old geodetic engine, is that all data is converted to WGS-84 (EPSG:4326) before storage and then forward projected into the project datum before display or export. In other words, all datum transformations in current and earlier versions of SonarWiz force a pivot through WGS-84. This makes it difficult to support datums that do not support pivots through WGS-84, such as the Ordnance Survey National Grid (OSGB) in the UK, the Geocentric Datum of Australia 2020 (GDA2020) and other grid-based datum of this nature.

Figure 1: Newer versions of SonarWiz will not store the data in WGS-84

In SonarWiz 7.11.0, all sonar file formats will record geodetic positions using the datum of the raw data as specified on the import dialog. When SonarWiz reads and processes this data, it will transform directly into the project coordinate system using the specified transformation pipeline. There is no forced pivot through WGS-84 required or implied.

The new SonarWiz import dialogs have been simplified to reflect the new procedure (see Figure 2). All the user needs to do is specify the SRID of the raw data. As an example, compare the import settings between old sonarwiz and new sonarwiz for a dataset stored in NAD83(2011) using Geographic Coordinates.

In new SonarWiz, all you need to specify is the actual SRID of your raw data. No datum transformation is applied to the raw data on import, we merely need to specify the SRID of the raw data. Datum transformations are handled automatically by the program if necessary.

Figure 2 – Comparison of Old and New SonarWiz import settings. (1) the map projection of the SRID must match the raw data, (2) there is no need for the Navigation data in XY grid format option.

What happens if you get the import geodesy wrong? In older versions of SonarWiz, the data needed to be properly converted into WGS-84 on import. If this step was bungled, the line was ruined and needed to be re-imported. In new SonarWiz, no datum transformation is performed on import, so it is possible to re-define the file SRID in the file properties window. SonarWiz will automatically reproject the data into the project coordinate system on the fly.

What about legacy data that does not have the SRID stored with each CSF file? In this case, new SonarWiz will detect that the CSF file is out of date. It will assume that the data was properly converted to WGS-84 on import and set the SRID of the line to EPSG:4326 (WGS-84).

You can review and change the SRID settings of the raw and projected files in new versions of SonarWiz by selecting the sonar file in the Project Explorer and reviewing the Imported SRID and Map Projection SRID settings in the Properties window (See Figure 3).

Once you get used to setting the SRID of your data before you import it, the new system has a lot of advantages. It is clear from the start what is happening to your dataset, there are no black boxes selecting the import datum based on incomplete data. More importantly, the new system does not require a pivot through WGS-84 which allows SonarWiz to support virtually all SRID in the EPSG database, including datum that were previously impossible to support.