The Swedish Maritime Administration has two hydrographic vessels, the Jacob Hägg and the Nils Strömcrona. The two vessels can operate in relatively shallow waters and the “three-metre curve” can usually be captured.The vessels work with double crews that work according to a 14-day rotation system.
In addition to the survey vessels, the Administration has two smaller boats for minor survey operations and control of reported errors in the charts. One of theses can be transported overland to the hydrographic survey area.
Bathymetric data is collected in digital form and can be processed onboard to a certain extent, allowing a preliminary depth chart to be produced in connection with the hydrographic survey. Bathymetric data is more carefully analysed at the head office and is reported to the editorial staff at Notices to Mariners.
Some multibeam bathymetric data is false and must be deleted. The cause of the false data may be that the acoustic pulse hit dynamic objects such as fish or grass. Small objects are often difficult to find, which makes frequent, full-coverage measurements extremely valuable.
Data is also corrected to another reference level in certain cases. When the evaluation is finished, the data is stored in DIS, the “digital portion of the Swedish depth archive” and appropriate products are developed. The chart is updated.