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Offset surveys

Offset surveys are surveys of hydrographic sensors within the mathematical system on board of platforms for hydrographic measurements.

The offsets are measured in order to establish the relation between the various sensors within the hydrographic data acquisition software.

There should be no significant negative influence of the geometry on the total propagated error within the hydrographic system.
A 15 metres hydrographic survey vessel during processing in photogrammetry software
The hydrographic survey vessel Amasus during a floating measurementStarmountain Survey & Consultancy has about 20 years of experience in ship's geometry measurements of objects up to about 150 metres, in all cases guaranteeing accuracies on sub-centimetre level, while sub-millimetre level is regularly achieved. Realistically an accuracy (1σ, 68%)of 1:20,000 (1mm at 20 metres object length) is feasible.

For vessels up to about 75 metres photogrammetry is used. With larger vessels, or when the situation is unsuitable for photogrammetry, tachymetry is used instead.

Not only the geometry is measured, but the attitude and phase centres of multibeam heads are calculated as well. Starmountain has developed a method that allows for measurement and calculation of the phase centre(s), including quality control, without loss of accuracy. Starmountain surveyed the following multibeam heads: Kongsberg EM950, Kongsberg EM3000 & EM3002 single and dual head installations, ODOM ES3, R2Sonic 2024, Reson Seabat 8101, Reson Seabat 8125 in both horizontal and inclined mountings, 250kc dual head GeoSwath transducer in both normal and reversed mountings.The hydrographic survey vessel Speurder with GeoSwath 250kc, Seabat 8101 and ADCP
The end product is a well-organized report showing all the offsets and calibrations as well as a clear insight in quality control and quality assurance.

Examples of objects measured by Starmountain can be found here.

For more on this subject also see our offset surveys web site.