Session Setup - Replay

The user can limit the maximum trigger frequency of the computations. This can be used to increase the replay speed for very fast updating positioning systems (e.g. Pos M/V, iXSea LandIns). What in fact happens is that raw positions are skipped from computation when the positioning system updates faster than the user defined limit.

If too many positions are skipped then the accuracy of the results may be affected.

In most cases the observations for attitude do not correspond to the actual time of the singlebeam/multibeam ping time. In order to derive the correct coordinates (X,Y and Z) for the time of the actual ping QINSy uses position interpolation for X and Y and the so-called “delta heave” method to derive the Z.
The position for the time of the ping is derived by interpolating between the 2 adjacent positions using the speed and COG.
For the height the “delta heave” method is used. First the height difference for the 2 adjacent height observations is computed. For the same period of time the heave difference is computed from the heave observation. In principle these differences should be the same. However, the heave observation may be affected by drift, so there a scale factor is computed which is applied to the heave observation so that heave and height show the same difference. The height for the time of the ping is computed by adding the scale factor corrected heave to the last height observation.

High position and height updates, for example from a 50Hz Hydrins or POSMV, do not often contribute to the horizontal and vertical accuracy of multibeam and laser data. High position and height updates however can cause performance issues during replay of the data: replay becomes a lot slower. Therefore it is recommend to reduce the number of position and height updates.
The above methods ensure that the position and height quality is not jeopardized. In case the recording is done with high update rates, it is recommended to use 5Hz to speed up the replay process.