Survey Calibrations & Checks
Before starting a survey (or during longer surveys) it is best practice to regularly check the
performance and alignment of the sonar system. The following procedures are vital in maintaining
quality control of depth measurements.
6.1
Patch Test
The Patch test resolves small misalignments between the sonar and the navigational system. Failure
to not resolve these misalignments results in inaccurate depth measurements. A patch test is
performed by running a series of reciprocal lines at varying speeds, depths and bottom terrain. A
patch test should be performed over an area with a slope or prominent features. It is recommended
that a patch test be performed whenever the sonar is re-mounted, repositioned, or has been
displaced in any way.
There are four tests that must be run for a patch test: latency, roll, pitch and Yaw. For best results,
patch tests should be carried out when conditions are calm and vessel motion is at a minimum. It is
important to have good vessel handling as the reciprocal lines need to be as close possible. Deeper
water is preferred as the misalignment angles will be more apparent. Positioning should, at a
minimum, be SBAS corrected. However, it is strongly recommended that RTK corrections be used
as positional errors will decrease the repeatability of patch test measurements. For processing patch
tests refer to the manual of the chosen acquisition or processing software.
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