Data Acquisition and Processing Report (DAPR) - Center for Coastal ...

V. Corrections to Echo Soundings
Instrument corrections
An initial leadline confidence check was measured against the EM1002
multibeam echosounder prior to the start of field operations on March 28, 2009 while tied
up at Isla De Vieques, Puerto Rico. The purpose of this check was to verify the system
during static conditions by confirming that the digital depths being recorded reflected the
actual depths. A sound velocity cast and leadline was performed on the starboard side of
the multibeam system. The sonar’s acquisition system was logging data while the
leadline were performed. The CARIS swath editor was then used to verify the depth
soundings. Soundings were queried approximately 4.1 meters to starboard of the nadir
beam and verified to the leadline values. No instrument correction was necessary
because of insufficient evidence of systematic error.
A leadline was unable to be performed for the 7125 due to sight limitations and
deployment issues. Vertical elevations were verified with a check line against the
EM1002.
Sensor Offsets
On the February 7 th , 2006 the NOAA Ship NANCY FOSTER had her sensor
offsets surveyed by the Power & Control Systems Group of L3 Communications. The
IMU, GPS antennas, EM1002 transducer and the center of rotation were surveyed with
respect to the RP of the vessel. The values obtained from the vessel survey are
documented in Appendix G & H.
On March 10, 2009 the NGS Field Operation Branch conducted an additional survey for
the second IMU. The field crew based there observations on alignment cubes and the
coordinate report from the L3 Communications survey. The values obtained from the
vessel survey are also documented in Appendix G & H.
These offsets were entered into the MERLIN acquisition software, POS/MV
software and into the Caris vessel configuration file in the appropriate areas. The offsets
used for the sonar’s and positioning systems are documented Appendix A, C and E.
Static and Dynamic Draft Corrections
Static draft values were obtained from visual observations of the projection draft
marks on the starboard side of the NANCY FOSTER for the EM1002. The static draft
recorded on March 28, 2009 was 3.66m while tied up at Isla de Vieques, Puerto Rico.
Subtracting the initial draft value of 3.66m from the fixed offset (1.68m) for the
Reference Point to the EM1002 gives the final draft reading of -1.98m, which was
entered into the MELIN software and confirmed with the leadline procedure previously
discussed. The final EM1002 draft reading at the end of the cruise (April 2 nd ) was
negligible.
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