WHAT EXACTLY DO YOU KNOW ABOUT PATCH TEST?
HydroCharting NG
Survey Solutions for Hydrographic, Aerial and Oceanographic Surveying
In Hydrographic Surveying, there are many factors that are out of the control of the surveyor. These factors may be weather, bottom composition or shipping traffic among many other factors that cannot be controlled. There are other factors that can be controlled by the surveyor, and it is important to dedicate time to fully control those factors otherwise it can lead to questionable data and more time spent on trying to determine what might be causing an issue.
Two areas that demand the most attention are the measuring of offsets and the multibeam calibration or patch test.Accurate offsets have to be determined before the patch test data collection.
However, to obtain correct offset measurements, do the following:
a)Measure offsets from the vessel reference point to the sensor’s reference point, then
b)Measure from the sensor’s reference point back to the vessel reference point
These two measurements should match within a few centimeters or less.
Check and re-check offset measurements as this will save you time later on
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Once the offsets have been determined and the patch test data collection starts, sufficient data has to be collected to derive a statistically viable result for the angular offsets.
In the early days of multibeam, there were no patch test programs to automatically solve for the angular offsets. One set of data collection would be performed, say the two roll lines, and then the data would be processed and two charts produced. The surveyor would overlay charts and, using a scale, dividers and depth would determine the offset angle. That angle would then be put into the equipment and the lines would be run again, with the corrections, with the same chart production and overlay process; this would continue until the correct value was derived. A full patch test could take days!
Nowadays, all software packages have programs that automatically solve for the patch test, which solves the patch test in minutes! With this being the case, there is no reason to then reduce the amount of data collected.
Now, what exactly is PATCH TEST?
A patch test is the systematic approach used for calibrating the various sensors used in multibeam data acquisition. There are three main sensors needed to map the bathymetry of the seafloor: the navigation sensor, attitude sensor and the echosounder (SONAR). The navigation sensor measures ship speed, heading and position. The attitude sensor measures the motion of the ship (i.e. pitch, roll, heave and yaw). The echosounder is used for transmitting and receiving sound to determine water depth and seafloor characteristics.
The purpose of calibration is to correct for systematic errors created by the positioning and mounting angles of the different sensors. A correctly calibrated system will show the same bathymetry in repeated tests, regardless of variables such as speed, direction and ship motion.
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