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TOPIC: SideScan Mosaic Water Column offset

SideScan Mosaic Water Column offset 8 months 2 weeks ago #11059

  • MarsArtis
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I've recently upgraded to a new laptop capable of graphic HW acceleration and the water column offset option became available.

I'm wondering about how you guys handle this option.
For the sea area I've been logging it seems that putting the offset all the way down and therefore reducing the water column to nothing makes the mosaic much more accurate. This because the seabed is randomly filled with rocks or massive rises and the water depth changes suddenly. So the inner "junction" of the right and left side-images is too much deformed.

Second some rocks, or other items laying on the bottom result shifted by about 1,5 meter or so.

The counterpart is that I will need to make a parallel pass to fill in the missing swath.

How do you prefer to deal with this option?
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SideScan Mosaic Water Column offset 7 months 1 hour ago #11163

  • MarsArtis
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Nobody has make any test or experiment with this function?
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Last Edit: 7 months 1 hour ago by MarsArtis.
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SideScan Mosaic Water Column offset 7 months 54 minutes ago #11164

  • Matt
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Sorry, I missed your earlier post.
The central section of the swath uses very few "pixels" to generate an image over a relatively large horizontal area. This is due to the way that slant-range correction works, and the fact that there is very little difference in the length of the direct route of the sonar beam vs horizontal distance when the angle of incidence to the bottom is high (approx 90 degrees directly underneath the boat). This gives reduced resolution, but not reduced accuracy as such. Conversely, areas further from the boat (horizontally) have an ever reducing angle which means that each "pixel" of the beam correlates more closely to the distance travelled along the sea floor, giving a higher effective resolution.

Although it may be beneficial to remove the lower resolution centre of the swath, increasing the water level offset to produce a gap in the middle of the swath is probably not the best way to do it. Instead, just use the upper range function to truncate the swath image and leave your gap, because otherwise you are adjusting the slant-range calculation for the entire image by changing the depth used underneath the boat.

The water-column offset feature is really supposed to be used as sparingly as possible, just to remove any last holes of water column that may appear e.g. to an incorrect depth reading or a local feature on the seabed. Ideally, offsets should be applied to as short a section of swath as possible - do this by cutting a track into segments in the Sonar Mosaic view.
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SideScan Mosaic Water Column offset 6 months 4 weeks ago #11165

  • MarsArtis
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Thank you for clarifing that.


Too bad that we don't have a reset button for all the adjustment sliders (watercolumn offset in this case) instead of having to move them and try to achieve the default value.
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