Humminbird Side Imaging Forums
General => General Discussion => Topic started by: fox_hunter on June 24, 2011, 04:09:11 PM
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Hi to All !!!
I use My 798 ci Si to build garmin Maps, with submerged structure, depth gap.. and all good or possible spot to catch fish .
So.. afther a lot of test .. i look that side imaging say a wrong depth !!!
My Englisch is not so perfect so.... i use jpg image to explain the problem !!
(http://www.rlab.it/pesca/fiumi.jpg)
I do this trach on the middle of submerged gap , you can see the different type of bottom "clarity" , but the instrument... humviewer... and all software .. measure the same depth along the horizontal line !!
I think that it's owful !
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Hi Fox_hunter,
I think you have discovered a basic feature of sidescan images, they can only tell the depth beneath the boat and not the depths to the sides. This has to do with how the images are created and there is nothing to do about it, I'm afraid. Depth measures to the sides require a multibeam bathymetric system.
Regards,
Rickard
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Thanks Richard it makes a lot of sense. First thoughts we may have jumped off of the deep end looking for problems that are not real. You are right only under the boat is going to be accurate. Accuracy decreases as it it goes to the side and it will not be able to dips, etc.
George
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The image below illustrates the problem of measuring depth vertically below the vessel with a wide angle transducer. There is just no way to know the direction where the first echo is coming from. The sonar typically assumes that the depth is shallower than it really is, since it picks up on any closer-range echos from each side.
(http://www.sonartrx.com/documents/images/rangeordepth.png)
Comments: The red "range-line" is pointing vertically down below the transducer. The green line is towards the port side (first echo return).
Then again, since a typical side imaging transducer has a very focused angle in the front/back direction (i.e. the direction of travel), it knows very well that the range-measurements are precisely in the vertical plane to each side or down, not forward or aft of the vessel.
Consequently, a side imaging system has a much better ability to pick up on bottom roughness along track than a wider angle downward facing DI transducer. Bottom roughness in the direction along track is represented by depth variations, while bottom roughness in the direction perpendicular to the track can be represented by the "shadow-picture" produced by the SI sonar.
Tore
Tore@sonartrx.com
http://www.sonartrx.com (http://www.sonartrx.com)
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Hi Guys !!! Thank to All !!
I suppose that a multy beam "sonar" can function "well" ,a multibeam s. width a lot of head , each "haed" can have a narrow beam angle and different type of axial angle , so you can build a perfect image width few distorsion.
However if the side imaging don't know the "real" distance from bottom and transductor how can tell to us the correct distance from a submerged object ?!?!?
If it's undestand that there are a rock or a submered three at "5 meters" on the left side of the boat (and different hardness of object) , but it can't read the correct depth on the side bottom.
If i think at the eco "like" a "timing" model for me is the same. It can be right ??
(A have the same distance, B is much longher than A , so how it can be accurately ?? )
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Fox hunter Read this Robert explains it all. Gary http://forums.sideimagingsoft.com/index.php?topic=715.msg3259#msg3259 (http://forums.sideimagingsoft.com/index.php?topic=715.msg3259#msg3259)