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Author Topic:   Marking Waypoints on Targets in Side Imaging Images
rnvinc posted 02-27-2015 05:09 PM ET (US)   Profile for rnvinc   Send Email to rnvinc  
I have read that the consensus of marking a waypoint on structure in an [side imaging] image is to place the waypoint at the point on the display where the structure touches the bottom detail in the [side imaging] image. It seems this method is more accurate when returning to the waypoint with the GPS. Why is this method more accurate than marking the waypoint anywhere else on the structure in the [side imaging] image?

There's got to be something involved here like mathematics or slant range correction in how the waypoint latitude and longitude calculation is being derived at a point in the SI image history. The waypoint created not at the [side imaging] centerline boat position.

Rickie

jimh posted 02-28-2015 08:14 AM ET (US)     Profile for jimh  Send Email to jimh     
What does the acronym "SI" stand for?
rnvinc posted 02-28-2015 05:46 PM ET (US)     Profile for rnvinc  Send Email to rnvinc     
SI is the common-speak acronym for side imaging, regardless of each brand's trademark designation or title for their respective side imaging technology.--Rickie
jimh posted 03-01-2015 11:06 AM ET (US)     Profile for jimh  Send Email to jimh     
There must be a lot of geometry and mathematics going on to compute the location of some target seen on a raster display of echoes from a side scanning SONAR. It seems to me that the calculations would have to follow an algorithm like this:

--get the boat's position from the GPS sensor

--get the boat's heading

--get the offset of the GPS sensor from the SONAR sensor

--get the location of the target echo as being either to Port or Starboard of the SONAR sensor

--get the distance of the target echo from the SONAR sensor; this might involve some more calculations based on the depth of the water and the distance away; I am not sure how you would calculate that.

Now to find the position of the target:

--take the GPS sensor position, and move to the SONAR sensor position by applying the boat heading and the offsets; now you know the orientation of the SONAR sensor Port-Starboard axis and its position

--move to the target position by following the the distance vector either Port of Starboard from the SONAR sensor location and axis heading.

As for doing all of that from data stored in the SONAR echogram history, I wonder if all the necessary data is recorded.

I suppose you could move backwards in the boat's track by stepping back in the stored track points. Then you could assume the heading was the same as the course over ground. I don't know where the offset between GPS sensor and SONAR sensor is applied. Do these side-imaging SONAR devices require that you input that data?

I suppose it is possible with a modern high-speed navigation computer to calculate all of this geometry and come up with a latitude and longitude for the SONAR target from its echo. If that all takes place, it is darn impressive.

I am not sure I understand what is the variation mentioned in the process of selecting the location on the screen of the echogram presentation. The screen is a two-dimensional display, and the presentation of these echograms on it is usually as a two-dimensional display. The two dimensions are time (in the vertical axis) and offset Port or Starboard in the horizontal axis. Where would you think to mark the position of a target echo other than at the echo itself?


jimh posted 03-01-2015 11:11 AM ET (US)     Profile for jimh  Send Email to jimh     
[Replaced the acronym SI with its intended meaning per the author. What is being discussed here is also called side scan SONAR.--jimh.]
rnvinc posted 03-01-2015 05:19 PM ET (US)     Profile for rnvinc  Send Email to rnvinc     
I'm not nearly as versed in mathematical equations and GPS applied verbiage, which is exactly why I came here with your (Jim) knowledge in the GPS field especially.

Standing structure being plotted into an side image view will typically show the bottom of the structure closer to the centerline (transducer position at that point in the side-imaging SONAR history) than the top of that same structure, even if that structure is perfectly vertical.

The brands give us the capability to activate the cursor and place waypoints directly on the side-imaging image on points of interest in that side imaging SONAR history. Depending on the structure actual height in the water column, placing the cursor on the structure for marking a waypoint could be "at the point where the structure seems to touch bottom" or anywhere along the structure's vertical height representation in the SI image . Tt is the common consensus among side imaging users that placing the cursor and marking the waypoint at "the point where the structure seems to touch bottom" seems to be more accurate (for returning to the waypoint) than placing the cursor and marking the waypoint higher on the vertical structure representation in the SI image, but no one has an explanation as to why this is true, and the brand's engineers are not going to tell us. I was hoping that your knowledge in GPS function and design might be coupled with additional knowledge in side imaging SONAR and side imaging display properties to shed some light in this instance.

I am a curious sort and enjoy learning new things.--Rickie

rnvinc posted 03-01-2015 05:23 PM ET (US)     Profile for rnvinc  Send Email to rnvinc     
I will study your reply (just above my last post). I assume I will eventually find out that the calculations involved will be way too complex for me to absorb to a capacity that I could share with others.

Rickie

jimh posted 03-01-2015 05:46 PM ET (US)     Profile for jimh  Send Email to jimh     
Rickie--I am not sure what you mean by "GPS verbiage." I don't think I used any words about the Global Positioning System in my reply, except to refer to the position of the GPS sensor. Let me explain what I mean by the GPS sensor position.

The GPS sensor is the device that receives the signals from the GPS satellites, and when it computes its position, it computes the position where it is located, that is, the position of the actual GPS sensor, or, perhaps I should say the antenna of the GPS receiver. So if you are trying to define a position of some SONAR target that is shown relative to the SONAR sensor, and the GPS sensor and SONAR sensor are not co-located, then you will have to account for the difference in their position. This is not any sort of concept related to the Global Position System. It is just the concept that there might be a difference in the position of the two sensors.

Believe me, there are a lot more words (or verbiage) that can be used about a global navigation satellite system than I used in my reply. So I am a little confused why you described what I wrote as being "GPS verbiage."

jimh posted 03-01-2015 05:55 PM ET (US)     Profile for jimh  Send Email to jimh     
Rickie--I think your real question boils down to something very simple: how to interpret the echogram of your side scan SONAR. As far as I know, a side scanning SONAR is using the same principles as the more traditional downward scanning SONAR. The distance between the sensor and a target is a function of the time between the sending of a sound burst and the return of an echo of that sound. If a target is so large that echoes from different parts of the target return at different times to the SONAR, the presentation of those echo in a rasterized screen makes those echoes larger or longer. To interpret how an underwater object might create a string of returning echoes, you just have to look at the geometry of the situation.

As for interpreting an echo on a side scan SONAR, I agree with your initial assumption. The best way to mark a location is to mark the part of the echo that upon interpretation of the echogram is likely to be the part of the object that is in touch with the bottom, or perhaps in vertical alignment with the bottom.

I bet that some manufacturer of side scan devices must have some tutorials about how to interpret their echograms. Have you looked at any tutorials about this?

jimh posted 03-01-2015 11:54 PM ET (US)     Profile for jimh  Send Email to jimh     
What is the depth of the water in which you are using the side scan SONAR?

There is a nice recorded tutorial presentation on interpretation of side scan SONAR images at

https://www.youtube.com/watch?v=yNucS1bniIw

The tutorial explains the basics of interpreting the sonograms resulting from a side scan SONAR. It is worth a few minutes to view.

rnvinc posted 03-03-2015 02:03 AM ET (US)     Profile for rnvinc  Send Email to rnvinc     
Thanks, Jim, for your advice and explanations. I have a lot of learning to do, but that's ok; I enjoy learning. --Rickie
jimh posted 03-09-2015 09:57 AM ET (US)     Profile for jimh  Send Email to jimh     
The recorded presentation (linked above) does a very nice job of explaining how the screen presentation is related to the position of targets relative to the SONAR transducer. In all SONAR, the target distance away from the transducer is proportional to the time delay between the sending of the signal and the reception of an echo from a target. In SONARs that are aimed only downward to the bottom, the bottom directly under the boat is usually the last echo to arrive. In SONARs that are aimed sideways, away from the boat, the echo from the bottom directly under the boat is the first echo to arrive, and echoes from targets farther out sideways from the boat arrive later and later as the distance from the boat increases. This concept is nicely explained in the recording by using a metaphor of a folded piece of paper.

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