Thread: Structure location vs boat location

I am using a Lowrance Elite 4 DSI. Here is my question. The transducer looks left and right of the boat. On the screen, how do you know if the structure you see is left, right or directly under the boat. I do not know where the image I see moving from right to left is in relation to the boat.
Thanks!

No, you really can't tell which "side of the boat" the image is coming from. The best you can do is calculate how far out to each side your sonar ping reaches, then figure it must be somewhere within the fan shape of that sonar ping. Depending on which setting (455kHz or 800kHz) you have dialed up, your fan shaped sonar ping would be either ~45deg or ~75deg wide to either side of dead center (directly below the transducer).



And since your fan shaped sonar ping is only ~6deg thick, your transducer is practically right over top of whatever your screen is showing AS IT COMES ON THE SCREEN. Anything showing on the screen, to the left of the right hand edge of the screen, is "history" ... as in it's already been passed over by your sonar ping. So, depending on your speed, by the time the image registers on the screen & you can identify it .... it may already be behind you, either under the boat or behind the boat.

And since the sonar ping is coming from a single point, and fanning out from there, and the sonar ping can only tell you the distance from that single point to the closest object that reflects the ping back .... the top of the cover is that distance from your transducer, whether it's directly below you OR out to either side. In other words, it may or may not be that far under the water. It could just be that far away from the side of the transducer.

That's one reason I always use the 800kHz setting ... so that I know that if I'm in 10ft of water, my sonar ping spread is about 10ft wide.

( I have the Lowrance Elite 5DSI unit ... bow mounted ... and this 87pg discussion is where I learned about this Down Image stuff : http://www.crappie.com/crappie/fishi...lotter-87.html )

Thank you crappiepappy! Best explanation I have found. The manual did not talk about it. Now it makes perfect sense and I can start sleeping at night instead of it always playing in my mind.

Please permit me to chime in. I am very glad I accidently opened this thread. I am considering purchasing a Garmin Striker 4DV which they just came out with which is in my price range, but just could not understand whether I could make it beneficial to me as I could not comprehend how to understand where what it was seeing is located in relationship to my canoe location. I have been all over crappie.com and bassresources.com and nobody explained it or provided reference material in such an understandable manner as did CrappiePappy. Having no experience with the electronics of fishing, and just moving into Virginia and not being familiar with the lakes and reservoirs here I am very glad to see there are people on these sites that are willing to go the extra mile to help others. I just assumed if the image appeared on the right portion of the image it had to be to the right of the canoe if I had the transducer mounted to the bottom of the TM which would be mounted on the port side as I sat in the canoe.

Great explaination Pap ...



Rickie

So with keeping what you have explained in my head could you use SI to put a waypoint on this piece of structure and then return to said piece and be over it? If this is the case then the value of SI would be immense.

Originally Posted by Snoball

So with keeping what you have explained in my head could you use SI to put a waypoint on this piece of structure and then return to said piece and be over it? If this is the case then the value of SI would be immense.

I mark waypoints on my SI all the time, but I usually stay slightly to one side of the structure and then put my mark on a spot where the tree or pile appears to touch bottom

You can use DI to find a pile in front of you as well. If you don't see it on 2d, you can turn the trolling motor side to side and watch for the pile or school to appear. If you know it is in front of you, and you know the beam goes out perpendicular to the direction of your trolling motor arrow, you know when you see it appear that you need to cast 90 degrees to the direction of the trolling motor arrow.

Thanks CrappiePappy
Several of us considering a purchasing finders
base our decisions on your advise.