Understanding Sonar

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Sonar – The word comes from SOund NAvigation & Ranging. Some call them fish-finders and some just call them depth-finders. Call them what you may, but here I’ll clarify a common misunderstanding about what you see on the screen.

What’s the difference between distance and depth when referring to sonar? There is a huge difference, as I’ll explain.


Recently, I was trolling with a deep diver and I had 35 feet of line out which was dragging at a 45 degree angle.  Geometry says that puts the diver down 16 feet. (taking into account a couple extra feet above the water’s surface.)

As I dragged my diver, this is what my Sonar looked like. The blue horizontal bar shows the diver at 32 feet, but I know that actually means it was 32 feet from the sonar’s transducer and not necessarily that deep.

By the looks of the sonar picture, my diver crashed into the edge of the drop-off! I knew better though and didn’t worry about it and continued to troll.

So remember, your sonar measures distance and can display the depth, but the marks you see on the screen do not necessarily show how deep they are.

The same goes for fish, that show up as arcs (when you’re moving)… a fish (arc) that looks to be 30 feet deep could be much less than that, because it’s off to one side or the other. For a complete explanation of how sonar works and how to read it properly, see: Explanation of how sonar works

It would be worthwhile to read the entire article to get a thorough understanding of what you are seeing on the screen. When you understand how to read it effectively, you will become a better fisherman.

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About Brian Helminen

Brian is the owner and Captain of Sand Point Charters, LLC. He also owns and operates Designotype Printers, Inc. with his wife, Margaret. They have a cottage on Lac La Belle that can be rented by the week in the summertime.

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