How to Determine Targets On December 22, 2013

How To Determine Target Distance With Sonar

Sonar displays targets based on the distance of the target from the transducer.
I will use 3 examples to help you understand sonar.
  1. If your sonar says 10 feet how deep is it really?
  2. Weed patches below the “bottom”.
  3. Why is the tail of a fish arch deeper than the top of the fish arch?..

How Deep Is The Water?

If you have depth as a digital overlay and it says 3 feet it is actually 4 -4 ½ feet deep since most transducers are mounted 12-18 inches below the waterline and sonar can only measure the distance from the bottom from the transducer.

Why Are The Weed Patches Under The Bottom

This Image has the Lowrance StructureMap as an overlay.
The right side is DownScan.
There are 2 patches of weeds, one on each side of the boat. The StructureMap is easy to interpret but the DownScan is confusing since the 2 weed patches look like they are under the bottom.
One weed patch is about 52 feet from the transducer on the right and the second weed patch is about 56 feet from the transducer on the left. Since the bottom is 33 feet the sonar can only display the targets at the depth they appear which places them “under the bottom”.

Why Is The Tail Of The Fish Arch Deeper Than The Top Of The Arch?

The top of a fish arch is the closest the fish is to the transducer. The tip of the tail is the farthest the fish is from the transducer and reflecting enough sound for it to be displayed.
On this image the fish didn’t change depth but the distance from the transducer changed as I drove over it. The tails are the same fish on the edge of the sonar cone but the distance is greater.
It appears the tail is 6 feet deeper on the display since sonar can only measure distance of the target from the transducer.
by Bruce Samson under lowrance lowrance sonar

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