Thread: Side image distance measurements

Originally Posted by RustyJig

I was watching a fellow on YouTube, who seemed very knowable, give a class on side image distance measurements and how to determine where a brush pile, stump, etc. is from your boat and he demonstrated it by placing his boat in a parking lot and towing a cooler out to the side and behind his trailer. He proceeded to show how the fish finder displayed this objects distance from the transducer. Everything was clear to my understanding until he said that if the object showed 25 ft to the side of the boat and that you were in 10 ft of water then the object is physically only 15 ft to the side of the boat. He said the transducer shoots a beam to the bottom and then to the side and you must deduct the depth from the displayed distance displayed.

This would have to be taken into account if you threw a marker bouy for instance. But, he said if you use the “mark” button on the fish finder it would mark the correct spot.

Let’s start by acknowledging that calculating distance to any target in the SI image is not simply “subtract water depth from the SI range to get distance to target”...

It is physically and scientifically impossible for any Sonar beam (sound traveling in water) to travel to point A and then change directions to get to point B (as in the common misconception of measuring from the xducer to the lake bottom and then measuring out from that point) ...

Horizontal Distance to any target in the SI image is a ”slant range CALCULATION using the Pythagorean Theory” ...

Using the OP scenario of 10’ water depth and seeing a target in the SI image 25’ away from the SI image center line ...:


Next we will delve into the HB software engineer’s minds to explain why the HB software engineers designed the SI image layout as they did ...

But 1st I will give space for questions ...

Rickie