Thread: Difference between depth and range?

I had ask this question in another thread but I felt like I was hijacking so I deleted it. Let's see if I understand when you see a Lowrance structure scan picture the numbers displayed on bottom it is the depth overlaying from 2D? I always thought that was the range. On a Humminbird you only have the option of showing range?

Looking at DonG's Lowrance Touch 7 thread, the numbers at the bottom of the side image screenshot are range numbers. Not sure if he can change that or not.

Originally Posted by Shellback

Looking at DonG's Lowrance Touch 7 thread, the numbers at the bottom of the side image screenshot are range numbers. Not sure if he can change that or not.

I guess I should have left my question in that thread. If the bottom numbers in SI is range how could determine the depths are not the same between the 2 views?

Water depth isn't determined by SI. Not sure what Don meant when he asked about the difference in DI and SI depth. What I did notice, is the digital depth reading doesn't match the bottom contour depth in the DI snap shot. I guess a Lowrance expert will have to answer that one.

Depth is how deep the water is and range is the distance you shoot off to the side with structure/side scan.

Sonar measures 2 things....

* "Time of the sound pulse ("ping")....thru the water and back"...and
* "Strength of individual echoes in the return" ...

"Time of the sound pulse thru the water and back" ...is what the processor uses to determine distance (range) to the object....

The "Strength of individual echoes" ...is what the processor uses to determine what the processor "thinks" that echo is bouncing off of...

Even the digital depth number is a mere "Range" calculation of the "Strongest reoccuring echo return" (in the beam shape)...to get back to the xducer...

The processor "interprets" this strong reoccuring echo return as [i]"Hey that echo must be the bottom because that echo return is very strong and it's occurring in every ping"...

The "Range Lines" at the bottom of the SI image are just that..."range" (distance)....to a point away from the xducer...

If we tie a 10ft rope to a ball and attach one end of the rope to the xducer....and then we swing the ball in an "arc" (within the confines of the SI beam shape)... this "arc-ed" path that the ball traveled would be the "10ft Range" from the xducer....

So any "echo target" that falls into this "arc-ed" path of the ball ...should show in the SI image directly on the displayed 10ft "Range Line"...

So...in a nutshell...

"Range" is a mathematical derivative of the "time" it took for the sound pulse to travel thru the water, bounce off something, and return to the xducer....

"Depth" (the digital depth number)... is a processed derivative of the "strongest reoccurring echo return" in the emitted sound pulses ("pings")...

Rickie

Thanks for the explanation Rickie.

Rickie's explanation also shows why you get "depth" readings way short of actual bottom depth, when you run across a seriously thick pile of brush. The sonar is picking up the topmost edge of the brush & "interpreting" it as the strongest signal, so it "thinks" it must be the bottom. It even, occasionally, happens when your transducer passes over a submerged log or big tree limb and all the sonar pings are returned from that "depth" ... even though there may be many more feet of water below the obstacle.

I'm referring, of course, to 2d & DI sonar pings ... as SI pings would likely not be affected in the same way ( but, because I don't have SI )

...cp

One additional note...

Most all sonar units that have 2d (HB SI, HDS, etc)....get the "digital depth" from the cone shape 2d beam (and usually from just 1 of the 2 frequencies)...

Dedicated DI units that do not have a 2d piezo...get digital depth from the DI beam...

Some HB DI units...(dedicated DI HB's)...get digital depth from the DI piezo until a certain depth and then switches to get digital depth from the 2d piezo...(I still don't understand this one myself)...

Regardless of which piezo's sound pulse the unit gets digital depth from, the concept is the same...

Digital depth comes from the "strongest reoccurring echo return" in the emitted sound pulses ("pings")...then the processor "interprets" those "reoccuring" returns as the bottom...

And (as Pap said)...it is easily confused by dense brush, surface clutter, thermoclines, thick bait balls, etc...

Rickie