Sonar interpretation?

[Larry3215]

Hopefully, these next pictures wil help visualize how that 3D situation gets converted into a 2D screen image.

Ive taken a series of pictures of a napkin drawing

Imagine the boat is following the green path across the bottom. You are going to drive across a log sitting on the bottom at an angle to your path. The dotted blue lines represent the side to side width of the downview cone.

Every single thing on the bottom that is between those dotted lines will be "seen" and drawn on the screen - in the order of their distance from the transducer. Things in the center will be closest and drawn at the shallowest depth on the screen. Things out to the sides will be drawn deeper on the screen. The left and right sides will be drawn on top of each other.

So this series shows how the screen images goes from a 3D situation to a 2D screen shot - I hope..

[Larry3215]

I can only do 3 images per post, so here are the next two.

This one represents how the two sides get mashed together so they can be drawn on the screen.

This last one shows how it looks from the side as it gets drawn on the screen in 2 dimensions. The left part of the original image AND the right part both get drawn on the screen on top of each other.

Sometimes you get some useful information from this and sometimes not so much.

As before, a flat bottom works much much better. Sloped or irregular bottoms mess up downview the same way they mess up sideview and 2D images.

[TrackerPro16]

Good info! It was hard to concentrate on the lesson as I was looking for stains on the napkin...

[Larry3215]

LOL!

[Amx]

He probably had a fishwich. With tartar sauce.

[Larry3215]

He probably had a fishwich. With tartar sauce.

LOL Good thing I didnt use that red sauce ;)

[Larry3215]

Ok, so lets look at how to minimize the problems a sloped bottom causes. Once you understand whats going on, you can take steps to get better side and downview images.

Most of the problems/distortions on side and downview come from running parallel to a slope with the bottom rising on one side and dropping off on the other. Sideview and downview both assume the bottom is flat side to side.

You can reduce the distortion to a very large degree by running the boat uphill or down hill instead of along the side of a slope. By running up or down hill, the side/down cone will see a relatively level bottom side to side - which is where it counts.

Imagine you are the transducer and you are putting out a wide cone to each side of the boat. Think of that cone as a long pole sticking 50 ft out to each side that needs to stay level to work properly.

Then think about running UP or DOWN a hill. The pole will not hit the ground on either side and will stay relatively flat and level to the ground all along its length.

Now try to take that 50 ft long pole and run along the side of the hill. The pole will hit the ground on the UP side and stick way out in the air on the down side.

The best way to decide which way to drive the boat for the best images is to look at your chart.

For the best sidevew and downview images you need to be traveling across and at right angles to the contour lines. If you are driving parallel to the contour lines, Your images will be distorted tot he degree the bottom is sloped.

This is a shot from American Lake. You will get the best images from traveling along the green lines and the worst when traveling along the red lines.

Of course, sometimes you really dont have any choice - especially for looking at things close to the shore. Plus, a lot of the time you will want to be trolling at a constant depth - which means going parallel to the contour lines.

[Larry3215]

To go along with that, here is a portion of Lake Washington.

Where the contour lines are far apart, the bottom is relatively flat and it wont matter much which way you are traveling.

The closer the contour lines are to each other, the steeper the bottom slope is and the more it matters as far as image quality.

In this example, where I have the red arrow, it wont be distorted nearly as much as it would running closer to shore where the lines are much closer together, but it will be more distorted than the green arrow.

[Sideburns]

Bump!

Just wanted to reassure you that there is at least 1 interested student checking regularly for updates! Don't want you to confuse lack of response for lack of interest!

[Amx]

There is?

[kokapaw]

Bump!

Just wanted to reassure you that there is at least 1 interested student checking regularly for updates! Don't want you to confuse lack of response for lack of interest!

Diddo !!

[Larry3215]

Thanks guys! Really appreciate the feed back

Unless there are some questions, comments or arguments, Im just about done. I want to talk about the new multi-beam sonar options briefly but I need to get my notes in better shape first and take a closer look at the new Furuno multi-beam offering that was just announced. It looks like Garmin finally has some competition in that area.

[TrackerPro16]

Wife and I did the maiden voyage with the new toys on the boat this morning. The boat ramp was crowded with two other trailers in the parking lot!!!
It was fun playing for the couple hours I had but the wind Made it difficult to be consistant. The info in this thread was put to good use understanding what I was seeing on the screen. Thanks!

[Larry3215]

Glad it helped!

[Sideburns]

oh no, You cant be done! But what about 3d, and phased array technology, and 360 imaging and

[Larry3215]

oh no, You cant be done! But what about 3d, and phased array technology, and 360 imaging and

LOL. I will cover a lot of that when I get to multi-beam and Panoptix.

360 is easy - its sideview spinning in a circle. Typically less range than regular sideview and subject to all the same issues and problems and distortions that regular sideview has plus a few of its own quirks. The distortion gets weird at much of a distance because of the built in skewing that goes on with the circular scanning.

It can be useful under the right conditions but I think its even worse for seeing individual fish than regular sideview - especially at any distance. For the cost, I personally dont think its worth it, but some people absolutely love it.

A forward looking multi-beam sonar like Panoptix, that you can turn in any direction, beats it by a mile as far as finding fish.

[TrackerPro16]

Here is my removable transducer mounts for my boat I made out of 1 1/4" PVC Schedule 40 and two removeable fishing pole holders. The PVC seems to be plenty strong enough up to the 4 mph I tried it with. The Panoptix mount I made by boring out a 'T' fitting so it will swivel completely around but is tight enough it will hold it in any position I put it in. With this set upI can put them on my smaller boat if I want to. I put a smallish 36 amp hour sealed battery in a 4 gallon square bucket, made a wood platform That sits down in that and mounted my sonar on that so I can move it around wherever I want. So far so good!

[Larry3215]

Very creative setup!

Be sure to get some screen shots for us!

[Larry3215]

It occurred to me that I forgot to cover a few things on the subject of 2D sonar that might be of interest. They will also help with the lead in to multi-beam sonar.

I mentioned cone angles and went over that some, but here are a series of drawings and screen shots that cover that area a little better and, I hope, illustrate the differences in how things are shown on the screen and how you can use that info to locate fish.

This is a screen shot I got from a user on another board where we are having a similar discussion. This screen shot is split screen with 200 khz on one side and 77 khz on the other - both running at the same time covering the same water. AS you can see, the screens look very different even though they are both covering the same water at the same time.

The key thing to remember here is that lower frequencies - like the 77 khz here, have wider cone angles than the higher frequencies - 200 khz in this case. Those differing cone angles are why the two sides look so different.

The first thing to notice is the fact that there are a lot more and larger arches and returns showing up on the 77khz side than on the 200 khz side. Thats almost certainly due to the different cone angles. The 77 will have a much wider cone angle than the 200. Those extra arches showing on the 77 screen are probably fish off to the side of the boat that are mostly or all the way outside the cone of the 200 khz frequency.

Even though you are looking at the same fish on both screens, the arches are larger on the 77 khz side because the fish stay in the wider cone longer than they do in a narrower cone as you pass over them.

There are fewer fish in the 200 cone because its narrower and only sees the fish more directly under the boat.

[Larry3215]

Here it is again. There are a couple of more subtle things going on that you can see in this screen shot along with a couple more of my crude drawings to illustrate whats happening.

Imagine 3 fish all exactly equal size and equally spaced under the boat and all at the exact same depth. The red fish is directly under the transducer and the two green fish are the same distance off to each side.

Notice that you only see two fish in the 77 screen shot, but it gets even worse on the 200 screen shot because you only see one fish. On top of that, none of them look to be the same size or at the same depth.

The arches on the two green fish are shorter on the 77 side because they dont stay in the cone as long as the red fish. They get drawn on top of each other - and at the wrong depth - and blend into a single arch because they are at the same distance from the transducer. So two fish blend into one arch that is shorter and thinner than the red fish arch.

On the 200 side, you only see the red fish. That fish spends the shortest time in the cone so it has the shortest arch. That arch also isnt as tall as the arches on the 77 side bacuse the difference in distance from the edge of the cone to the center is less with a narrower cone. The top of the red arch will be at the same depth on both sides, but the start and end of the arch will be deeper on the 77 side.

The main point here is you cant really tell how big the fish are or where the fish sit in relation to each other, or the boat, based on where the arches are drawn on the screen. The wider the cone angle, the greater the differences can be. The narrower the cone angle the less differences there will be.

To sum up - narrow cone angles give more precise information but show fewer fish and cover less bottom and less water. Wider cone angles will show more fish, but you will have less of an idea where they are side to side and less idea of their exact depth.