SideScan Basics-Part 1

By Lance Valentine

Over the past few seasons SideScan sonar has become much more popular and for those who understand how to interpret and use SideScan, it has become a tool we could not fish without. The basic idea of SideScan is the ability for the angler to actually see the bottom, fish, structure and other details up to 300 or more feet to each side of the boat.

Using a large, multiple element transducer (the Lowrance LSS-2 uses 3 elements: right, left and down) anglers can get an entirely new view of the underwater world they are fishing. The “cone angle” of the side looking elements are comparably small (about 3 degrees) to traditional sonar to focus the sonar ping and get high definition returns on the screen.

Let’s get started on some basics to interpreting your SideScan picture. Take a look at the picture on the right. This is a pretty typical and detailed SideScan screen showing fish, timber, rocks and bottom transitions. Before we get into how to interpret what we see, let’s take a minute to understand HOW we are seeing the lake bottom on the screen. Our current location is at the top center of the screen and everything heading to the bottom of the screen is history; bottom we have driven over. Spot A1 is the location of the boat (transducer). Line A is the path of the SideScan transducer. Lines B and D are the bottom of the lake at the inside edges of the SideScan elements. The best way to demonstrate what you are seeing is to do a little origami! You can either print the picture (best way) or simply grab a piece of paper and draw the 3 lines on it. Now fold the paper in half at the middle line and then fold the paper at the other lines so the outside edges of the paper is folded toward the middle line. Hold the paper flat in front of you the way it would look on the screen. Push the 2 outside lines together under the middle line, which should cause the middle line to be forced “up” (see example). Now rotate the paper 90 degrees away from you and you now have a better idea of what part of the bottom you are seeing.

Since the screen displays a 2-dimensional representation of a 3D world, the picture can be tough to interpret and understand, especially the first few times on the water. Here are a few guidelines to help you. First, think of the transducer as the sun and the lake bottom as the earth. In our above water world, anything between the sun and the earth will cast a shadow; the same is true of SideScan. Any object that is standing up off the bottom, will cast a shadow directly relative to the location, position and height of the object. The further a shadow extends behind an object on the screen, the higher off the bottom the object is. In this picture you can see multiple trees off to each side and each casting a different length shadow, meaning they are different heights off the bottom. This function also helps in determining what types and how high the weeds you see are, if you are seeing gravel, rocks or boulders and if an object is attached to the bottom (structure) or on the bottom (fish). If an object is attached to the bottom (wood, weeds, rock etc.) the shadow will actually touch the return of the object at its furthest point from the transducer (see pic). If an object is on the bottom (fish) the shadow and the return for the object will NOT touch (more on that in the next segment.

Second, the brightness of the “mark” will determine its relative “hardness”. Bigger fish and harder bottom will be “brighter” on the screen than smaller fish and softer bottom. Notice the change in bottom hardness shown by the bright to dark line on the picture shown. Being able to see these transitions off to the side of the boat is one of the key angling advantages to using SideScan. We can now position the boat EXACTLY where it needs to be to run our lures on the bottom transition line! Also by combining SideScan imagery with the Route function on the Lowrance HDS, we can create a route exactly on the transition line and have our MotorGuide Xi5 follow exactly on the transition! When interpreting fish returns, be careful about confusing how they are made with 2D sonar versus SideScan. Because the cone is so narrow on the SideScan elements, big fish may be very “small” marks since they will not stay in the cone very long. Focus on the relative “brightness” of the return to help determine fish size. We have included some example pictures to help make this clear.

This is the first part of a multi part series on SideScan interpretation. Understand the basics covered here, get on the water and practice and you should start making more sense of your SideScan returns. See you next time!

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