How To Read A Fish Finder: 2D Sonar

How To Read A Fish Finder: 2D Sonar

Thanks to unparalleled advancements in technology over the last three decades, we now have unprecedented access to what lies beneath the water. From the most basic 2D sonar to the latest in forward-facing technologies, there’s a wide range of capabilities available across a vast spectrum of price points for anyone who wants a better look at the world below and around their boat.

Here’s a shot from Humminbird’s website of their HELIX 5 SONAR G2. The great thing about 2D sonar is the price point, this graph priced at $219. Though 2D is the most basic version of sonar, it can tell you a lot for a little. Let’s look closely at this shot to see what all we can see.


The basics -


We can immediately see the readout in the bottom left corner that shows the depth, speed and water temp. Looking at the right of the screen, there’s a scale that shows the depth range from 0 to 40 feet. These depths are broken up into 10-feet increments with horizontal lines across the graph to make it easy to identify the depths of objects under the water. This reading is being generated by a transducer mounted either below the trolling motor or near the outboard motor, likely near the outboard since the speed reading is 4.5 mph.

The most recent reading is to the far right of the screen. This is what was directly beneath the transducer at the moment this image was taken. Everything to the left is what has already been passed over by the transducer, a recent history reading if you will. To breakdown this image then, we can see that the water directly beneath the boat is between the 20- and 30-feet depth lines, which supports the reading in the bottom left corner that states the boat was in 28.2 feet when this image was taken.

If you move to the left of the screen, you can see that the boat passed over water that was around 33 feet deep, then what looks like a piece of cover or a hump that stuck up a few feet off the bottom, with baitfish and fish above it from 10 feet deep down to about 24 feet. This is what is possibly indicated by the blob of red, yellow and purple color. The white is water void of objects and the solid yellow line near 30 feet is the bottom of the fishery.

It’s important to note too that the yellow reading indicates a “harder” return, the red being a “softer” return. This can tell us a few different things. One, that the angler has driven directly over something soft or small, like a small fish or mucky bottom. Or, there’s a chance that the angler didn’t drive directly over something but instead just off to the side of the object, and thus the return of the sonar ping was a little lighter. This uncertainty is possible because of the shape of the sonar beam that’s being shot out of the transducer.



A cone, not a thin beam -


It’s important to understand that the transducer is actually shooting a cone down to the bottom, not a straight-line beam. Don’t think of the sonar as a laser pointer. Think of it more as a flashlight beam. The same way the beam of a flashlight spreads out the farther it is from the face of the flashlight, this is how the cone of a sonar works. If you shine a flashlight directly at something, it’s much easier to see a clear image of the object. But something on the edge of the flashlight beam is a little harder to see. This is the same concept.

Taking this into consideration, we can look at this image again and see that one of two things is possible. Either the transducer passed directly over a school of smaller fish (indicated by the red and purple blob) with a few bigger fish among them (indicated by the 7 or 8 more defined yellow markings). Or it is possible that the angler actually idled just off to the side of a tree top, and these yellow readings are 7 or 8 larger fish sitting in the limbs of the tree.

If you were in this situation, there would be two simple ways to find out. You could turn the boat around and move over about 10 feet and idle back by the same area a couple times, to see if you could idle directly overtop of whatever it is. If it was a tree, you’d get a clearer, harder reading revealing even the base of the tree making contact with the bottom. Or, you could shutoff the engine, take a deep diving crankbait out and reel it through the area where the reading was. If it was in fact a tree, you’d make contact with the branches within the first few casts.


My interpretation -


My guess after having used 2D sonar a lot over the years, this is a tree that was just off to the side of the transducer as the angler idled by. The bottom composition is pretty hard as indicated by the thick yellow reading. With only an inch or two of softer sediment on top of it, indicated by the thin red line. There are at least 8 crappie or bass in the tree, shown by the yellow readings between 15- and 25- feet.

These are also most likely crappie, since they group up in larger numbers and most bass would be too territorial to sit in a treetop with at least 7 competitors— likely more out of sight of the sonar on the other side of the tree. All that being said, it would still warrant a cast with a crankbait or a drop with a dropshot. Even though most of these fish are likely crappie, there is likely a bass or two in the mix as well.


What we don’t know -


One of the challenges to 2D sonar is that there is no horizontal scale. Again on the right of the screen, there’s a vertical scale that can be used to determine the depth of an object. But there is no horizontal scale to know how far behind the transducer the object is. This is simply impossible to show on the screen, since the boat can be moving forward throughout a wide range of speed.

To use this image again as our example, let’s just say that the treetop was 15 feet behind the boat at the exact moment this image was taken. Based on the reading in the corner we know that the boat was traveling at 4.5 mph at this moment, and let’s assume it had been for the last few minutes.

So at 4.5 mph the treetop was 15 feet behind the boat. If the image looked the same but the boat had been moving at 9 mph, the treetop would be 30 feet behind the boat at the moment this image was taken. If the boat had been moving at 2 miles per hour, it would be closer to 7 feet behind the boat. The point is the distance is constantly fluctuating based on the speed the boat is traveling.

So there is no scale to indicate how far back the structure is, but is certainly close enough to figure out. By marking a waypoint on the graph as you pass over the cover, or by tossing a marker buoy overboard shortly thereafter, you can really hone it on the location of the cover pretty quickly. Sonar

In conclusion -


Reading 2D sonar is fairly easy to get the hang of once you get out on the water and try it for yourself. Don’t be intimidated by the technology as it is very user friendly and you can learn a whole lot from it. Remember the basics, whatever is to the far right of the screen is the most recent information about what is below the boat.

And the rest to the left is a recent history of what the transducer has passed over. Know what the colors on your particular graph indicate when it comes to the strength of the signaled returned, so that you can better tell what you’ve crossed over and how directly you crossed over it. These are the basics, and they’ll help you become an expert at reading sonar in time.

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