THE BASICS TO INTERPRETING SONAR
By Lance Valentine
Anglers spend thousands of dollars each season on buying the latest fishing electronics. Obviously, good electronics will help you find more fish faster and should increase your catch. But so many anglers have top of the line electronics and don’t understand how to interpret what they are showing. Below are a few things to think about to help you better understand what your sonar units is showing you on the water.
Understand Coverage Area
While sonar looks into the water to show anglers what lurks below, it does it in a very defined, small area. Cone angles are usually measured in degrees of coverage, and coverage area will vary with transducer design and power. Traditional 2D sonar mostly uses cone angles from 8 degree up to about 45 degrees. The wider the angle, the more coverage area BUT the less definition is shown. Narrower cone angles show amazing detail, target separation and resolution, but cover a smaller area. Wider cone angels cover more area but show less detail. A good trade off for 2D sonar is about a 20-degree cone angle. So how much of the water column are you covering with a 20-degree cone angle? The diameter of the cone is only about 1/3 the water depth. So, in 20 feet of water your 2D sonar coverage is only about a 7’ diameter circle!
This small coverage area means that if you see something on your sonar screen, it is happening IN THE CONE, and not too far away from the transducer. It also dictates how fish show on your screen as long marks, arches or just short marks. Understanding how small the cone angle is also explains why seeing fish arches is not a good thing most times.
Know How Sonar Works
In the three decades I have been teaching sonar, I would confidently say that only about 5 percent of anglers really understand how sonar works. Sonar sends a sound wave, that we call pings or pulses, down into the water at a steady 4800 feet per second in fresh water. That ping hits an object and returns to the transducer. When it does, the sonar receives, measures, interprets then displays two different pieces of information for each ping. First, is the TIME it took the ping to return. That time is then calculated, converted into a number then displayed on the screen. Second, is the intensity of the return, or how much of the signal came back to the transducer.
It is very important to understand that sonar measures DISTANCE NOT DEPTH and therefore we see what we see on our screen. Remember, sonar travels through the water in a “cone” that is narrower at the transducer and gets wider as the water gets deeper. If we look at it from the side, it looks like a triangle. Now we need to add some geometry into our understanding of sonar. If we draw a line straight down the middle of our coverage triangle, then draw a horizontal line out the edge of our cone we have created a right triangle which lets us do some easy math. The picture on the right explains it very well. The distance from the transducer to Point A is FURTHER than the distance from the transducer to Point B, EVEN THOUGH POINT A AND POINT B ARE AT THE SAME DEPTH!!!
So, when a fish enters the cone, it will show as “deeper” on the screen than when that fish is in the middle of the cone where it will show as “shallower”, even though the depth of the fish NEVER changes! Hopefully you just had a light bulb go off in your head about fish arches. Fish arches are ONLY created when a fish enters the cone (further away from the transducer), swims through the middle of the cone (closer to the transducer) then continues out the other side (further away from the transducer).
Understand How Sonar Displays Distance (Depth) Information
Okay, we have covered some basics of sonar, so how does this show up on our screen? Remember those pings, well the sonar unit is sending about 15-18 per second, and each one gets the distance and intensity information and displays it on the screen.
Your sonar screen is made up of small squares (pixels) arranged in rows and columns. The most current information (ping) is displayed on the column that is on the furthest right of the screen. Anything to the left of that column is “history”. So, when the sonar unit gets distance information from a ping it “turns on” a pixel in the far-right column that corresponds to the “depth” scale on the unit. This then continues 15-18 times per second, resulting in the continuous display we see.
Back to fish arches. Think about what we just learned and how fish arches are created. A fish hits the outside edge of the cone, and the first ping hits it. The distance is recorded (21’4” in our picture) so the display turns on a pixel at 21’4” on the screen. As the fish moves through the cone, the fish is closer to the transducer as each ping hits it, and each new ping turns on a pixel at a closer distance. Finally, the fish is in the middle of the cone, and the ping records the closest distance to the transducer the fish will be and turn on a pixel correlating to that distance (depth). As the fish moves out the other side of the cone, the same thing happens in reverse. We have a fish arch!!!
Understanding that Fish Arches aren’t Always Good!
Let’s wrap all this up for now. To make a fish arch, a fish must hit the outside edge of the cone, travel in a straight line through the middle of the cone and out the other side. Remember, our cone angle in 21’ of water is ONLY 7’ across with the transducer in the middle (3.5’ from the transducer to any part of the outside edge of the cone). What are the chances of a fish actually traveling in a straight line completely through that little 7’ circle? Chances are slim.
But, if we do see an arch, here is what we know….the fish traveled through the cone’s circle, which means it DIDN’T MOVE as we traveled over it, and fish that aren’t moving are INACTIVE fish that probably won’t bite! So, while they look “cool” on the sonar screen, they are not a good indicator of moving, active, feeding fish!
Hopefully this gives you a better understanding of the science behind sonar and what you are seeing (or not seeing!) on your screen. In the future we will dig deeper into more sonar science, but for now take the time to learn these sonar facts and understand what you are seeing. It’s the first step to getting the most from your sonar unit!