Questions on how to properly adjust sonar units and decipher what is shown on the screen have come up many times on this board. The following is what I have learned from using several different graphs over the years and may be of some help to folks trying to get the most out of their sonar units. A lot of this is pretty basic, long and drawn out (boring) but I've tried to make it helpful for someone that has never used a graph before.

Fish Arches - What Am I Seeing?

In order to get a “fish arch” (like the manufactures lead us to believe we should be seeing on our "fish finders") a fish must pass completely through the “cone” or view of the transducer from one side to the other within a narrow range of speed depending on several variables without changing depth when either the boat passes over the fish or the fish swims under the boat.

Think of your graph screen as a composite picture of individual sonar returns. The transducer sends intermittent sound waves or “pings” down into the water and records how long it takes for each ping to return to the transducer as well as how loud that signal is when it bounces back up from the bottom or suspended objects and returns to the transducer.

The returning sound waves ("return") are "painted" on the graph screen by filling in pixels with shades of gray or colors depending on whether you have a "Grayscale" or a color graph. The sound waves are broadcast into the water in somewhat of a "cone" or circle that is narrow at the transducer and widens until it meets the bottom. Only objects that are in the cone or view will return sound waves and show up on the screen.

Think of each horizontal pixel on your screen as one ping or one still photograph of what was in the view at that instant. As long as the object being painted on the screen stays in the view of the transducer the pings will continue to paint or fill in the pixels across the screen. Think of each vertical pixel as one volume setting. The louder the ping comes back the more vertical pixels it will paint and the darker or "louder" colors (on a color graph) it will paint them depicting higher volume. Higher volume means the sound wave bounced off a larger and/or denser object.

Also, note that the display on your screen is simply a short history of what your boat passed over or what passed under your boat. The most recent information comes into view on the right side of your screen and then moves across and off your screen to the left. Depending on your boat speed and scroll speed (speed at which information passes across your screen), what you're seeing on your graph isn’t necessarily under your boat and can actually be quite a ways behind you if you are moving.

Arches are created because as a fish passes through the view or “cone” of the transducer it is farther from the transducer when it is at the edge of the cone and closer to the transducer when in the center of the cone. The width of the arch depends on several factors including, how fast the fish passes through the cone and also how fast your Scroll Speed is set.

The cone angle also affects fish arches because at a given speed (boat or fish) and scroll speed it takes longer for a fish to pass through a wide cone than a narrow cone at any given depth. Varying depths of fish also affects the arches because the cone gets wider as it gets deeper so the deeper a fish is the longer it takes it to pass through the cone and visa versa because of the difference in the distance across the cone at different depths.

The faster the fish passes through the cone and/or the slower your Scroll Speed is set the shorter the arch is going to be up to the point where it is just a blob instead of an arch. The slower the fish passes through the cone and/or the faster your Scroll Speed is set the longer the arch is going to be up to the point that it goes all the way across your screen.

If the fish stays in the view of the cone - either because the boat is not moving or the fish is moving at the same speed and direction as the boat – and doesn’t change depth then you’re going to see a solid line all the way across your screen instead of an arch.

If you’re sitting still or moving very slowly over a brushpile the only way you’re going to see fish arches is if a fish swims through the view of the transducer fast enough – probably somewhere between .5 and 3-mph depending on a variety of factors as mentioned above.

If you get over an active school of fish that are moving up and down in the water column you're going to see lines that go up and down as they scroll across your screen. Crappie however are not usually that active and if you’re sitting or moving very slowly over a brushpile you can expect them to show up as solid lines across your screen rather than arches.

If a fish enters the view of the transducer and leaves in less time that it takes for that information to move across the screen but not fast enough or centered enough to make an arch then you’ll have a line on your screen that starts and stops.

To see actual sonar returns - lines or arches - and no fish symbols turn your Fish ID off. Set your overall sensitivity so the bottom shows up fairly dark (hard return) and cover – brushpiles, bamboo condos or weed beds, etc. – show up a lot lighter (soft return). Most graphs now have several different “views” – Whiteline or Grayline, Inverse, Structure ID, Bottom Black, etc., which are a variety of ways information is displayed on the screen. I have mine set to Whiteline (Humminbird) or Grayline (Lowrance), which seem to be the best for differentiating between the bottom and cover.

How Do I Adjust My Graph For Optimum Performance?

First, make sure your transducer is pointing straight down and be sure there is no oil or wax residue on the face, which could cause small air bubbles to stick to the face and interfere with the sound waves. Also, make sure you have a good connection where your transducer cable connects to your graph - there should be no moisture or corrosion in this connection.

Depth or “Keel” Offset

The very first adjustment you should make on your graph is for Depth Offset or Keel Offset. This is an adjustment to correct for the difference between the depth of your transducer and the surface or the difference between the depth of your transducer and lowest point of your boat that is apt to run aground. For fishing purposes you’ll probably want to make adjustments so your depth reading is the actual depth from the surface to the bottom of the lake.

The easiest way to accurately make this adjustment is to position your boat over a smooth, level, hard bottom and measure the depth with a tape measure or string with several ounces of weight at the bottom and a mark for a known depth. Then adjust your Depth Offset or Keel Offset up or down until your depth reading matches the actual measurement from the surface to the bottom.

Changing the Depth Offset or Keel Offset may or may not correct the Target Depths when using Fish ID or the Grid Lines on your screen depending on the graph you have. To find out, put a 2-ounce bank sinker on a length of fishing line or string that is marked at 15 or 20-feet and matches a Grid Line on your screen. Then lower the sinker directly under your transducer to your measured mark and see if the return from the weight shows up at the Grid Line of the same depth.

You can also check your Fish ID to see if the Target Depths are corrected by turning up your Fish ID sensitivity until fish symbols start showing up on the sonar return from your weight. If the depth of the Grid Lines and Target depths are accurate then they have been corrected with your Depth Offset or Keel Offset. If not then you’ll need to mentally adjust for the depth of fish and cover returns if you want to know exactly how deep they are, which can be very important when crappie fishing.

Adjusting Sensitivity

This is probably the next most important adjustment on your graph. I think one mistake folks make is to assume that a higher sensitivity setting means you see more on your graph, which isn't necessarily true. Increased sensitivity simply makes everything darker (or more of the colors that depict more solid objects on a color graph) and decreased sensitivity makes everything relatively lighter (or less of the colors that depict solid objects on a color graph).

With sensitivity properly set, harder (rockier) bottom will show up darker because stronger signals are coming back to the transducer and mud or silt bottom will show up lighter because some of the signal is absorbed and less of the signal is returned to the transducer. Increased sensitivity also shows more surface clutter and electrical noise and visa versa.

You can distinguish between cover and the bottom because wood, bamboo, weeds, etc. have a different density than rock, silt or mud. Knowing what type of cover you are over, either because you put it there or have seen it when the water level was down, helps a lot in learning to identify what you’re seeing on your graph.