Interpreting your display means more fish

Left: Vertical sonar, ceates exaggerated blurry images. Right: SI beam, fish and branches are visible.

Ever since the Lowrance "green box" hit the market, dedicated anglers have had one or two sonar units in their boats. One transducer typically shoots through the hull or off the transom with the display on the console, and another is attached to the trolling motor, screen facing the angler in the bow. This configuration is so common a casual observer might assume most anglers understand how sonar works and know how to use these units effectively. But, if the questions I get from anglers are any clue, this isn't so.

What Sonars Show

and Fail to Show

Less experienced users don't realize that vertical sonars can't make an actual picture of the bottom. Their images are composites, made up of echoes received from everything within a given range at a given time.

Sonar as a Distance-Measuring Device: As sonar pulses go out, anything an equal number of microseconds away is combined into a single blob on an LCD screen or a flash on a flasher. The size of the blob is usually determined by the pixel size of the unit and proximity of objects, including fish.

Two, 5, or 10 fish at the same distance from the boat all look like one fish and create the same size blob unless the machine has an automatic filter to eliminate weaker echoes. The closest point on the bottom, or a suspended object, marks first. Anything nearby is merged into the bottom return unless a large gap exists. For example, a fish a few inches off bottom, say 32 microseconds away from it, will be hidden if the nearest peak of the bottom is 31 microseconds away.

Unit Features: Contours displayed on the screen vary widely and depend on boat speed, pulse or ping speed of the unit, and chart display speed. A steep gradient underwater can look like a gradual slope to a slowly moving boat with fast ping and chart speed. A gentle slope can appear like a cliff to a fast moving boat with a slower ping and chart speed. Although it sacrifices memory, most freshwater anglers who aren't fishing very deep water find a maximum chart and ping speed optimal. Some units are sold with a factory setting of about 50 percent of maximum speed, so every user should check this feature and reset manually if needed.

If neither the boat nor a target fish is moving, vertical sonars receive echoes from the same objects over and over, creating a horizontal line across the screen. A large tree or anything else off the bottom will look like a flat object or false bottom. It takes boat or fish movement to change the shape of sonar images.

Reflectivity: All objects are not equally reflective. Most anglers understand that a rock or hard bottom returns a strong echo signal, while a thick mud bottom sends back a weak signal. This is revealed by the grayline feature, or differences in the bottom signal width. The size, shape, and angle of an object relative to the transducer also affect the strength of returning echos.

Consider so-called "stealth" aircraft. Their surfaces are canted at an angle away from radar so they return weak echoes that are hard to detect. The same is true of sonar, except that a density difference from water is more important to sonar reflectivity than the hardness of the reflective object.

Understanding differences in reflectivity of various materials and angular orientation can help anglers identify and see the difference, for example, between a buoy cable and a tree trunk or telephone pole, and differentiate between hard underwater objects like roadbeds or rocks, and vegetation, even though these objects may create identical single pixels on a black-and-white LCD or video sonar screen.

Many branches of a tree or brushpile will be the same time-distance or number of microseconds away from the transducer. Anything closer to another object than a pixel-width in time is blended into one pixel. Trees, brush, and shad schools turn into blobs, even though there are individual branches and fish within them. In contrast, the much narrower beam of a side-­imaging (SI) unit can depict individual branches and fish inside brush and trees. This is virtually impossible with even a narrow-beam vertical sonar.

Shape of Sonar Pulses: Perhaps the least understood factor is the shape of sonar pulses. For the sake of convenience, and to form a more easily understood image, manufacturers have illustrated the outgoing signals as cone or tear-drop shaped. In reality, the area of peak ­signal strength is so shaped, but the edges of that cone are not sharply defined. Total coverage is more like that of a half-bowl under and around the transducer, partially blocked by the boat itself.

The bowl of outgoing pulses can create a cloud of returning weak echoes that gradually fades with distance from the transducer. If a unit is tuned to receive weak echoes, the coverage of a vertical sonar can be up to 60 degrees or more around the boat. With high intensity settings, users have reported echoes from buoys and swimmers at the surface. Sonar manufacturers use a variety of electronic filters or clippers to reduce echoes from objects well to the side of vertical sonars.

When sonars are operated in the fish-symbol display mode, the filters tend to eliminate so much clutter that they may not even detect small schools of bait. Most sonars detect baitfish, thermoclines, and lures more distinctly with fish symbols off.

Variation among Sonar Units: Many units can operate using universal transducers, so differences in the electronics themselves create differences in the sensitivity of units that anglers observe. There also are differences in the effect of the gain or sensitivity control among units. Some, like high-end Lowrance products, seem to use fewer filters to remove weak returns. As a result, they tend to pick up the returns from drop-shot rigs and spoons worked within the cone of maximum coverage better than units with greater filtering.

The downside of such sensitivity is that the coverage of these sonars tends to be much wider and can be far to the front, sides, and rear of the boat. This can distort the apparent sizes of cover objects and the shape of underwater structures, making it harder to locate objects depicted by these units since everything they detect is shown to be below the boat. Units with less sensitivity due to greater filtering and those with narrow cones can help anglers locate specific objects under or near the boat, but are less useful for scanning around the boat for baitfish and suspended bass, or for vertical fishing.

When an angler wants to see small items like echoes from a spoon, top-quality sonars generally perform better with the sensitivity turned way up. It may also help to place a small shim under the rear of a transducer mounted on the trolling motor to aim the "cone-of-coverage" slightly forward. This shift requires some mental re­adjustment concerning where fish and cover displayed on the screen are actually located, relative to the boat.

With a widened cone of returns, a fish that appears close to a lure may actually be 10 to 15 feet away, but at the same time difference from the transducer. This disparity helps explain times when anglers fishing vertically see fish echoes that apparently refuse to strike a lure presented on their noses. In reality, the  fish weren't near enough to see it.