May 22, 2023
By Shane Beilue
The two down-facing sonars offered on most chartplotters are two-dimensional (2D) and down imaging, branded as Down Imaging (Humminbird), Down Scan (Lowrance) and DownVu (Garmin). The 2D and down-imaging systems share some similarities in perspective, yet the manner in which they emit and interpret the sonar signal differs dramatically.
How it Works
Both downward sonars display a vertical orientation, as though you were standing on the lake bottom looking at the image from ground level (as opposed to side imaging, which provides a bird's eye view from above). Both 2D and down imaging refresh the screen from right to left, with the newest data (immediately under the transducer) being on the far-right side of the screen and that which is behind the boat scrolling progressively to the left.
Around for decades, 2D sonar emits a lower frequency sonar signal in a cone-shaped pattern—like an upside-down funnel. Any object within that cone, such as the lake bottom, a brush pile or a fish, will bounce the sonar signal back to the transducer. It will be displayed as a two-dimensional image with no depth of field to that image. One of the drawbacks of this 2D image is it can be hard to distinguish specific features within the cover. Also, different underwater objects can look quite similar on a 2D sonar display. For example, submerged brush rising 5 feet off the bottom can be hard to distinguish from a clump of vegetation or even a dense ball of shad near the bottom. However, one big advantage of 2D sonar is in identifying fish, as they are easily recognizable by the classic arch that really pops off the sonar screen. The bigger the fish, the larger the arch it will create on 2D sonar.
Bottom composition can be determined on 2D sonar by the thickness of the bottom band, representing the lake bottom. Harder bottom compositions will provide stronger echoes, creating a thicker bottom line. Occasionally it'll produce a double bottom signal over rocky bottom, whereas soft mud bottoms will absorb much of the echo and display a thinner band representing the lake bottom.
Down imaging emits the sonar signal in a very thin, high-frequency beam fanning out from the transducer, creating picture-like quality with incredible target separation and resolution. The result is that brush can not only be clearly distinguished from other cover types, but you can actually count the limbs within the brush. Hard rocky lake bottoms create a brighter bottom signal, whereas softer silt bottoms show up as a darker bottom image.
Bait balls of smaller threadfin shad will be displayed as dense clouds in the water column with both 2D and down imaging. However, the increased resolution and target separation of down imaging can even show individual fish in larger species such as gizzard shad. Larger gamefish such as bass will be displayed as vertical dashes or dots on down imaging, with larger fish creating larger returns on screen.
Because of the incredible image quality of down imaging, one might assume that 2D sonar is obsolete; however, I still find use for both technologies. I’ll typically run both views side by side on a split screen when idling and searching for offshore bass. As mentioned, down imaging provides a much clearer picture of the type of cover, yet I still like the way 2D sonar displays the fish arches that easily pop off the screen. Because of how quickly gamefish enter and exit the razor-thin sonar beam of down imaging, I can occasionally overlook the small dots or dashes of individual fish if I’m distracted while idling. A fish will be within the conical 2D sonar beam considerably longer. Therefore, the sonar return for fish with 2D sonar is a much stronger display on screen.
That being said, the conical sonar pattern of 2D sonar will produce "false positives" on fish returns, displaying a horizontal limb or underwater cable as an arch on screen. Down imaging provides a "fact check" to the 2D readout by recognizing the object for what it is, thereby allowing me to continue my search for fish elsewhere.
Another great use for 2D sonar is when fishing vertically under the boat in water of 20 feet or greater. Prior to the development of live sonar, 2D was the technology of choice for watching the fish and the lure in real time due to the wider coverage area of the conical 2D signal. Since the boat is stationary, the continuous echo from a bass holding under the transducer is displayed as a horizontal line across the screen, as opposed to an arch. A lure such as a jigging spoon or drop shot rig can be clearly seen descending toward the fish as long as it remains within the cone of the sonar signal. If the vertical line rises to meet the lure, the bite is imminent.
While down imaging can reveal fish, cover and the lure while the boat is stationary, these objects don’t remain within the thin wall of the down-imaging beam for very long, making down imaging a better choice for when the boat is moving forward.
With the advanced sonar available to the angler today, there’s really no excuse for fishing unproductive water. Without the presence of game fish or their food source on our sonar screen, we should continue the search elsewhere until we find them.