Running multiple graphs at the dash and at the bow allows you to collect as much information as possible, all at once, which helps determine how and where to present lures.
February 27, 2025
By Thomas Allen
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Having already evaluated 2D and side-scanning sonar in this series , another technology—down-scanning sonar—brings perspectives together to increase recognition of how structure plays out. That collective information will assist in achieving the best boat position to make more accurate casts, which means catching more fish.
Incorporating down-scanning sonar into your fishfinder readouts helps establish a digital picture of what’s below. Then utilizing down-viewing in conjunction with standard 2D sonar adds dimension to what you’re looking at or searching for. Short of draining the lake, these combined perspectives provide a tremendous amount of critical information.
Regardless of the brand of gear you have mounted on your boat, understanding the lingo is the first step. For example, Humminbird calls their down-scanning technology Down Imaging , Lowrance refers to it as DownScan , and Garmin’s is DownVü . Each version basically accomplishes the same thing, but with variances in color and some minor differences in settings adjustments.
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2D vs. Down Scanning In general, down-scanning relies on sonar returns provided by the 2D transducer, which are then inserted into the product’s algorithm to produce the most accurate, high-definition image possible. Consider this visualization: Cut a LifeSaver-shaped sliver out of the middle of a golf ball—that’s what the inside of a 2D transducer looks like. The center of that slice produces an image that starts out as small as the point on a finely sharpened No. 2 pencil. That tiny point expands downward into a cone-shaped perspective to the bottom, and it views a circle-shaped area of the bottom, the relative diameter of which depends on the cone-angle of the transmitter. For a 20-degree cone angle, for example, the diameter of the area scanned is about one third of the depth. Cone angle is related to frequency, with higher frequencies producing narrower cone angles.
In the middle of this screenshot, you can see the shift in image clarity, accomplished by changing the frequency from 800 kHz to 455 kHz, increasing contrast, and decreasing sensitivity settings. Perhaps there’s a middle ground in this scenario that could be better? Experiment—that’s part of the fun. Down-viewing sonar returns come from a pair of elements located behind the internal 2D sonar and down the middle of the transducer. They’re about to the length of a business card inside the elongated transducer mounted to the boat’s stern. It scans the bottom on a single plane and generates a history as the images move across the screen to create an upright picture from the bottom up. The two technologies work together to generate the final high-definition image. It’s not a matter of picking one technology over the other; it’s understanding how 2D contributes to the down-scanning perspective—both are necessary.
Years ago, late professional bass angler Aaron Martens said: “2D sonar shows you that something is there, and Down Imaging proves what it is.” I’ve found that statement to be 100 percent true. Down-scanning adds tremendous detail so the user can identify what they’re looking at, and if fish are staged within cover. For example, if you go over a standing tree, 2D sonar shows a line up from bottom to the top of the tree with marks coming off either side, but with no real detail. With down-viewing sonar, branches are clearly identified, and perhaps even crappies or bass are separated from the branches and indicated by bright dots. Or maybe nothing is there, and you need to move onto the next spot.
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Transducer Positioning Some anglers place a unique 2D transducer inside and on the bottom of the hull of a fiberglass boat, as the sonar will read though fiberglass (not aluminum). Others might place the transducer on the pad of the hull where it can collect readings from clean, bubble-free water while running the boat. An application for this might be marking at speed so you can collect 2D data while the boat is in motion. Note: it’s only accurate up to about 25 to 30 mph before the images become quite distorted.
This is a fine example of how to use 2D alongside down-scanning sonar. You can see a cluster of something staged near a drop-off, but the unit’s Down Imaging creates more separation and shapes, helping the angler identify the cluster as baitfish. There are also times when having depth lines turned on is helpful. The side-scanning and down-scanning transducer is typically mounted on the boat’s transom at a level above the water when the boat is on plane and running so it doesn’t kick up a “rooster tail” of water, but the transducer is then fully submersed when the boat is at rest or idling. Plus, it’s not as productive as 2D at producing legible images while the boat is on plane. Being able to collect information while running is important, so often mounting a separate high-speed 2D transducer is an option, but the side- and down-scanning transducers come standard with built-in 2D sonar. Consider your personal fishing preferences and electronics applications, and rig accordingly.
Down-Scanning Options Much like every other form of fishing electronics, there are settings adjustments to consider when hunting for the next spot. As with side-scanning sonar, contrast and sensitivity are settings you should work with regularly. If the water is seemingly cluttered indicating an algae bloom or maybe mud from runoff, adjusting sensitivity can increase image quality. Or, perhaps the body of water you’re fishing has stratified, with denser, cooler water below the thermocline showing more color. In this case, contrast and sensitivity adjustments can help sharpen the edges, but focus on the top half—or what’s above the thermocline—as that’s most likely where your targeted species are swimming and feeding.
In both scenarios, if you’re looking to increase structure and cover viewability, increasing sensitivity might make larger objects pop more, while increasing contrast will adjust the lights and darks in the image to help isolate fish or structure and cover elements.
It’s a give and take: If you increase sensitivity, more clutter may appear making it more difficult to see denser items like fish. Increasing contrast might make fish returns appear smaller.
Plenty of color palettes are available to look through. Some work better under harsh light while others might be best during twilight hours or cloudy days. Experiment with what produces the best viewing for you.
In the case of Humminbird’s Down Imaging, you can select between several frequencies, including 455 kHz, 800 kHz, and Mega, which is 1,200 kHz. Largely depending on water depth and temperature, you can work through different frequencies to produce the best image. In deeper water, say 40 feet and deeper, 455 kHz might return the best resolution while 800 kHz might be best in middepth ranges of around 16 to 30 feet and Mega in ranges of 6 to 12 feet. These aren’t hard-and-fast rules and vary from lake to lake. Again, experiment to your liking.
This is another example showing 2D sonar alongside down-scanning sonar—you can see something obvious on the bottom on 2D sonar, and the unit’s Down Imaging separates and clarifies what’s there. Bringing It All Together The goal is to create a clear, discernible image of contours and structure and cover you plan to fish, and if fish are present, to identify how deep they are and how they’re relating to the structure or cover you’re scanning. The best way to master this is time on the water. I like to run a split screen with 2D on one side and Down Imaging on the other to help locate and identify specific structural and cover elements, while the second dash unit will feature Side Imaging and LakeMaster mapping .
I look for structure with Side Imaging, then once I’ve found something interesting, I drop a waypoint and turn the boat around and drive directly over the waypoint and view the structure with Down Imaging. The point is to create an above-surface perspective of what’s beneath the surface, which helps in bait selection and presentation.
Regardless of the species of fish you’re after, comparing multiple sonar perspectives generates useful information to consider when it comes time to fish. The more information you can gather along the way makes each cast more productive than the last.
