Home Fishfinders Down-Imaging Vs. Side-Imaging Finally Explained!

Down-Imaging Vs. Side-Imaging Finally Explained!

Down-imaging vs Side-imaging


Many anglers who are interested in adding a fish finder to their arsenal of tools become frustrated and confused with the extensive options that they have to choose from, not to mention the distinct features that each product has.

This is unfortunate as fish finders differ pretty significantly from one unit to the next. So you need to select the best sounder for your needs and circumstances if you are to derive the maximum benefit possible.



One particularly confusing aspect of fish finders that many anglers struggle to understand is the direction in which it emits the sonar waves.

Down-imaging fish finders feature a transducer that directs sonar waves down beneath your boat, while side-imaging fish finders direct their beams to the sides of your boat. Both type is inherently better than the other, and both can help you find (and therefore catch) more fish.

In fact, some units rely on both types of imaging and allow you to select between the two. Accordingly, it is important to know what circumstances favor the use of side-imaging technology and when down-imaging units obtain better results.


Understanding the Jargon

Part of the reason side- and down-imaging is difficult concepts to understand is that these terms are often replaced with proprietary names for marketing purposes.

For example, Garmin (one of the leading fish finder manufacturers) calls their down-imaging systems “DownVu,” while Lowrance uses terms like “DownScan” and “SideScan.” Meanwhile, Humminbird directly identifies their units as down-imaging or side-imaging.




• Side-imaging fish finders allow you to scan more water quickly. Because they “look” in two different directions at the same time, they effectively cover twice as much water as down-imaging units do.

• They provide a better picture of the marine environment, as they offer a better point-of-view than down-imaging finders do.

For example, if you are using a down-imaging fish finder, different piles of rocks to the left and right of your boat show up as a single patch of rocks. However, a side-imaging fish finder displays the rocks as being in distinct groups on either side.

• They are very helpful in finding fish behind shallow creeks and bays, where their orientation provides a better image than down-imaging scanners do. Often, anglers with both types of imaging systems switch to their side-imaging finder as they enter shallower water.



• Side-imaging fish finders are typically more expensive than down-imaging fish finders. Although they are worth the additional expense, anglers live in the real world and everyone has a budget.

• These fish finders do not provide a defined image of things below your boat. This can leave you wanting a down-imaging fish finder, should you be interested in learning about a specific piece of cover beneath you.

• They must be operated while the boat is traveling at low speed. This makes it hard to use when you are trying to race across the lake to reach another potential fishing location. This is, however, offset by the ability of side-imaging fish finders to cover more water at a given point in time, thanks to the greater amount of water they scan.




• They are often more helpful when fishing in deep waters. Since you are trying to find the fish in the vertical rather than horizontal plane, down-imaging fish finders are suited for these situations.

• Down-imaging fish finders continue to produce reasonable images, even while traveling at high speed. By contrast, side-imaging fish finders work better at slower speeds.



• Most down-imaging fish finders only rely on one transducer. This provides less information for the computer to work on and therefore produces a lower-resolution image.

• They fail to provide you with as much horizontal information as side-imaging fish finders do. For example, you may be able to determine that a big fish is somewhere underneath your boat about 15 feet deep but you can’t tell what side of the lake he is on – he may be 50 yards to your left, 50 yards to your right or anywhere in between.


Think It Over

Both technologies excel in different ways so be sure to select the model that will improve your ability to find fish. Consider the depth that you’ll fish on and the speed you like to travel around the boat before making a decision.

But also think of other factors like the size of the fish you’ll pursue and the typical cover you’ll fish on.



  1. Hi Ben,
    Your comprehensive analysis is very helpful!

    Can you help me pin down the best sonar technology and manufacturer for my unique situation?
    I’m buying a boat to use for cruising (not fishing, at least not Yet!) on a freshwater lake in North Carolinainthe during the summer, and then in coastal South Carolina salt water in fall/winter/spring.

    The challenge, especially of the freshwater lake is avoiding submerged trees and rocks in shallow water near my house and the inlet to my Boathouse. I’ve got to navigate the hazards under and side to side. During the coastal cruising avoid shallow sand bars at low tide.

    Is side-imaging or down imaging the best for me?….or a combo?….and whatever your recommend for the best unit. I’ll be ordering the boat and specifying the unitfordash mount.

    Thanks so much for a helpful recommend!

    • Hello Rick,

      Unfortunately, neither down-imaging or side-imaging transducers are designed for navigating underwater hazards.

      Of the two, side-imaging systems will work best, but those still only detect what’s to the sides of your boat. What you want is a forward-looking or 360-degree sonar.

      These will be more expensive than normal fishfinders and you’ll need to go slow while using them. Really slow. 3 knots, not 30 knots.

      You do have a few options.

      The Simrad ForwardScan can give you a look at what’s directly in front of you. It’s not known to be very accurate, though it is the cheapest option.

      The Lowrance SpotlightScan Transducer is a side-imaging sonar that mounts to your trolling motor. It connects to Lowrance’s HDS Gen2 display. Rotate your trolling motor to get 360-degree imaging.

      Finally, there’s the Hummingbird 360, which is a side-imaging sonar that rotates on its own. I haven’t heard good results from people who’ve used them, though.

      Of these, I would most recommend the SpotlightScan. Keep in mind that all of these will be slow and short-ranged, so they’re more of an augment to your navigational skills than a replacement.

      Good luck on the water!

  2. Hello,
    I’m looking for the capability to map and understand structure and depth as well as fish finding on small often very shallow rivers. I need to be able to see structure and depth that is often not directly under the boat since I fish from from a drift boat and no techniques I know of actually fish anywhere close to under the boat although getting a good understanding of the bottom contour as I go over it is incredibly important. The article mentions side imaging only works well at slow speeds, can you tell me how slow it needs to be and does the water movement, the boats speed in relation to the bottom or both effect the accuracy? Durability, weather proof, smaller size screen, and GPS locating as well as charting is important, screen size being the lowest priority are important. Obviously putting a $2000 unit on a $6000 oar powered boat is not realistic so cost is a factor like it is for most of us but function is most important. Any recommendations?

  3. Hope you can help
    I recently purchased a humminbird helix 5 chirp si gps. You mentioned shallow water for side imaging , deep water for down imaging. At what depth should I change it from one to the other? I fish in water up to 30 ft. deep.
    When should I use the contour mode? Every video i see, no one seems to use it. Why is that?

    Thanks so much! Lou

  4. Side imaging units also have down imaging. So it’s not like you will have to go without di if you buy a si unit. You buy an di unit and you only get di. You buy an si unit, you get both si & di.


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