Sonar
Live sonar or side imaging: which should you add to the boat first?
Quick answer
Side imaging searches, live sonar watches. Side imaging scans a wide swath of bottom as the boat moves to find structure fast and costs far less. Live sonar shows fish and lure movement in real time in a narrow beam you aim, and it is the pricier upgrade to add once you already know where to stop.
Garmin LiveScope Plus System with GLS 10 and LVS34
$1,445.00Real-time forward, down and perspective mode sonar showing fish and lure movement as it happens
Check before buying: It needs a compatible Garmin display, a mount, and in practice a dedicated battery. The head unit is not included.
Best for: A first live sonar setup on a compatible ECHOMAP or GPSMAP
Check price on Amazon
Humminbird Helix 7 CHIRP MEGA SI GPS G4N
$999.99 Published specsMEGA Side Imaging scanning a claimed 125 feet either side of the boat as you drive
Check price on AmazonBuy side imaging first. It is a fraction of the price of live sonar, and it does the job of finding water worth fishing in the first place: a wide, fast scan of the bottom as you idle along, flagging brush piles, drop-offs and baitfish you would otherwise drive past. Live sonar, sold as LiveScope, MEGA Live or ActiveTarget depending on the brand, shows fish and lure movement in real time in a narrow beam you point and hold, which is a different job entirely: watching how a specific fish reacts to a specific bait right now, at a spot you have usually already found some other way.
This comparison is built from each system's own published range and coverage figures alongside the published trigonometry behind how a sonar beam actually covers water.
Two different jobs, not two versions of the same tool
Side imaging is a search tool. It scans continuously as the boat moves, building a wide picture of the bottom on both sides so you can cover a shoreline or a flat quickly and mark anything worth returning to. It answers the question: where is the structure? Live sonar is a targeting tool. It renders a narrow, real-time beam aimed wherever you point the transducer, usually forward or straight down from the trolling motor, and it answers a completely different question: is there a fish there right now, and how is it reacting to what I just cast at it?
Because they solve different problems, most serious anglers eventually own both rather than choosing one permanently over the other. The order matters more than the final answer: side imaging first, because it is both cheaper and the one that finds the water live sonar then gets pointed at.
Why a narrow beam matters more than it sounds
Both technologies depend on the same published trigonometry that governs every sonar beam: coverage diameter equals two times depth times the tangent of half the beam angle. In shallow water a narrow beam covers almost nothing, which is exactly why side imaging and live sonar both exist rather than just running a single traditional cone.
A standard 20 degree cone in 10 feet of water covers a circle only about 3.5 feet across, which is why a boat idling over shallow structure with only a traditional cone is looking at almost nothing beneath it.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Water depth | Coverage diameter of a 20 degree cone |
|---|---|
| 5 ft | 1.8 ft |
| 10 ft | 3.5 ft |
| 20 ft | 7.1 ft |
| 40 ft | 14.1 ft |
Coverage diameter equals two times depth times the tangent of half the cone angle. A wide-swinging side imaging beam and an aimed live sonar beam both exist because a single traditional cone covers this little in shallow water.
Buy side imaging first: it is cheaper and it finds the spots
The Humminbird Helix 7 MEGA SI G4N at $999.99 covers a claimed 125 feet either side of the hull as you idle along, turning a lake you have never fished into a mental map within an afternoon. Nothing about live sonar does that job as efficiently, because a live sonar beam only shows what it is currently pointed at, not a wide swath of everything nearby.
Add live sonar once you already know where to stop
Garmin's LiveScope Plus system at $1,445.00 needs a compatible Garmin display and, in practice, a dedicated mount and often its own battery, which is a meaningfully bigger commitment than adding a transducer to an existing head unit. What it buys is the ability to watch a specific fish decide whether to eat, in real time, once side imaging or local knowledge has already put you on the spot. That is a genuinely different kind of information than a wide scan ever provides, and it is why live sonar is usually the second purchase rather than the first.
Frequently asked questions
Should I buy side imaging or live sonar first?
Buy side imaging first for most anglers. It costs a fraction of a live sonar system, and its job is finding structure and fish-holding water across a wide area as you move, which is exactly what you need before live sonar's narrow, aimed beam becomes useful. Live sonar is the natural second purchase once you already know where to point it.
How much more expensive is live sonar than side imaging?
Considerably more. A capable side imaging unit like the Humminbird Helix 7 MEGA SI runs under $1,000 complete. A full live sonar system like the Garmin LiveScope Plus system runs close to $1,450 for the black box and transducer alone, and that price assumes you already own a compatible display, mount and often a dedicated battery on top of it.
Can live sonar replace side imaging entirely?
Not effectively for searching. Live sonar shows a narrow, real-time beam wherever it is currently aimed, which is excellent for watching a specific fish but poor for scanning a wide area quickly. Side imaging's wide swath as the boat moves is a fundamentally different, faster way to cover water and find structure than repositioning a live sonar beam ever is.
Why does live sonar need such a narrow beam?
The same trigonometry that limits a traditional sonar cone in shallow water applies here: a narrow beam concentrates transmit power and resolution into a smaller area, which is what makes individual fish and a falling bait visible in real time rather than as a blur. Widening the beam would trade that clarity for coverage, which is what side imaging already does instead.
Does live sonar work well in shallow water the way side imaging does?
Live sonar works in shallow water, and its Perspective or Down modes are specifically useful there, but its narrow beam still only shows what it is aimed at rather than a wide search area. Side imaging remains the faster tool for covering a shallow flat or shoreline, with live sonar better suited to confirming and watching a specific target once found.
Do I need a new display to add live sonar to my boat?
Often, yes. Garmin's LiveScope requires a compatible ECHOMAP or GPSMAP display, and Humminbird's MEGA Live only works with XPLORE, APEX and SOLIX G3 head units rather than a Helix. Lowrance's Eagle Eye 9 sidesteps this by bundling its own live sonar transducer with a dedicated display, which is worth checking before assuming an existing head unit will support a live sonar add-on.
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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.