Category: Fish Finders & Chartplotters

Your phone already has GPS, a mapping app, and a bigger app ecosystem than any marine electronics company will ever build. So why do serious boaters still spend hundreds to thousands of dollars on a dedicated chartplotter? The answer comes down to reliability, integration, and a few capabilities phone apps fundamentally can’t match on the water.

What a Phone GPS App Does Well

Apps like Navionics Boating or similar marine navigation apps are genuinely good for casual, close-to-shore boating — they’re inexpensive, the charts are regularly updated, and most boaters already know how to use a phone interface. For a weekend on a familiar lake or a short coastal run with cell coverage nearby, a phone app is a perfectly reasonable primary tool, and even boaters with a full chartplotter often keep one installed as a backup.

Where Phone Apps Fall Short

The gap shows up in conditions that matter most: direct sunlight readability (marine displays are built for full-sun visibility; phone screens generally aren’t), waterproofing (a chartplotter is rated for spray and rain; a phone in a case is a compromise), and battery life (a chartplotter runs off the boat’s power system all day; a phone burns through its battery fast with GPS and screen-on navigation running continuously). More importantly, a dedicated unit like the Raymarine Axiom Pro 9 S integrates directly with your boat’s sonar, radar, and autopilot over a single network — a phone app operates in isolation from all of that.

Signal and Cell Coverage

This is the dealbreaker for many boaters: most phone navigation apps rely at least partly on cell data for map tiles, live weather, and AIS traffic overlays, even if base charts are cached offline. Once you’re a few miles offshore or in a coverage dead zone, that functionality can degrade exactly when you need it most. A standalone chartplotter’s charts, GPS, and (if equipped) radar and AIS all run independently of cell signal.

Integration With Sonar and Radar

If you already run a fish finder or are considering marine radar, a combo chartplotter/sonar unit or a networked MFD system shares one screen and one set of controls for everything — overlaying your route on the same display as your sonar returns or radar picture. Our guide to how fish finders work covers how the sonar side of that integration functions.

Frequently Asked Questions

Can I use a phone app as my only navigation on the water?
For casual, near-shore boating in good conditions, many boaters do. For anything offshore, in variable weather, or where reliability genuinely matters, most experienced boaters treat a phone app as a backup to a dedicated chartplotter, not a replacement.

Do I need a chartplotter if I already have a fish finder with GPS?
Many combo fish finder/chartplotter units already include full navigation charting — check whether your unit has chartplotting built in before assuming you need a separate dedicated MFD.

What size chartplotter display should I buy?
Bigger is almost always better for on-water readability, especially if you’ll split the screen between charts and sonar or radar — 9-inch units like the Axiom+ 9 are a common sweet spot, while serious offshore setups often run 12-16 inches, like the Garmin GPSMAP 8616.

Does a chartplotter need an internet connection to work?
No — charts are stored on the unit or a memory card, and GPS positioning doesn’t require cell or internet signal at all, which is the core reliability advantage over a phone app.

Ready to compare actual chartplotter models? See our best marine chartplotters compared guide, or browse the full fish finder & chartplotter collection.

Buying a good fish finder is only half the battle — the other half is actually knowing what you’re looking at once it’s mounted and running. Here’s how to make sense of the screen instead of just staring at colorful noise.

Arches, Blobs, and Dots: Reading Fish Targets

On traditional 2D sonar, a fish swimming through the transducer’s cone shows up as an arch — the fish enters the edge of the cone (further away, weaker signal), passes under the center (closest, strongest signal), then exits the other edge. A full, symmetrical arch usually means a fish swam directly under the boat; a half-arch means it clipped the edge of the cone. On down imaging and side imaging, fish usually appear as bright dots, dashes, or small fish-shaped marks rather than arches, since those modes render a photo-like image instead of a traditional sonar return.

Reading the Bottom

The bottom shows up as a thick, continuous line across the screen. A hard bottom (rock, sand) returns a strong, sharp, often doubled line (the “second echo” as the pulse bounces bottom-surface-bottom again). A soft bottom (mud, silt) returns a thinner, fuzzier line since the sound energy absorbs into it rather than bouncing back cleanly. Learning to read bottom hardness matters for both fishing (many species prefer specific bottom types) and simply understanding what your anchor will grab.

Understanding Side Imaging Layout

Side imaging splits the screen into left and right halves, with the boat’s track running down the center. Distance from center represents actual distance from the boat — features closer to the centerline are closer to your hull. Brighter returns mean harder, denser objects (rock, wood, metal); darker areas are softer bottom or open water. Shadows behind objects (just like a shadow cast by sunlight) tell you the object has real height — a stump or rock pile casts a visible shadow, while a flat depth change doesn’t.

Color Palettes: Does It Matter Which One You Use?

Most fish finders offer several color palettes (blue, white, amber, etc.) purely for screen visibility in different lighting — amber and high-contrast palettes are often easier to read in direct sunlight, while blue palettes feel more natural in low light. The palette doesn’t change what the sonar is detecting, only how it’s displayed, so pick whatever’s easiest for your eyes on the water.

Split Screens: What’s Worth Running at Once

Most anglers settle into one of two layouts: sonar + chartplotter side by side (so you always know your location while watching sonar), or down imaging + side imaging together (for actively searching structure). Running three or four panels at once on anything smaller than a 9″ screen usually makes everything too small to read quickly — better to cycle between two clean views than cram in extra data you’ll ignore anyway.

Frequently Asked Questions

Why do I see fish arches sometimes but only dots other times?
This is usually a difference between 2D sonar view (arches) and down/side imaging view (dots and marks) — they’re different rendering modes of the same underlying sonar data, not a malfunction.

Why does the bottom depth jump around or disappear at speed?
At higher boat speeds, especially in choppy water, the transducer can lose a clean read on the bottom. This is normal and usually resolves once you slow down; if it happens constantly at low speed, check your transducer mounting angle and depth.

Should I adjust sensitivity manually or leave it on auto?
Auto mode works well for most conditions on modern units. Manual sensitivity is worth learning if you fish very deep water (turn it up) or very shallow, clear water with excess surface clutter (turn it down).

How long does it actually take to get comfortable reading a fish finder?
Most anglers get a working intuition within a handful of outings once they stop trying to read every detail and instead learn to recognize the handful of patterns above — arches, hard vs. soft bottom, and shadows on side imaging.

New to fish finders entirely? Start with our how fish finders actually work guide, or browse the best fish finders by budget if you’re still shopping.

Walk into any tackle shop and ask “what fish finder should I buy?” and you’ll get a wall of jargon back — CHIRP, down imaging, side imaging, transducer frequency, beam angle. None of it makes sense until you understand what a fish finder is actually doing underneath your boat. Once you get the fundamentals, choosing between models (and reading the screen once you’re on the water) gets a lot easier.

The Basic Idea: Sound, Not Sight

A fish finder doesn’t “see” anything. It sends a pulse of sound energy down through the water from a transducer mounted on your hull or transom, and listens for the echo bouncing back. The time it takes for that echo to return tells the unit how far away an object is; the strength and shape of the echo tells it roughly what that object is — the bottom, a school of baitfish, structure, or a single big fish suspended in the water column. Your display just translates thousands of these echoes per second into a scrolling picture.

CHIRP vs. Traditional Sonar

Older, single-frequency sonar sends one pitch of sound (say, 200kHz) and reads the echo. CHIRP (Compressed High-Intensity Radiated Pulse) sends a sweep across a range of frequencies in every ping, which returns far more detail and separates targets that would blur together on traditional sonar — two fish holding close together, or a fish sitting right on the bottom. Nearly every fish finder worth buying today runs CHIRP as standard, including budget units like the Lowrance HOOK Reveal 5.

Down Imaging, Side Imaging, and Live Sonar

Traditional 2D sonar shows a cone-shaped slice directly under the boat. Down imaging uses a wider, flatter beam to produce a near-photographic picture of what’s directly below — brush piles, stumps, and individual fish show up as sharp, recognizable shapes rather than arches. Side imaging aims that same flat beam out to the sides, letting you scan a corridor 100–200+ feet wide without moving the boat, which is how experienced anglers find structure and schools without blind-casting the whole lake.

The newest step up is forward-facing live sonar (Garmin LiveScope, Lowrance ActiveTarget, Humminbird MEGA Live) which updates in real time instead of scrolling historically, so you can literally watch your bait fall past a fish and see it react. We cover the difference in more depth in our live sonar vs. traditional sonar guide.

Why Transducer Placement Matters

A high-end display is only as good as the transducer feeding it. Transom-mount transducers are the easiest DIY install and fine for most recreational use; thru-hull transducers sit flush with the hull for cleaner readings at higher speed; trolling-motor-mounted transducers (common with side and live imaging) move with the bow of the boat so you’re always scanning where you’re actually pointed. If you’re mounting your own, our fish finder & chartplotter collection lists the transducer each unit ships with so you know what you’re getting before it arrives.

GPS and Chartplotting: The Other Half of the Screen

Almost every fish finder sold today is really a combo unit — sonar plus a GPS chartplotter sharing one screen. The GPS side lets you drop waypoints on productive spots, follow contour lines that reveal drop-offs and ledges, and in many cases overlay lake maps or coastal charts. If you’re trying to decide whether you even need a dedicated unit instead of a phone app, that’s a separate question we answer directly in chartplotter vs. phone GPS apps.

Frequently Asked Questions

Do I need CHIRP, or is traditional sonar fine?
CHIRP costs little to nothing extra on current-generation units and gives meaningfully better target separation, so there’s rarely a reason to buy traditional-only sonar today.

What frequency should I look for?
Lower frequencies (50kHz) penetrate deeper water and cover a wider cone; higher frequencies (200kHz+) give sharper detail in shallower water. Most CHIRP units sweep a range and let the software pick the best return, so this matters less than it used to — but if you fish deep offshore water regularly, check that the transducer is rated for it.

Does a bigger screen actually help?
Yes, more than most people expect. Side-by-side split screens (sonar + chartplotter, or down + side imaging) eat screen space fast, and a 5″ display gets cramped quickly if you run more than one view. Most serious anglers land on 7″ to 9″ as the sweet spot.

Can I add live sonar to a unit I already own?
Sometimes. Garmin LiveScope and Lowrance ActiveTarget systems can often be added as a second transducer feeding into a compatible existing display — check your specific model’s compatibility before buying the add-on system.

Ready to compare actual models instead of specs sheets? Our best fish finders by budget guide breaks down real picks at every price point, and if you’re torn between brands, see Garmin vs. Lowrance vs. Humminbird.