Sonar
How to read a fish finder screen: arches, cones and imaging
Quick answer
A fish finder screen scrolls sonar history from right to left, and a 20 degree CHIRP cone covers a circle only about 3.5 feet across in 10 feet of water. Fish appear as arches rather than fish shapes because the cone narrows over a stationary target as the boat moves, and Down or Side Imaging replaces that arch with a picture-like image instead.
A fish finder screen is not a photograph of what is under the boat right now. It is a scrolling history: each new column of pixels on the right edge is the latest sonar return, and everything already on screen shifts left as older data. Understanding that one fact resolves most of the confusion new owners have about arches, shadows and why the picture keeps changing even when the boat has not moved far.
The second fact that resolves the rest of the confusion is that the sonar cone is a real, physical cone with a real width, published as trigonometry rather than as a marketing claim, and that width shrinks fast in shallow water. This guide works through both, plus how Down Imaging and Side Imaging change what you are looking at once you switch views.
On this page
- The screen scrolls sonar history, it does not show a live snapshot
- Why fish show up as arches instead of fish icons
- How wide the cone actually is at your depth
- Why shallow water makes imaging matter so much
- Reading Down Imaging and Side Imaging instead of the sonar view
- Step by step: reading a new sonar screen on the water
The screen scrolls sonar history, it does not show a live snapshot
Every fish finder screen works the same way underneath its own color palette: sonar pings go out, echoes come back, and each ping becomes one vertical column of pixels drawn at the right edge of the screen. As the boat moves and new pings arrive, the older columns scroll left. What you are reading at any moment is a strip of recent history, wide enough to show how the bottom and any targets changed as the boat passed over them, not a single frozen instant. This is also why holding still over one exact spot for a while draws a flat, repeating picture rather than new information, and why speeding up compresses more history into the same screen width.
Why fish show up as arches instead of fish icons
The cone is widest where it meets the water surface below the transducer and narrows to a point directly beneath it. As a fish swims through that cone while the boat moves over it, the sonar first catches the edge of the cone at a longer, weaker range, then the center of the cone at the shortest, strongest range directly under the boat, then the far edge again as the boat continues on. That sequence of longer-shorter-longer range draws a curve, which is the arch. A tight, full arch means the fish held still and the boat passed directly over it. A short, choppy or incomplete arch usually means the fish moved, the boat's course only clipped the edge of the cone, or the boat's speed changed mid-pass. Many units also overlay a fish icon feature on top of the raw arches as a beginner aid, but the arch itself is the real data and the icon is an interpretation layered on top of it.
How wide the cone actually is at your depth
Sonar cone coverage diameter
Published figurecoverage diameter = 2 x depth x tan(cone angle / 2)
Depth is the water depth under the transducer. Cone angle is a property of the transducer and frequency in use, not of the display.
Cone coverage is published trigonometry rather than a rule of thumb: coverage diameter equals twice the depth times the tangent of half the cone angle. The consequence surprises a lot of new owners.
A 200 kHz, 20 degree cone covers about 3.5 feet across in 10 feet of water, while an 83 kHz, 60 degree cone covers roughly 11.5 feet at the same depth.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Frequency | Typical cone angle | Coverage at 10 ft | Coverage at 20 ft |
|---|---|---|---|
| 50 kHz | 45 deg | 8.3 ft | 16.6 ft |
| 83 kHz | 60 deg | 11.5 ft | 23.1 ft |
| 200 kHz | 20 deg | 3.5 ft | 7.1 ft |
| 455 kHz imaging | 16 deg | 2.8 ft | 5.6 ft |
| 800 kHz imaging | 10 deg | 1.8 ft | 3.5 ft |
Cone angle is typical for the frequency and varies by transducer model. Your own transducer specification governs.
Why shallow water makes imaging matter so much
The table above is the whole reason Down and Side Imaging exist. In 10 feet of water a standard 200 kHz cone is only looking at a patch of bottom about 3.5 feet across, which is almost nothing compared to the water actually around the boat. A boat idling over a flat in that depth range is covering a narrow strip and driving past most of what is there. Side Imaging's wedge shaped coverage on either side of the boat, and Down Imaging's higher frequency picture straight down, both exist specifically to cover the water a narrow traditional cone misses in exactly this kind of shallow, structure-heavy water.
Reading Down Imaging and Side Imaging instead of the sonar view
Down Imaging replaces the arch with a picture-like image directly beneath the boat, rendered in shades rather than a line graph, and it reads brush piles, stumps and suspended fish with much more shape than a CHIRP arch does. Side Imaging shows a wedge on each side of the boat as if you sliced the water open and looked at it from above, with the boat's own track running down the middle of the screen. A raised object on the bottom, such as a rock or a stump, casts a dark shadow away from the boat's track on a Side Imaging screen exactly the way a real object casts a shadow in sunlight, and the length of that shadow is a rough clue to the object's height. Both imaging views read water temperature and bottom hardness less directly than a traditional CHIRP view, which is why many anglers run traditional sonar and imaging side by side rather than choosing one.
Step by step: reading a new sonar screen on the water
- Confirm the transducer is reading before you trust anything on screen. Check that a stable bottom line and a depth number both appear at idle before assuming any target above the bottom is real.
- Read the bottom line thickness before you read anything else. A thin, sharp bottom line usually means soft bottom such as mud or sand. A thick, fuzzy bottom line usually means hard bottom such as rock or gravel.
- Watch for full, tight arches rather than every mark on the screen. A complete arch that rises and falls cleanly is the strongest sign of an actual fish holding in that spot as the boat passed over it.
- Switch to Down or Side Imaging in shallow water. In under about 15 feet, a traditional cone covers very little bottom. Imaging views cover far more of the water actually around the boat.
- Mark a waypoint the moment you see something worth returning to. Sonar history scrolls off the left edge of the screen quickly at speed, and a mark made a few seconds late is often already well behind the boat.
Sources
- Published sonar cone trigonometry and typical cone angle specifications by frequency
- Humminbird published MEGA imaging range specifications
Frequently asked questions
Why do fish show up as arches instead of fish shapes on the screen
The sonar cone is widest at the surface and narrowest directly under the transducer. As a fish passes through that cone while the boat moves, the sonar first catches it at the wide, weak edge of the cone, then at the narrow, strong center, then at the far edge again, which draws a rising and falling curve rather than a single dot. A complete, symmetrical arch means the boat passed directly over a stationary fish. A choppy or partial arch usually means the fish moved or the boat only clipped the edge of the cone.
How wide is my fish finder cone at the depths I actually fish
It depends on frequency and depth, using coverage diameter equals twice the depth times the tangent of half the cone angle. A common 200 kHz, 20 degree cone covers only about 3.5 feet across in 10 feet of water and roughly 7 feet at 20 feet of water. Wider beams like an 83 kHz, 60 degree cone cover far more, around 11.5 feet at the same 10 foot depth, at the cost of less detail and weaker bottom separation than the narrower beam.
What does a thick versus thin bottom line mean
A thin, sharply defined bottom line generally indicates a soft bottom such as mud or sand, because soft material absorbs more of the sonar signal and returns a weaker, cleaner echo. A thick, fuzzy or doubled bottom line generally indicates a harder bottom such as rock, gravel or a heavily compacted lakebed, because hard material reflects sound more strongly and can even produce a second, weaker echo below the true bottom on some units.
How is Side Imaging read differently from a traditional sonar view
Side Imaging shows a wedge of water on each side of the boat as if the water were sliced open and viewed from above, with the boat track running down the center of the screen. Raised objects on the bottom cast a dark shadow away from the boat track, similar to a real shadow in sunlight, and the shadow length is a rough indicator of the object height. A traditional CHIRP view only shows what passed directly under the boat, drawn as a line graph and arches rather than a picture.
Why does the sonar picture change so much with boat speed
The screen draws one column of pixels per sonar ping and scrolls older columns to the left. At a slower speed, more pings and more screen width cover the same stretch of bottom, which spreads out arches and shows more detail per foot traveled. At a faster speed, the same stretch of bottom is compressed into fewer columns, which shortens and can distort arches. This is a display effect from ping timing rather than a change in what is actually under the boat.
Can two fish finders on the same boat interfere with each other
Yes, when both transducers run overlapping frequencies close together, which shows up as random noise, false arches or a jumpy bottom line on one or both screens. Running the units on different frequency bands, staggering their sonar timing where the unit supports it, or spacing the transducers further apart on the hull all reduce it. Most manufacturers publish interference guidance for pairing their own units, which is worth checking before assuming a new unit is simply defective.
Get the complete rigging shopping list
Every item in the beginner, intermediate and expert rigs for both a boat and a kayak, with running totals, in one printable list. No charge.
One list, no sequence of daily emails, and you can leave at any time.
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.