Side Imaging vs Down Imaging vs CHIRP: Sonar Types Explained

Fish finder boxes list sonar types like a feature menu: CHIRP, down imaging, side imaging, sometimes 360 and live. Each is a different shape of sound beam aimed in a different direction, and each answers a different question about the water. This page explains what each one shows, using the beam angles and frequencies the makers publish.

What are the main fish finder sonar types?

There are three standard types: traditional 2D sonar (now usually CHIRP), down imaging and side imaging. Two newer types sit on top of them: 360 imaging, which rotates a beam around the boat, and forward-facing live sonar.

Live sonar is a separate purchase and its own topic; see forward-facing sonar compared.

What is 2D CHIRP sonar?

2D sonar sends a cone-shaped beam straight down and draws everything it hits as a scrolling history, with fish showing as arches. A fish makes an arch because its distance to the transducer shrinks as it enters the cone and grows again as it leaves.

CHIRP is a way of sending that pulse: instead of one fixed frequency, each ping sweeps a band of frequencies. Humminbird says the result is better target separation, clearer fish arches and a higher signal-to-noise ratio. Its HELIX Dual Spectrum CHIRP offers a narrow 25 degree beam for detail and a wide 42 degree beam for coverage. Garmin’s Striker Vivid units run CHIRP at 50, 77 and 200 kHz (mid and high bands).

2D sonar is still the only view that clearly shows a fish suspended in the water column as a fish. Every fish finder has it, and it is the view to trust for depth.

What is down imaging?

Down imaging is sonar that uses a very thin, high-frequency beam, wide from side to side but thin from front to back, to draw a picture-like image of what is under the boat. Brush piles look like brush and rock looks like rock, where 2D shows only blobs.

Humminbird’s HELIX manual gives the trade-off between frequency, detail and depth:

Garmin’s ClearVu runs at 260, 455 and 800 kHz on the Striker Vivid line. Down imaging scrolls right to left like 2D sonar, with the newest returns on the right, so it is easy to read next to the 2D view in a split screen.

What is side imaging (side scan sonar)?

Side imaging is sonar that fires thin, fan-shaped beams to the left and right of the boat and stitches each slice into a continuous picture of the bottom as the boat moves. Humminbird introduced it to consumer fish finders in 2005; the same principle has been used for seabed survey since the 1960s, typically at 100 to 500 kHz, where higher frequency gives sharper images but less range.

Range depends on frequency. Humminbird’s Side Imaging page quotes up to 400 ft each side, 800 ft in total; the HELIX manual gives the limits per setting:

Garmin SideVu uses 260, 455 and 800 kHz on the Striker Vivid line; Lowrance lists 455 and 800 kHz imaging for the HDI nosecone transducer on its Recon motor. Humminbird’s MEGA frequency is 1.2 MHz, which it suggests switching back to 455 kHz when suspended sediment or pollen clutters the picture. Water color itself does not affect sonar.

How do you read side imaging and down imaging screens?

On both views, bright means hard or rising bottom, dark means soft or falling bottom, and a shadow means something is standing up. The HELIX manual’s reading key:

• Light shades: denser terrain such as timber, rocks or compacted sediment, or rising terrain. A very hard bottom can show white.
• Dark shades: soft returns such as sand or mud, or descending terrain.
• White streaks or clouds: possibly fish.
• Shadows: not caused by light but by missing sonar return behind an object. The longer the shadow, the taller the object, and a fish’s shadow shows how far above the bottom it is.

Side imaging adds one layout rule. The boat sits at the top center of the screen, history scrolls down, and the dark band in the middle is the water column under the boat. Its width shows the depth as you pass. Everything on a side imaging screen is water you have already passed, so mark a waypoint on the target, then turn around.

What boat speed does each sonar type need?

Side imaging needs trolling speed, straight lines and minimal turning, per Humminbird’s HELIX manual; 2D sonar keeps working at running speed.

This is why side imaging and a trolling motor go together: the motor holds the slow, straight pass the sonar wants. For a picture while holding position, Humminbird’s MEGA 360 sweeps up to 125 ft in every direction around a stationary boat, with six rotation speeds and a sweep you can narrow from 360 degrees to 10.

What does each sonar type cost to add?

At Tackle Warehouse in October 2026, a 2D-only unit started at $119.99, CHIRP with down imaging at $199.99, and side imaging at $519.99.

Upgrading an existing unit means a new transducer if the display supports it: Lowrance StructureScan HD $259.00, Humminbird Side Imaging transom transducer $299.99 and Garmin GT54UHD-TM $499.99 at the same retailer. Prices change often. Which transducer fits which display, by connector and generation, is covered in fish finder transducers. Screen size, GPS and mounting on a small hull are covered in fish finder for a small boat.

Which sonar type does a small boat need?

For most small boats, 2D CHIRP plus down imaging is the best value, and side imaging is worth adding when you search big water for structure.

• Kayaks and canoes: CHIRP plus down imaging. Side imaging wants steady, straight passes that are hard to hold under paddle; see kayak fish finders.
• Jon boats on rivers and small lakes: CHIRP plus down imaging shows the brush, rock and drop-offs you fish over.
• Bass and bay boats on big reservoirs, flats or coast: side imaging finds structure, schools and wrecks far faster than passing over them one at a time.
• Rental boats and bank fishing: a castable or portable unit instead; see portable and castable fish finders.

Who is this explainer not for?

This page explains recreational fish finder sonar on small fishing boats. It is not a guide to commercial or survey side-scan systems, towed sonar, or offshore multifunction networks, and it does not rank forward-facing sonar units. Product availability and prices differ outside the US.