CHIRP, down imaging and side imaging: what each one actually shows you

The single rule that explains all three

Higher frequency gives finer detail and less depth. Lower frequency gives more depth and a coarser, wider picture.

That is it. Every difference between these three sonar types follows from where each one sits on that trade, and the published numbers make it visible.

Read down that depth column against the frequency column. 77 kHz reaches 1,900 feet. 1.2 MHz reaches 75. The relationship is not subtle, and it is the whole story.

What CHIRP adds to 2D sonar

Traditional sonar pings at one frequency. CHIRP sweeps through a range on every ping — which is why the published specs give bands rather than single numbers: 75–155 kHz and 130–250 kHz on a HELIX 5 DI.

Sweeping puts more energy into the water per ping and lets the unit separate returns that arrive close together. In practice you get better target separation — two fish holding a foot apart show as two marks rather than one blob, and a fish sitting tight to the bottom separates from the bottom return.

Humminbird’s Dual Spectrum implementation publishes three usable modes on the HELIX 5: narrow at 180–240 kHz for maximum detail, wide at 140–200 kHz for maximum coverage, and full at 150–220 kHz. That is a genuine control, not a marketing switch.

Cone angle: the arithmetic worth doing once

The beam radiates as a cone, so the area it covers grows with depth. The diameter is roughly 2 × depth × tan(angle/2).

Two consequences worth internalizing. First, in shallow water every cone is small — at five feet even a 60-degree beam sees under six feet across, which is why shallow-water sonar feels useless until you accept that you are sampling a narrow strip.

Second, a fish drawn at the edge of a wide cone is not directly beneath you. It could be seventeen feet to one side in fifteen feet of water. Anglers drop a lure straight down on a mark and wonder why nothing takes it.

Why fish appear as arches

A fish enters the edge of your cone at maximum range from the transducer, passes through the middle at minimum range, and exits the far edge at maximum range again. Plotted against time, that traces an arch.

Which means an arch requires relative movement. A stationary boat over a stationary fish draws a horizontal line. A narrow cone produces short, thin arches simply because the fish is inside it briefly. Neither is a fault.

Down imaging: identifying, not finding

Down imaging uses a much higher frequency in a thin fan rather than a cone. That fan is narrow front-to-back and wide side-to-side, which is what produces a picture rather than a profile.

It is genuinely good at telling you what something is — branches versus rock versus weed. It is not good at telling you a fish is there, because a thin fan samples much less water than a cone.

The standard way to use both: run 2D and down imaging split-screen. 2D finds; down imaging identifies.

Side imaging: covering water

Side imaging fires two fans out to either side, nearly parallel to the surface. The published reach varies substantially by philosophy: Lowrance’s TripleShot HD claims up to 600 ft per side, Humminbird’s MEGA SI publishes 250 ft per side at much higher frequency.

Two conditions it needs, both of which people get wrong. It needs a steady straight track, because the image is assembled from consecutive sweeps and a turn skews it. And it needs consistent moderate speed — too fast leaves gaps, too slow stretches the image.

It also creates a blind spot directly beneath the boat, where the two side fans do not reach. That is what the down imaging beam is for.

Which do you actually need?

For a first unit on a small lake: 2D CHIRP with a wide beam option and GPS. That covers depth, bottom, fish and returning to a spot.

Add down imaging when you want to understand structure you already know is there. Add side imaging when you are covering large open water and searching. Neither replaces 2D, and both add menu complexity that genuinely slows down a first season.