Side imaging looks like an aerial photograph of the lakebed rolling past on both sides of your boat — and once you can read it, it's the fastest way to find fish-holding structure across a huge area. The key is learning to read shadows.
This is the standalone deep dive on reading side imaging (SideVü, MEGA SI, SideScan). For how it fits among the other sonar views, see understanding sonar, and compare it directly with down imaging in down vs. side imaging.
01How the image is built
Picture your boat traveling down the center of the screen. The unit fires thin beams out to the left and right, and as you move forward it paints a continuous strip of the bottom on each side. The result reads like a top-down map: left side of the screen is the water to your left, right side is the water to your right, and the bottom of the screen is the most recent pass.
02The center strip (and the surface clutter)
The bright column running down the middle is the water column directly beneath the boat — not the bottom. The point where the bright center meets the darker "bottom" on each side is roughly where the bottom begins under you. The fuzzy band near the very center is surface return and the water column; the real structure detail is out to the sides.
03Read the shadows — that's the secret
Side imaging works like a low sun casting long shadows. An object standing up off the bottom — a stump, rock, brush pile, dock post — blocks the sonar beam and throws a dark shadow behind it (away from the boat). Those shadows are where the information is:
- A tall object casts a long shadow; a flat one casts almost none. Shadow length tells you height.
- The object itself is the bright return; the shadow is the dark void behind it.
- Fish suspended off the bottom show as bright dots or short dashes — often with a small shadow offset behind them, which helps separate them from bottom debris.
Beginners stare at bright returns; experts read shadows. A long shadow means something is standing tall on the bottom — exactly the kind of structure that holds fish.
04Bright vs. dark returns
Hard, reflective surfaces (rock, gravel, timber, metal) return strong and show bright; soft surfaces (mud, sand, weed) return weak and show darker. A bright, hard patch surrounded by soft bottom is a classic fish-holding spot. Boulders, riprap and standing timber all pop once you know to look for the bright-plus-shadow signature.
05Range, detail and speed
- Detail is sharpest close to the boat and softens with distance. Don't over-set the range — a tighter range gives a crisper image of nearby structure.
- Go slow and straight. Idle speed (roughly walking pace) and a straight line produce the cleanest image; turns and speed distort it.
- Higher frequency = more detail, less range. MEGA / high-frequency imaging is stunningly sharp but reaches less far; see frequencies explained.
06A practical workflow
- Scan a flat, ledge or shoreline at idle to locate structure and bait fast.
- Mark a waypoint on anything promising — see using GPS waypoints.
- Confirm with down imaging and 2D directly over the spot.
- Fish it — or, if equipped, switch to live sonar for the cast.
For the clarity leader in side imaging, Humminbird's MEGA Side Imaging is hard to beat:
Humminbird HELIX 7 CHIRP MEGA SI GPS G4N
$$The only 7″ unit with MEGA Side Imaging — the high-frequency 1.2 MHz imaging widely regarded as the clarity leader — plus Dual Spectrum CHIRP and LakeMaster basemap. A superb way to learn to read side scan.