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Mode 06 · On water

The picture is behaving badly. Start here.

Whether a device's claim survives inspection is a question you ask once. Whether the screen in front of you right now can be trusted is a question you ask every trip. Pick what you are looking at; work the cheapest check first.

Devices49 · 18 makersLocal-only · condition context
Every check below states what a clean result proves and what it still does not. A two-minute test that comes back fine is not a clean bill of health, and nothing here will pretend otherwise.

06AStart from the complaint

Which one am I looking at?

Thirteen symptoms is a list you read sitting down. If you are standing in water, start here instead: pick the complaint in your own words and answer two to four questions by looking at the screen already in front of you. It ends on a symptom below, and it never ends on a conclusion about the water.

06On water

What are you looking at?

Pick the symptom on the screen in front of you. Nothing here diagnoses the water — every answer is about the instrument and what it is looking through.

Session

This only hides checks that cannot apply where you are. It never adds a conclusion.

30 revision-covered catalog entries now have model/generation-specific field branches. If the current local claim draft names one of them, it is carried here automatically. Changing this selector does not alter the saved claim or send device information anywhere.

The bottom, or the whole picture

Speckle and clutter

Something added to the picture

Returns that repeat

Where marks are drawn

The depth number

What the unit chooses to draw

The installation itself

→Cone geometry

How much water is in the beam right now

Trigonometry, not a fish model. Coverage diameter is 2 · depth · tan(θ/2), so the circle the beam paints grows with depth at a fixed multiple. This is the number that explains a mark you cannot find under you.

Circle painted

10.6 ft

Furthest off to the side

5.3 ft

Multiple of depth

0.35×

Sonar cone at 20 degrees and 30 ftSide elevation and plan view drawn to one scale. At 30 ft below the transducer a 20 degree beam paints a circle 10.6 ft across, so a return can sit up to 5.3 ft to either side and still be drawn as though it were directly beneath the transducer.SIDE ELEVATIONsurface30 ftbottom10.6 ft across20°a return out at the edge is drawn at the same depth, under youPLAN — SAME SCALE5.3 ftinside this circle, everything reads as “under the boat”0.35× the depth, at every depth — the circle grows with thewater, and the angle never changes.
  • At 30 ft the beam paints a circle about 10.6 ft across — roughly 0.35× the depth, at every depth.
  • A target can sit up to 5.3 ft to the side of the transducer and still be drawn as though it were directly beneath you.
  • Return strength is intensity, not identity. Nothing about this geometry lets a screen name a species.

The angle is the maker's stated total included cone, normally quoted at −3 dB. A weaker return from outside that boundary can still paint, so treat this as the honest geometric case rather than a boundary the water respects.

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