The vessel fires a downward sonar ping at a steady rate as it moves. Sound leaves the transducer, reflects off the seafloor, and returns; the round-trip time t gives the depth directly from d = c·t/2, where c ≈ 1500 m/s is the speed of sound in seawater. Only the travel time is ever measured — depth is always inferred, never read off a ruler.
A real transducer doesn't emit a single ray: it radiates a cone of half-angle θ. At depth d the footprint illuminated on the bottom is roughly 2·d·tan(θ) wide — the same beam covers far more seafloor over a trench than over a shallow shelf, which is exactly why deep-water surveys need fewer, wider-spaced ping lines than shallow coastal ones.
- Nadir depth — the return straight beneath the ship, used for the live readout.
- Edge beams — the outer rays of the cone, solved by iterating on the seafloor slope until the ray and the bottom agree, which is what draws the widening cone you see.
- The map itself — every ping's measured depth (with a touch of acoustic noise) is plotted right where it was taken; nothing ahead of the ship is drawn, because a real survey only knows the seafloor it has already swept.
This is the flat 2D companion to the 3D acoustic-sonar-mapping simulation: the same round-trip timing and beam-cone geometry, viewed as a live cross-section as the vessel advances.