Overlap Loss from Swath Spacing

Overlap is the ground each pass shares with the one before it. A little is deliberate — a small overlap is insurance against a skip, and skips are worse — but it is paid for on every hectare of every pass, in chemical, seed, fertiliser, fuel and time, and on a large programme the total is substantial.

There are two conventions for stating it and they give different numbers, which is the trap on this page. This calculation uses the MACHINE-WIDTH convention, $(w - s)/w$: the share of the machine's swath that lands on treated ground. It is the convention a guidance display uses and the one behind the familiar claim that manual marking gives 5–10% overlap. The alternative expresses the wasted input against the ground actually gained, $(w - s)/s$, which is always the larger figure. On a 12 m boom at 11.4 m spacing the first gives 5.0% and the second 5.3% — close enough at small overlaps that nobody notices the difference, and far enough apart at large ones to matter: at 9 m spacing the two read 25% and 33%. Quote which one you mean.

The reason guidance systems sell is the gap between what operators think they achieve and what they do. Marking by foam or a disc marker in good visibility gives perhaps 5–10%; in dust, at night, or on a hillside it is worse, and it degrades exactly when the work is most urgent. A lightbar takes it to 3–5%, and RTK autosteer to well under 1%. On a 2,000 ha spraying programme, moving from 8% to 2% stops 120 ha worth of chemical being applied twice, every pass, every season — which is the calculation that justifies the equipment far more reliably than the fatigue argument does.

One caution about accuracy specifications. A receiver quoted at ±2 cm is quoting PASS-TO-PASS accuracy over about fifteen minutes, which is the relevant figure for adjacent passes and the reason it is the one advertised. Year-to-year repeatability — coming back to the same line next season for controlled traffic or strip-till — is a different and much looser specification unless the system is corrected against a fixed base. And whatever the receiver does, the implement can still wander: a trailed tool on a side slope tracks downhill of the tractor, and no amount of satellite precision fixes an implement that is not steered.