offset

An offset is a measured perpendicular distance from a surveyed baseline to a point that lies away from that line. The surveyor runs a main chain or tape along a convenient path, then measures sideways at right angles to capture the true position of an irregular boundary, building, or other feature. This method lets surveyors work efficiently along straight lines while still recording the exact location of obstacles and irregular edges without having to chain directly to them.

Offsets are essential for mapping boundaries that zigzag, curve, or contain fixed structures. A surveyor might run a chain along one side of a property while measuring offsets across to a fence line, riverbank, or cliff face at regular intervals, say every 10 or 20 metres. These perpendicular measurements create a series of points that, when plotted, recreate the actual boundary on the final plan. The perpendicular requirement is strict: any angle other than 90 degrees distorts the recorded position and introduces error.

Common practice and measurement

In traditional surveying, offsets are measured using an optical square, a hand held device that ensures the perpendicular is true, combined with a tape held normal to the baseline. Modern practice sometimes uses laser squares or electronic theodolites to establish the right angle more precisely. Offsets are typically kept short, usually under 20 metres, because measurement error accumulates as the perpendicular distance grows. For longer offsets or where accuracy matters most, surveyors may set out auxiliary lines or use trig to control the work.

The offset method works well for linear surveys of irregular features, but has limits. If boundaries curve sharply or the terrain is very broken, a single offset every 10 metres may miss detail. Surveyors must judge the interval spacing: too close and the work is laborious; too sparse and the plot is inaccurate. Offset distances are recorded in a fieldbook alongside the chainages along the baseline, then plotted at right angles from the baseline on the plan. Any blunder in the perpendicular or in the distance measurement will put the boundary in the wrong place, so care and cross checking are essential.