trigging

Trigging is the practice of performing triangulation to determine the relative positions of points over a region, by constructing a network of survey stations and measuring angles and distances between them. The technique emerged in the 18th and 19th centuries as a practical method for basemapping large areas before the advent of precise satellite navigation. In many countries, fixed trig points were erected on prominent natural or man-made features, such as hilltops, or on monuments, capped with steel discs or pillars that could be observed from afar.

In a typical trigging operation, surveyors select a network of primary stations, determine a baseline distance, and measure the angles to adjacent stations using a theodolite or contemporary total station. Distances may be measured with tapes, EDM, or laser ranging. The resulting angular data are combined with the baseline to compute a network of coordinates through triangulation, often refined with least-squares adjustment to minimize errors and ensure consistency with an existing datum.

With the rise of global navigation satellite systems, direct GNSS methods have reduced the dependence on traditional trig points for new surveying. However, trig networks remain important for historical mapping, geodetic control, and as reference markers. Access to trig points is sometimes restricted to preserve their condition and surrounding environment.