Know your Benchmarks from your Trig Points

Know your Benchmarks from your Trig Points

All will become clear, thanks to Richard Mogg, surveyor in our Wimborne office

Have you ever spotted a strange marking on the side of a church or building and wondered what it was? If it had a chiselled horizontal line with an arrow head below, it could be a benchmark.

These are used by surveyors so that an angle-iron could be placed to form a ‘bench’ for a levelling rod, thus ensuring that a levelling rod could be accurately repositioned in the same place in the future.

The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark – a point with a precisely known relationship to the vertical datum of the area, typically mean sea level.

You may have seen Trig Points these on Ordnance Survey (OS) maps. Trig points are the common name for triangulation pillars. These are concrete pillars, about 4’ tall, which were used by the OS in order to determine the exact shape of the country.

They are generally located on the highest piece of ground in the area, so that there is a direct line of sight from one to the next. By sitting a theodolite (an accurate protractor built into a telescope) on the top of the pillar, accurate angles between pairs of nearby trigpoints could be measured. This process is called triangulation.

A major project to map out the shape of Great Britain began in 1936. The network of triangulation pillars, with accurately known positions, led to the excellent OS maps we enjoy today.

The coordinate system used on these maps is known as the national grid, and it is essential that you are familiar with this system if you are to get the most out of OS maps, or their website .

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