Frant has a high ridge, a church on top of it and long views in most directions. In the 1780s those three facts briefly made it scientifically useful.
An argument between observatories
In 1783 the director of the Paris Observatory sent a memoir to the Royal Society in London. He doubted the accuracy of Greenwich’s figures for its latitude and longitude, and proposed that the matter be settled by joining the two observatories with a precise chain of survey triangles across the Channel. The French had already surveyed their side.
The Astronomer Royal, Nevil Maskelyne, was confident Greenwich was right, but saw that a survey would be valuable in its own right. Sir Joseph Banks, President of the Royal Society, proposed that it be led by William Roy, a Scottish military surveyor then in his late fifties who had spent decades arguing for a national survey of Britain.
How triangles measure a country
The principle is old and elegant. Measure one line on the ground with great care: the baseline. From each end, measure the angle to a distant third point. With one length and two angles, simple trigonometry gives you the other two sides of the triangle without walking them. Each new side becomes the base of the next triangle, and the chain can be carried across a whole country.
Roy’s baseline was measured on Hounslow Heath in 1784, a little over five miles long, eventually using glass rods because wooden ones swelled in the damp. A second check base followed on Romney Marsh in 1787. The angles needed an instrument of unprecedented precision, and Jesse Ramsden took so long to build his great theodolite that, as Roy dryly reported to the Royal Society, nearly three years passed before there was anything to measure the angles with.
Why a church on a ridge
Triangulation stations need to see one another, often over many miles. That means high ground and, where possible, something tall on top of it. Church towers and steeples were ideal: already there, solidly built and visible from a distance. The angles were taken in 1787 and 1788 along a chain running from Greenwich across Kent and Sussex towards the coast near Dover, where it was joined to the French triangles.
Frant church was one of those stations. William Mudge’s account of the trigonometrical survey, published in 1799, illustrates the buildings where instruments were placed, and Frant Church appears among them, alongside the steeples of Goudhurst, Tenterden, Lydd and Swingfield, Severndroog Castle on Shooter’s Hill and the keep of Dover Castle.
There is a pleasing wrinkle. The church the surveyors used is not quite the church you can see today. A drawing of 1797 shows the medieval building with its tower; within twenty years it was described as ruinous and rebuilt, the present tower first, in 1819. The station stood on this site, in this view, but the stones at the top are later.
What it led to
Roy calculated his results between 1788 and 1790 and published them in the Philosophical Transactions of the Royal Society. He died in 1790. The following year, the Board of Ordnance began the trigonometrical survey of Britain that grew into the Ordnance Survey, which counts 1791 as its foundation and Roy’s work as its forerunner.
Every Ordnance Survey map in a walker’s pocket descends, at some remove, from that chain of triangles. On the south wall of St Alban’s you can still find a much humbler survey mark: a cut benchmark, from the later levelling that recorded heights above sea level. It is not Roy’s, but it is a reminder that this churchyard has been helping people measure England for a long time.
If you’re staying at Martins Farm
Stand on Frant green on a clear day and look at the horizon in different directions. You will understand at once why a surveyor with a very expensive theodolite would want to be up here. Then go and buy a map; it is, in a modest way, a local product.