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Tide

On the level

21 September 2006

How does Ordnance Survey measure the height of the British landscape? The secret lies on the Cornish coast.

Next to the lighthouse at the end of a pier in Newlyn, near Penzance, is the starting point for measuring the height of the entire landmass of Great Britain – from the gentlest slope to the tallest mountain.

There you'll find the Tidal Observatory – a room with a small metal plate in the floor. A few metres underneath that is the level that has been recorded by Ordnance Survey as zero height. This is a datum mark or a reference from which all other heights can be measured. Without such a mark it would be impossible to tell how high anything is. Where would you begin? 

This is how it works.

Zero or 'Ordnance Datum' was found by measuring the average sea level over a six-year period, from 1915 to 1921. Floats were located in a chamber beneath the floor of the room. These went up and down with the tides and their movements were drawn by a special machine called a tide gauge.

Every 15 minutes, for 24 hours a day, 356 days a year, the height of the water was marked out onto a graph. From this graph the mean sea level (MSL) was established – our starting point and our zero mark.

A bolt, called the tide gauge bolt, was then set in the floor 4.751 metres above the MSL, allowing our surveyors to see it easily and have a definite position from which to measure the height of the land.

To do this a surveyor holds a levelling staff (essentially, a very big ruler!) on top of the bolt. A colleague stands outside the lighthouse building with a tripod and levelling instrument on top. This instrument is similar to a telescope and allows the surveyor to look through it to exactly line it up with the staff.

When it is lined up the instrument can then be swung around and transferred to another levelling staff. This whole process continued out from Newlyn across the rest of Great Britain.

For example, the bolt is 4.75 metres high. The levelling instrument lines up with the staff (the ruler) at say 2 metres. So the height the surveyor has measured is 6.75 metres.

The surveyor then swings the tripod around to face a colleague holding another staff who is further away. Looking through the telescope he sees he is lined up with the 1–metre line on the staff.

Therefore the ground is 5.75 metres (the known height of the levelling instrument) minus 1 metre (the mark on the ruler).

The evidence of this measuring process, called levelling, is there for everyone to see. If you look carefully you can spot the signs, or bench marks as they are known, around the country. These are a permanent record of heights at different locations.

There are about 200 fundamental bench marks constructed at specially selected sites. These are granite pillars that have been set on very stable ground to minimise movement. You can also see brass plates and etchings on structures (usually walls and gateposts) every half mile or so. There are about 75 000 marks throughout the country. These are all catalogued along with their height, and some are uniquely numbered.

That is how a complete profile of the land was recorded, giving rise to the familiar spot heights and contour lines you see on maps today. This sort of height data is particularly useful for ramblers, but also for planners, civil engineers and insurance companies, to name but a few.

Of course, GPS (Global Positioning System) is now widely used to do the same job. But that is another story!

For more information on GPS see Ordnance Survey's GPS website

 
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