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Part Three.


The Mapping of Great Britain
by Chris Knights.


“WHAT ABOUT THE VERTICALS & WHERE'S SEA LEVEL?”

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riangulation was essentially concerned with calculating horizontal distances but vertical distances ie heights were also important and not just because the height differences had to be taken into account when doing the triangulation calculations. The measurements of heights was originally done using the same theodolite on a vertical plane as was used for the horizontal measurements of angles in the triangulation survey but this proved to be not very accurate especially over long distances as the measurements were affected by refraction. Later measurements of heights were carried out (originally in the 1830s) by means of ‘levelling’ where two ‘levelling staffs’ – or long vertical poles – were placed a few hundred yards apart with a telescope kept horizontal by a spirit level being set up between the staffs. The staffs were marked with heights in inches and the telescope was first trained on one staff and the height shown on the staff was noted, then the telescope was turned round and trained on the other staff and the height again noted. The difference in heights indicated the increase or decrease in the height between the two staff positions.
  

But height above what?  Sea level was the obvious choice, Britain being an island. It was initially thought that sea level was the same everywhere but after some experimentation it was soon discovered that sea level varied from place to place. Originally Liverpool was chosen, then Felixstowe, then Dunbar in Scotland but eventually experiments showed that Newlyn in Cornwall had the most constant mean sea level of all these. A new tide guage was established at the end of the stone pier at Newlyn harbour in 1915 and records of sea level were made hourly for the next six years. From this data ‘mean sea level’ was calculated and Newlyn became the ‘datum’ for all OS maps showing heights above sea level. The Newlyn datum is now accepted as the most accurate datum for sea level anywhere in the world, and it is noted on all OS maps (see the bottom right corner of your Landranger map legend) as ‘Vertical Datum Mean Sea Level Newlyn’ or‘ODN’ – Ordnance Datum Newlyn.

fundbenchmark1.JPG

Having established the datum, a more accurate levelling could take place and several did over the next  50 years. The latest levelling survey of the whole country took place in the 1950s and 1960s during which a hundred or so ‘Fundamental Benchmarks’ were levelled from Newlyn. Once these were established and marked, secondary levelling could take place based on the main benchmarks. Secondary levels were marked (diagram above) on any convenient and reasonably permanent feature such as walls, buildings, churches and so on.

 secondarylevel.JPG

The 100 or so fundamental benchmarks were meant to be completely secure and permanent and were usually chosen because the geology was very stable. The levellers dug down to the bedrock and marked the precise level in the bedrock itself, often with a brass bolt. The pit was then filled with gravel and sealed. Above ground the site was simply marked with a granite block about 2’ tall rather like a small trig point. These sites are not marked on the usual maps but I can reveal that the nearest fundamental benchmark to us is near Church Stretton at 137/467927. All the fundamental benchmarks are still used by the Ordnance Survey as ‘controls’ for  modern GPS surveying.

secondarybmark1.JPG

Secondary or lesser benchmarks, measured from the fundamental benchmarks, are usually marked with the standard OS benchmark sign – a horizontal line with an vertical arrow underneath – and they are to be found all over the place; all trig points have them and they are also carved onto buildings, walls etc. There are three in Pattingham and the one pictured left is  about two feet up the wall on the south east corner of St Chad’s church over the road from The Pigot. The OS database indicates that St Chad’s benchmark is 118.81 metres above mean sea level at Newlyn.


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