Metal detector

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Section 7.032 in The History of the Retriangulation of Great Britain [1] mentions that, for Buried Blocks, a metal ring was sometimes buried with the block so as to enable it to be found by mine detectors if the locating description should prove insufficient. A number of these rings, commonly known as Concrete Rings, have been rediscovered but many more are thought to exist. Elsewhere, the same purpose was met by embedding scrap metal in the block.

To date, metal detectors have not been widely used by baggers when searching for blocks. Some success has been reported in a very small number of cases, including Surface Blocks: Winstanley Stocks, Hordern Stoops and Clyne Wood. For the latter, a 2" bolt was detected at a depth of approx 3". There is a single instance of a Concrete Ring being detected at Middle Healey and a report of a successful find using a metal detector app on an android phone at Ty'N-Y-Fron.

With so few examples, it remains unclear as to whether a low-cost metal detector can be a general-purpose and effective tool for locating trig blocks.

It has also been reported that metal detectors have successfully been used when searching for old levelling rivets installed in the 50s 60s and 70s. These rivets are only the size of a finger nail, but generally on rocks etc that have grown over with moss, bracken, heather and peat bog. [2]


  1. The History of the Retriangulation of Great Britain 1935-1962, Section 4.00, p.226
  2. Email from Graham Pennington, Ordnance Survey, to trigonomy mailing list, 24/02/2013