Request for information – FOI13399
Thank you for your e-mail of 19 July 2013, requesting the following information from Ordnance Survey in accordance with the Freedom of Information Act (FOIA) 2000:
Could you please advise me of the date and method of survey of Mean High Water on the latest editions of 1:1250 scale maps TQ5180NW and TQ5181SW published before January 1976. If the method of survey was from aerial photography, can you please advise me if copies of the photography used for the survey are available for purchase.
Ordnance Survey does not hold the information you have requested. By inspection, the map references quoted are of the north-eastern shore of the Thames Estuary at the northern end of Erith Reach close to Frog Island. Given the date of mapping in question, we do not hold records to hand of the methods of survey used in our offices here. Any records relating to our surveys in that timeframe will have been deposited with The National Archives at Kew under the terms of the Public Records Acts, and may be accessed by approach to them.
The documents required will be the 'Object Names Books' (ONBs) for TQ 5180 and TQ 5181, which, in addition to containing the authority for all of the distinctive names appearing on each map, will also include information on all of the other types of information contained in the maps, and their survey methods. The ONB records should include information on the date and survey methods for the tide lines shown on these maps.
However, under Section 16 of the Freedom of Information Act 'the duty to provide advice and assistance' we are able to advise that it is likely that the mapping current in 1976 would have been the edition of 1966 or 1967, or a subsequent revision. Given that the river bank shown on that section of the River Thames on the 1966-67 edition and subsequently appears to be a long established sea wall, reinforced on the river side with rocks and shingle, it is highly likely that the Mean High Water mark was surveyed by field survey methods, marking the alignment of the water at a designated point in time correlated to when the tide was deemed to conform to Mean High Water, and then surveying the marked points along the shingle bank and sea wall once the tide had receded.
Mean High Water is related to 'Mean Sea Level', which in turn is determined by reference to the Newlyn Datum. This was established through many years of observations of fluctuations in tides recorded by the Ordnance Survey Tide Gauge established in the Harbour Wall at Newlyn in Cornwall from 1915, and with measurements recorded at 15 minute intervals over 365 days per annum until 1921. Observations for scientific purposes continued until the Tide Gauge was decommissioned in 1983.
It follows that it is unlikely that there is relevant aerial imagery available from which the tide line might be inferred. If however it transpires from inspection of the ONBs that it was the case that the tide lines were inferred from aerial photography, then all of the historical air photography related to Ordnance Survey mapping has been deposited with English Heritage Archive, and any images from this time which exist will be accessible there under Public Records Act deposition rules. You may contact English Heritage as follows:
The Engine House
Fire Fly Avenue
Telephone: 01793 414600
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Thank you for your enquiry.