River Dysynni – Normal Tidal Limit (NTL) (FOI2515)

We received your request on 8 March 2025

We have handled your request under the Freedom of Information Act (FOIA) 2000.

A copy of your request is set out in the extract below:

“Under the Freedom of Information Act would you please send me the details of the research and investigations that have been carried out [regarding a change in the Normal Tidal Limit on the River Dysynni in Gwynedd].” 

I confirm that Ordnance Survey does hold the information you have requested.

Below are the details of the investigations carried out.

November 2019

On 18 November 2019, a Ground Survey was completed to determine the NTL because of a customer complaint.

OS’s capture specification for the NTL is the highest point in tidal rivers to which mean tides (in Scotland, mean spring tides) flow at high water where there is no other feature present.

Therefore, a survey of an NTL is carried out on a date and time at which there is a Mean Tide; this is determined using tide tables. The UK Hydrographic Office (UKHO) indicated that 14 November was a Spring Tide, and 21 November a Neap Tide. Mean Tides are recorded halfway between Spring and Neap Tides, and the date of 18 November was the closest to this where other conditions were also suitable (see below). The Admiralty Tide Tables show the Mean High Tide for Barmouth to be 4.35m. On 18 November, the High Tide for Barmouth according to the UKHO website was 4.4m, so tidal range on the day of survey was very close to the mean, and well within our tolerances.

The method used to measure the tidal influence on the river was our standard method. Namely, to record how far upstream a floating object is carried on the rising tide, before it is counteracted by the flow of the river downstream. The measurement needs to be taken on a relatively still day in terms of wind, as (i) wind in an upstream or downstream direction can have an influence on how far the object is carried, (ii) a strong onshore wind would exaggerate the tide height, and (iii) a strong offshore wind would reduce the tidal influence. In addition, the river flow needs to not be abnormal; for example, we would not undertake a survey of an NTL in a time of drought or following a prolonged period of heavy rainfall. Atmospheric pressure also affects tidal range, so ideally extremes are avoided. It is for the surveyor to determine whether wind speed and direction, atmospheric pressure, and tidal flow are all acceptable, i.e. unlikely to negatively impact on an accurate measurement. On the day of the survey there was negligible wind, and both atmospheric pressure and tidal flow were normal.

The result of the survey was to update the position of the NTL approximately 1.1 km downstream. This new position of the NTL was published in our MasterMap Topography mapping data in January 2020.

2020

Following publication of the new position of the NTL, our Customer Service Centre (CSC) received complaints suggesting that the NTL position was incorrect.

In May 2020, CSC met with our Surveying team and a full review of matter was conducted. This review confirmed that the surveyor had performed a full survey of the site in November 2019 and that the location of the NTL as published in January 2020 was correct. Our remit is to reflect what is currently on the ground at the time of survey.

Following this review, further evidence was submitted to OS and a meeting was held between CSC, the Surveying team and our product specification team to consider the position of the NTL in light of the evidence supplied. Although the evidence highlighted potential errors with our foreshore attribution at the River Dysynni, it did not impact on the position of the NTL captured in December 2019 and published in January 2020.

In September 2020, an aerial survey was conducted to correct any inaccuracies in the data OS held for the foreshore along the river. Although the NTL itself was not directly investigated, we did consider its position when making the re-attribution of foreshore. No update to the position of the NTL was deemed necessary.

2021

We received a copy of a report prepared by Bangor university entitled “Characterisation of water levels on the lower Afon Dysynni” (PDF). Following receipt, we held a meeting with its authors to discuss their methodology and to consider whether any amendment to the NTL’s position should be made. Having reviewed the report, discussed it with its authors, and considered all available evidence, OS determined the location for the NTL is in the proximity of Mooring-1 and the most logical position for the NTL is at the entrance to the Dysynni River where it enters Broadwater.

This was updated in our MasterMap Topography on 5 October 2022.  This is where the NTL is currently depicted.

Advice and Assistance

Under the duty to provide information and assistance in accordance with section 16 of FOIA, we can provide the following information which may assist you in this matter:

• Detail on OS maps is always shown in its current real world position on the day a survey is made.
• Our topographic data represents physical features, captured to OS specification, and it does not reflect nor is it influenced by legal ownership.
• Ordnance Survey does not have the requirement to perform a consultation when repositioning physical features. However, we do have the right to choose which 3^rd Parties we seek additional information from. The Bangor University data was used in this instance because it used more advanced methodology and provided more accurate data than that available from standard OS survey.
• When our mapping is updated either by a surveyor on the ground or by photogrammetry, we do not keep a separate record or catalogue of these alterations; the map itself is our only record of the mapping decisions made at that time.