Mapping Mountains – Significant Height Revisions – The Welsh P15s

Tywyn Niwbwrch (SH 419 625) 

There has been a Significant Height Revision to a hill that is listed in The Welsh P15s, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Tywyn Niwbwrch (SH 419 625)

The criteria for the list that this height revision applies to are:

The Welsh P15s – Welsh hills with 15m minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s, with the criteria for this sub category being all Welsh hills with 14m or more and below 15m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list appearing on Mapping Mountains on the 10^th May 2019. 

The Welsh P15s by Myrddyn Phillips

The name the hill is listed by is Tywyn Niwbwrch and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the coast to its south-west and the A4080 road to its north-east, and has the village of Niwbwrch (Newborough) towards the north. 

When the listing that became known as The Welsh P15s was being compiled, this hill was not included in either the main P15 list or the accompanying P14 sub list, as it possessed no contours of significance on either the Ordnance Survey 1:50,000 Landranger or the 1:25,000 Explorer map. 

Extract from the Ordnance Survey 1:25,000 Explorer map

Since the initial compilation of this list started there have been a number of Ordnance Survey maps made available online, some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website, whilst others are current and digitally updated such as the mapping on the Magic Maps website. 

One of the mapping resources now available online is the Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  This map has many spot heights not on other publicly available Ordnance Survey maps and for this hill a 27m spot height is given on the area of its summit. 

Extract from the Magic Maps website

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR summit image of Tywyn Niwbwrch (SH 419 625)

The summit height produced by LIDAR analysis is 26.9m and this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are: 

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis. 

Therefore, this hill’s new listed summit height is 26.9m and this was derived from LIDAR analysis.  This is 26.9m higher than detail on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map which do not give any significant contours for this area, but its new listed summit height is similar to the 27m spot height that appears on the interactive mapping on the Magic Maps website. 

 ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Ynys Môn 

Name:  Tywyn Niwbwrch 

OS 1:50,000 map:  114, 115

Summit Height (New Height):  26.9m (LIDAR)                                                           

Summit Grid Reference:  SH 41937 62552 (LIDAR) 

Bwlch Height:  8.4m (LIDAR) 

Bwlch Grid Reference:  SH 41804 62760 (LIDAR) 

Drop:  18.4m (LIDAR) 

 

Myrddyn Phillips (October 2021)