Mapping Mountains – Significant Height Revisions – 30-99m Twmpau

Yr Ynys (SN 614 896) 

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau, 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 Yr Ynys (SN 614 896)

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, and the second sub category is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

30-99m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Yr Ynys and this was derived from the Tithe map, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road and the B4353 road to its west, and has the small community of Borth towards the south-west. 

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

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category, as with no contours on the Ordnance Survey 1:25,000 Explorer map it was difficult to know whether any hill of note existed.  However, although details for this list were initially taken from the 1:25,000 series of mapping the Ordnance Survey 1:50,000 Landranger map does have a 20m uppermost ring contour for this hill. 

Extract from the Ordnance Survey 1:50,000 Landranger map

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of 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 were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

Another resource now available online is the interactive mapping hosted on the Welsh Government website and entitled the DataMapWales.  This mapping has 5m contours and its detail matches that produced from the OS Terrain 5 product, which compliments much of that produced from LIDAR, and for this hill it has a 24m summit spot height. 

Extract from the DataMapWales

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 Yr Ynys (SN 614 896)

The summit height produced by LIDAR analysis is 25.45m and is positioned at SN 61402 89639, 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, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 25.45m and this was derived from LIDAR analysis, this is 25.45m higher than detail given on the Ordnance Survey 1:25,000 Explorer map, which has no significant contours of note for this hill.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Banc Llechwedd Mawr 

Name:  Yr Ynys 

OS 1:50,000 map:  135

Summit Height (New Height):  25.45m (LIDAR) 

Summit Grid Reference:  SN 61402 89639 (LIDAR)               

Bwlch Height:  3.0m (LIDAR) 

Bwlch Grid Reference:  SN 61317 89488 & SN 61319 89492 (LIDAR) 

Drop:  22.4m (LIDAR) 

 

Myrddyn Phillips (July 2025)