Mapping Mountains – Significant Height Revisions – 600m Twmpau and Welsh Highlands – Uchafion Cymru

Pt. 631.7m (SH 733 622) 

There has been a Significant Height Revision to a hill that is listed in the 600m Twmpau and the Welsh Highlands – Uchafion Cymru, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Aled Williams. 

Pt. 631.7m (SH 733 622)

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

600m Twmpau - Welsh hills at or above 600m and below 700m in height with 30m minimum drop, with an accompanying sub list entitled the 600m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 600m and below 700m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  The list is authored by Myrddyn Phillips and is available to download in Google Doc format from Mapping Mountains.

600m Twmpau by Myrddyn Phillips

Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

The hill is being listed by the point (Pt. 631.7m) notation and it is adjoined to the Carnedd Llywelyn 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 A5 road to its south-west and the B5106 and A470 roads to its east, and has the village of Capel Curig towards the south south-west. 

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

When the original list that later became known as the Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with an estimated c 36m of drop based on the 634m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 598m bwlch height based on interpolation of 10m contouring between 590m – 600m.  The drop value for this hill was later amended to 36m, based on the 598m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

LIDAR image of Pt. 631.7m (SH 733 622)

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 Pt. 631.7m (SH 733 622)

The summit height produced by LIDAR analysis is 631.7m and is positioned at SH 73366 62247 and SH 73365 62246, 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, the new listed summit height of this hill is 631.7m and this was derived from LIDAR analysis, this is 2.3m lower than the previously listed summit height of 634m, which was based on the spot height that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Carnedd Llywelyn

Name:  Pt. 631.7m

OS 1:50,000 map:  115

Summit Height (New Height):  631.7m (LIDAR)

Summit Grid Reference:  SH 73366 62247 & SH 73365 62246 (LIDAR)

Bwlch Height:  595.9m (LIDAR)

Bwlch Grid Reference:  SH 73256 62104 (LIDAR) 

Drop:  35.8m (LIDAR)

 

Myrddyn Phillips and Aled Williams (September 2024)