Mapping Mountains – Significant Height Revisions – 200m Twmpau

Chwarel Penrhyn (SH 621 658) 

There has been a Significant Height Revision to a hill that is listed in the 200m 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 Chwarel Penrhyn (SH 621 658)

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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. 

200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Chwarel Penrhyn, and it is adjoined to the Glyder Fawr 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 B4409 road to its north, a minor road to its west and the A5 road to its east, and has the town of Bethesda towards the north-east.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying Hills to be surveyed sub list, as with no significant contours of note on the contemporary Ordnance Survey 1:25,000 map, it was hard to know whether any hill with qualifying prominence existed and therefore it was considered not to meet the criteria then used for this sub category. 

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

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 Chwarel Penrhyn (SH 621 658)

LIDAR analysis gives the summit of this hill as 220.6m positioned at SH 62149 65875, 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 220.6m and this was derived from LIDAR analysis, this is 70.6m higher than the highest contour in the vicinity of this hill that appears on the contemporary Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Glyder Fawr 

Name:  Chwarel Penrhyn 

OS 1:50,000 map:  115

Summit Height (New Height):  220.6m (LIDAR) 

Summit Grid Reference:  SH 62149 65875 (LIDAR) 

Bwlch Height:  197.0m (LIDAR) 

Bwlch Grid Reference:  SH 61793 65966 (LIDAR) 

Drop:  23.6m (LIDAR)

Myrddyn Phillips (January 2026)