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

Parc Penrhyn (SH 601 720) 

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 Parc Penrhyn (SH 601 720)

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 that have 30m minimum drop, with an accompanying sub list 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.  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 listed by is Parc 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 A5 road to its west and the A55 road to its south-east, and has the city of Bangor towards the west.

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.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 19m of drop, based on an estimated c 53m summit height and an estimated c 34m bwlch height, with both heights based on interpolation of 10m contouring. 

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 Parc Penrhyn (SH 601 720)

The summit height produced by LIDAR analysis is 56.5m and is positioned at SH 60179 72091, 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 56.5m and this was derived from LIDAR analysis, this is 3.5m higher than the previously listed summit height of c 53m, which was based on interpolation of its uppermost 50m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

The full details for the hill are:

Group:  Glyder Fawr 

Name:  Parc Penrhyn 

OS 1:50,000 map:  115

Summit Height (New Height):  56.5m (LIDAR) 

Summit Grid Reference:  SH 60179 72091 (LIDAR)               

Bwlch Height:  31.5m (LIDAR) 

Bwlch Grid Reference:  SH 60168 71662 (LIDAR) 

Drop:  25.1m (LIDAR) 

 

Myrddyn Phillips (March 2025)