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

Penymorwydd (SH 384 912) 

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 detail on contemporary maps produced from Ordnance Survey data and LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Penymorwydd (SH 384 912)

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, 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, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Penymorwydd 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 encircled by minor roads with the A5025 road farther to its north-west, and has the village of Cemaes towards the north-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 26m of drop based on the 69m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 43m bwlch height based on interpolation of 5m contouring between 40m – 45m. 

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 Penymorwydd

The summit height produced by LIDAR analysis is 70.5m, 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 70.5m and this was derived from LIDAR analysis, this is 1.5m higher than the previous listed summit height of 69m which was based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Ynys Môn 

Name:  Penymorwydd 

OS 1:50,000 map:  114

Summit Height (New Height):  70.5m (LIDAR) 

Summit Grid Reference:  SH 38471 91255 (LIDAR) 

Bwlch Height:  c 43m (interpolation) 

Bwlch Grid Reference:  SH 38636 90826 (interpolation) 

Drop:  c 28m (LIDAR summit and interpolated bwlch) 

 

Myrddyn Phillips (July 2021)