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

Bryn (SH 782 798) 

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 Bryn (SH 782 798)

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, 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 Bryn and this was derived from the Tithe map, and it is adjoined to the Mynydd Hiraethog group of hills, which are situated in the northern part of North Wales (Region A, Sub-Region A2), and it is positioned with the A546 road to its west and the B5115 road to its south-east, and has the town of Deganwy towards the north.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list with a summit height of 90m, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

When 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 23m of drop, based on the 90m summit spot height and an estimated c 67m bwlch height, based on interpolation of 5m contouring between 65m – 70m. 

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 Bryn (SH 782 798)

The summit height produced by LIDAR analysis is 87.2m and is positioned at SH 78234 79892, 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 87.2m and this was derived from LIDAR analysis, this is 2.8m lower than the previously listed summit height of 90m, which was based on the spot height 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:  Mynydd Hiraethog 

Name:  Bryn 

OS 1:50,000 map:  115

Summit Height (New Height):  87.2m (LIDAR) 

Summit Grid Reference:  SH 78234 79892 (LIDAR)               

Bwlch Height:  65.6m (LIDAR) 

Bwlch Grid Reference:  SH 78372 79790 (LIDAR) 

Drop:  21.6m (LIDAR) 

 

Myrddyn Phillips (June 2024)