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

 

Tyddyn Gwydryn Llain Fan (SH 495 681) 

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 Tyddyn Gwydryn Llain Fan (SH 495 681)

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 Tyddyn Gwydryn Llain Fan and this was derived from the Tithe map, 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 positioned with its summit adjacent to the A4080 road, and has the village of Brynsiencyn towards the south-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 59m spot height adjoined to a triangulation pillar positioned at SH 49980 68424 and the 59m spot height positioned at SH 50109 68526 and an estimated c 33m bwlch height based on interpolation of 5m contouring between 30m – 35m that appear on the Ordnance Survey 1:25,000 Explorer map. 

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. 

The summit height produced by LIDAR analysis is 60.4m, 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 60.4m and this was derived from LIDAR analysis, this is 1.4m higher than the previous listed summit height of 59m which was based on the spot height adjoined to a triangulation pillar positioned at SH 49980 68424 and the spot height positioned at SH 50109 68526 that appear 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:  Tyddyn Gwydryn Llain Fan 

OS 1:50,000 map:  114, 115

Summit Height (New Height):  60.4m (LIDAR) 

Summit Grid Reference:  SH 49584 68102 (LIDAR) 

Bwlch Height:  31.9m (LIDAR) 

Bwlch Grid Reference:  SH 51324 71408 (LIDAR) 

Drop:  28.5m (LIDAR) 

 

Myrddyn Phillips (May 2021)