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

 

30-99m Twmpau – Significant Height Revisions

The 30-99m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 30m and below 100m in height that have a minimum drop of 30m.  Accompanying the main P30 list are two sub lists entitled the 30-99m Sub-Twmpau with the qualification to 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, and the Double Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 20m and below 30m in height with 20m or more and below 30m of drop.

The list is authored by Myrddyn Phillips and the posts that have appeared on Mapping Mountains detailing the significant height revisions to the main P30 list and the sub lists appear below presented chronologically in receding order.

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

Needle Rock (SM 975 379) - 36th significant height revision

 

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Needle Rock (SM 975 379)

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. 

The 30-99m Twmpau by Myrddyn Phillips

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Needle Rock, and it is adjoined to the Mynydd Preseli group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it is positioned with the A487 road to its south, and has the town of Abergwaun (Fishguard) towards the west 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 included in the main P30 list with a summit height of 40m, with an accompanying note stating; Approximate height of tidal sea stack from Dave Viggers. 

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

This hill is a sea stack and is cut off from mainland Wales at high tide.  As the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map give this hill no spot height or contours 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 Needle Rock (SM 975 379)

The summit height produced by LIDAR analysis is 33.9m and is positioned at SM 97569 37997, 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 33.9m and this was derived from LIDAR analysis, this is 6.1m lower than the originally listed summit height of 40m, which was based on an approximation given the author by Dave Viggers.

 

The full details for the hill are:

Group:  Mynydd Preseli 

Name:  Needle Rock 

OS 1:50,000 map:  159

Summit Height (New Height):  33.9m (LIDAR) 

Summit Grid Reference:  SM 97569 37997 (LIDAR)               

Bwlch Height:  N/A sea level (LIDAR) 

Bwlch Grid Reference:  SM 97566 37987 (LIDAR) 

Drop:  33.9m (LIDAR) 

Dominance:  100.00% (LIDAR) 

 

Myrddyn Phillips (August 2023)