Mapping Mountains – Significant Height Revisions – 30-99m Twmpau, Y Trechol – The Dominant Hills of Wales and Y Pellennig – The Remotest Hills of Wales

North Bishop (SM 670 280) 

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

North Bishop (SM 670 280)

The criteria for the three listings 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

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

Y Pellennig – The Remotest Hills of Wales - Welsh hills whose summit is at least 2.5km from the nearest paved public road and the hill has 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available as a downloadable e-booklet and print-booklet version on Mapping Mountains Publications with the up-to-date master list available to download on the Mapping Mountains site in Google Doc format.

Y Pellennig - The Remotest Hills of Wales by Myrddyn Phillips and Aled Williams

The name the hill is listed by is North Bishop, and it is adjoined to the Garn Fawr group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it forms part of an islet chain positioned to the west of mainland Wales with a minor road on the mainland to its east south-east, and has the city of Tyddewi (St David’s) on the mainland also towards the east south-east.

When the original 30-99m height band of Welsh P30 hills compiled by Myrddyn Phillips were published on Geoff Crowder’s v-g.me website, this hill was listed with a 44m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.  And it was this height that was given the hill when the listings of the Y Trechol – The Dominant Hills of Wales and Y Pellennig – The Remotest Hills of Wales were subsequently published. 

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 image of North Bishop (SM 670 280)

The summit height produced by LIDAR analysis is 40.0m and when compared to the previously listed summit height, this comes within the parameters of the Significant Height Revisions used within this page heading, and 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 40.0m and this was derived from LIDAR analysis.  This is 4.0m lower than the previous listed summit height of 44m, which was based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Garn Fawr 

Name:  North Bishop 

OS 1:50,000 map:  157

Summit Height (New Height):  40.0m (LIDAR) 

Summit Grid Reference:  SM 67087 28056 (LIDAR)               

Bwlch Height:  N/A (sea level) 

Bwlch Grid Reference:  N/A (sea level) 

Drop:  40.0m (LIDAR) 

Dominance:  100.00% (LIDAR) 

Remoteness:  6.000km 

 

Myrddyn Phillips and Aled Williams (November 2025)