Mapping Mountains – Significant Height Revisions – 100m Twmpau

 

Banc (SN 568 757) 

There has been a Significant Height Revision to a hill that is listed in the 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

Banc (SN 568 757)

The criteria for the list that this height revision applies to are:

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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 100m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Banc and this was derived from the Tithe map, and it is adjoined to the Elenydd group of hills which are situated in the western part of Mid and West Wales (Region B, Sub-Region B2), and it has the coast to its west and the A487 road to its east, and the village of Llanfarian towards the north-east. 

When the original 100m 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 192m summit height, based on the spot height adjoined to a triangulation pillar that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and which is positioned at SN 56695 75938. 

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 Banc (SN 568 757)

The summit height produced by LIDAR analysis indicated a relocation and as the summit has subsequently been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised in preference to the LIDAR height and position for this hill, resulting in 194.2m positioned at SN 56817 75750 and this height 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 summit 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. 

The Trimble GeoXH 6000 gathering data at the summit of Banc

Therefore, this hill’s new listed summit height is 194.2m and this was derived from a Trimble GeoXH 6000 survey, this is 2.2m higher than the previously listed 192m height which was based on the spot height adjoined to a triangulation pillar that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and which is positioned at SN 56695 75938.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Elenydd 

Name:  Banc

OS 1:50,000 map:  135

Summit Height (New Height):  194.2m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 56817 75750 (Trimble GeoXH 6000)

Bwlch Height:  130.2m (LIDAR)

Bwlch Grid Reference:  SN 57845 73062 (LIDAR) 

Drop:  64.0m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (May 2021)