Mapping Mountains – Significant Height Revisions – 200m Twmpau

 

Quarry Field (SJ 155 075)

There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

Quarry Field (SJ 155 075)

The criteria for the list this height revision affects are: 

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Quarry Field and this was derived from the Tithe map, and it is adjoined to the Carnedd Wen group of hills which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with the B4392 road to its north-west and the A458 road to its south, and has the town of Y Trallwng (Welshpool) towards the east. 

When the original 200m 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 that accompanied the main P30 list, as it was considered not to meet the criteria then used for the main P30 category. 

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 25m of drop, based on an estimated c 262m summit height and an estimated c 237m bwlch height, with both heights based on interpolation of 10m contouring that appear on the Ordnance Survey 1:25,000 Explorer map.  With an adjacent hill now listed as Cefn Uchaf (SJ 157 072) given a summit height of 262m based on the spot height that appears on contemporary Ordnance Survey 1:25,000 Explorer maps. 

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for the area taking in this hill it also had contouring at 5m intervals with an uppermost 265m contour, with interpolation giving an estimated c 266m summit height and as this is higher than the 262m summit spot height given Cefn Uchaf the bylchau were swapped and each hill was reclassified. 

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 Quarry Field (SJ 155 075)

The summit of this hill has now been surveyed with the Trimble GeoXH 6000, resulting in 268.5m at SJ 15530 07552 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 Quarry Field

Therefore, this hill’s new listed summit height is 268.5m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SJ 15530 07552 and is 2.5m higher than the previously listed summit height of c 266m which was estimated from the uppermost 265m contour that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

 

The full details for the hill are: 

Group:  Carnedd Wen 

Name:  Quarry Field 

OS 1:50,000 map:  125

Summit Height (New Height):  268.5m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SJ 15530 07552 (Trimble GeoXH 6000) 

Bwlch Height:  209.6m (LIDAR) 

Bwlch Grid Reference:  SJ 16489 07601 (LIDAR) 

Drop:  58.9m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (June 2021)