Mapping Mountains – Significant Height Revisions – 200m Twmpau

 

Mynydd Bach Brechfa (SN 520 286) 

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 derived from detail produced by Joe Nuttall in his surface analysis programme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Myrddyn Phillips. 

LIDAR image of Mynydd Bach Brechfa (SN 520 286)

The criteria for the list that this height revision applies to 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. 

200m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Mynydd Bach Brechfa and this was derived from the Ordnance Survey series of Six-Inch maps, and it is adjoined to the Mynydd Pencarreg 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 B4310 road to its north and west, and a minor road to its immediate south-east, and has the village of Brechfa towards the north north-east. 

Extract from the Ordnance Survey 1:50,000 Landranger map

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 main P30 category and listed with a 294m summit height, based on the spot height that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

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

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

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 this hill it had 5m contouring, with the uppermost contour being 295m, resulting in an estimated c 297m summit height based on interpolation and a summit relocation to SN 51818 28761.

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 summit image of Mynydd Bach Brechfa (SN 520 286)

LIDAR analysis gives the highest natural ground on this hill as 296.1m positioned at SN 52039 28618, and when compared to its originally listed summit height of 294m 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 296.1m and this was derived from LIDAR analysis, this is 2.1m higher than the originally listed summit height of 294m, which was based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Mynydd Pencarreg 

Name:  Mynydd Bach Brechfa 

OS 1:50,000 map:  146

Summit Height (New Height):  296.1m (LIDAR) 

Summit Grid Reference:  SN 52039 28618 (LIDAR) 

Bwlch Height:  223.7m (LIDAR) 

Bwlch Grid Reference:  SN 51351 28147 (LIDAR) 

Drop:  72.3m (LIDAR) 

 

Myrddyn Phillips (March 2024)