Mapping Mountains – Significant Height Revisions – 100m Twmpau

Bryn Bach (SS 903 875)

There has been a Significant Height Revision to a hill that is now listed in the 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Bryn Bach

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 name of the hill is Bryn Bach, and this was derived from the Ordnance Survey One-Inch ‘Old Series’ map and it is this name that the hill is now listed by, and it is adjoined to the Cymoedd Morgannwg group of hills, which are situated in the central part of South Wales (Region C, Sub-Region C2), and is positioned with the A4063 road and the Afon Llynfi to its south-west and the A4064 road and the Afon Garw to its south-east, and has the small community of Betws towards the south. 

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 Hills to be surveyed sub list that accompanied the main P30 list, as it did not meet the criteria then used. 

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 31m of drop, based on an estimated summit height of c 164m and an estimated bwlch height of c 133m, with both heights based on interpolation of 10m contouring on the Ordnance Survey 1:25,000 Explorer map.

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 became available online; this mapping is entitled the Interactive Coverage Map.  This mapping gives a 164m summit spot height and as it has 5m contour intervals it enabled a better assessment for an interpolated value for the height of the bwlch.  It also has a 133m spot height at a junction of roads at the area of this hill’s bwlch, with interpolation favouring a different position for the critical bwlch and an estimated height of c 133m, with these values giving this hill c 31m of drop.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website

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.  

The summit height produced by LIDAR analysis is 167.0m, this is not a substantial revision compared to some revised heights, but it does come 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.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new listed summit height is 167.0m and this was produced by LIDAR analysis, this is 3m higher than the 164m spot height that appears on the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Cymoedd Morgannwg

Name:  Bryn Bach

OS 1:50,000 map:  170

Summit Height (New height):  167.0m (LIDAR)

Summit Grid Reference:  SS 90364 87431 (LIDAR)

  

Bwlch Height:  135.2m (LIDAR)

Bwlch Grid Reference:  SS 90511 87849 (LIDAR)

Drop:  31.9m (LIDAR)

Myrddyn Phillips (June 2019)