Mapping Mountains – Significant Height Revisions – 500m Twmpau and The Welsh Highlands – Uchafion Cymru

Twyn Crug yr Afan (SS 920 954) 

There has been a Significant Height Revision to a hill that is listed in the 500m Twmpau and The Welsh Highlands – Uchafion Cymru, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and Aled Williams. 

Twyn Crug yr Afan (SS 920 954)

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

500m Twmpau – Welsh hills at or above 500m and below 600m in height that have 30m minimum drop, with an accompanying sub category entitled the 500m Sub-Twmpau consisting of all Welsh hills at or above 500m and below 600m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  The list is authored by Myrddyn Phillips and is published on Mapping Mountains in Google Doc format.

The 500m Twmpau by Myrddyn Phillips

The Welsh Highlands – Uchafion Cymru – This is the revised draft title for the Welsh 500m P15s list that takes in all hills in Wales at or above 500m in height with 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams with the introduction to the list being published on Mapping Mountains in November 2015 and an update relating to the list appearing on Mapping Mountains on the 19^th June 2019.

The Welsh Highlands - Uchafion Cymru by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Twyn Crug yr Afan 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 it is positioned with the A4058 road to its north-east and the A4107 road to its immediate south, and has the connecting villages of Blaengwynfi and Abergwynfi towards the west north-west.

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

When the original list of Welsh 500m P15s that later became known as Yr Uchafion and latterly as The Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with 31m of drop, based on the 558m spot height that appears on the Ordnance Survey 1:50,000 Landranger map and the 527m bwlch spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

Since the original compilation of this list there have been a number of Ordnance Survey 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 are current and digitally updated such as the old Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and the mapping on the OS Maps, Magic Maps and WalkLakes websites. 

The details for this hill were reassessed 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 maps and for this hill it had a 559m summit spot height adjoined to the ancient tumulus and positioned at SS 92037 95445. 

One of the mapping resources now available online is the Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  This mapping also has many spot heights not on other publicly available Ordnance Survey maps and for this hill the 559m summit spot height is also shown. 

Extract from the Magic Maps website

The summit of this hill has now been surveyed with the Trimble GeoXH 6000, resulting in 560.2m positioned at SS 92035 95448 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 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 Twyn Crug yr Afan

Therefore, this hill’s new listed summit height is 560.2m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SS 92035 95448 and is 2.2m higher than its originally listed height of 558m which appears as a spot height on contemporary Ordnance Survey 1:50,000 Landranger maps, and 1.2m higher than the 559m spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and which now appears on the interactive mapping on the Magic Maps website.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Cymoedd Morgannwg

Name:  Twyn Crug yr Afan

OS 1:50,000 map:  170

Summit Height (New Height):  560.2m (converted to OSGM15)

Summit Grid Reference:  SS 92035 95448  

Bwlch Height:  526.0m (converted to OSGM15)

Bwlch Grid Reference:  SS 92019 95054

Drop:  34.2m

 

Aled Williams and Myrddyn Phillips (August 2021)