Mapping Mountains – Summit Relocations – 500m Twmpau and Yr Uchafion

Cistfaen (SN 867 775)

There has been confirmation of a Summit Relocation to a hill that is listed in the 500m Twmpau and the Yr Uchafion, with the summit height, bwlch height and their locations, drop and status of the hill being confirmed by a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips on the 28^th March 2019, and the height and position of the previously listed summit confirmed by LIDAR analysis conducted by Aled Williams.

Cistfaen (SN 867 775)

The criteria for the two listings that this summit relocation 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.

Yr Uchafion – Welsh hills at or above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams, with the Introduction to this list appearing on Mapping Mountains on the 4^th November 2015.

The name of the hill is Cistfaen, and it is adjoined to the Elenydd group of hills, which are situated in the central part of Mid and West Wales (Region B, Sub-Region B2), and is positioned with minor roads to its north and south and has the A44 road further to its north and the A470 road to its east, and has the small community of Llangurig and the town of Llanidloes towards the north-east.

When the listing that would later be referred to as Yr Uchafion was first compiled this hill was listed with a summit position placed at SN 86474 77145 based on the 535m spot height that appears on the Ordnance Survey 1:25,000 Explorer map.  This position is in a conifer plantation with the Ordnance Survey 1:50,000 Landranger map also giving a 535m spot height to an ancient cairn that is positioned just outside of the conifer plantation.

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

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

This hill’s summit was subsequently relocated to SN 86788 77502 based on the 536m spot height that appears on the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.

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.

LIDAR only covers the area around the ancient cairn and the 535m map heighted top, and its analysis resulted in the following:

Ancient cairn:  535.0m at SN 86367 77036

535m spot height:  534.9m at SN 86465 77153

The survey with the Trimble GeoXH 6000 at the 536m map heighted summit resulted in the following:

536m spot height:  535.1m at SN 86787 77515

All three results are close in height but as the result produced by the Trimble GeoXH 6000 survey is the highest it is considered the best available evidence for this hill’s summit position and height, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to any listed hill whose summit meets the following criteria; where there are a number of potential summit positions within close proximity and the highest point is not where previously given, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist, or when the summit of the hill is in a different field compared to where previously given, or when the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct.  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 relocations.

The Trimble GeoXH 6000 gathering data at the summit of Cistfaen

The summit height produced by the Trimble GeoXH 6000 survey is 535.1m and is positioned at SN 86787 77515.  This position is given a 536m spot height on the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map and is approximately 450 metres north-eastward from where the previously listed summit position is situated, and approximately 600 metres north-eastward from the ancient cairn that LIDAR analysis gives as 0.1m higher than the original listed summit position.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Elenydd

Name:  Cistfaen

OS 1:50,000 map:  135, 136 147

Summit Height:  535.1m (converted to OSGM15)

Summit Grid Reference (new position):  SN 86787 77515

  

Bwlch Height:  512.9m (converted to OSGM15)

Bwlch Grid Reference:  SN 86609 77748

Drop:  22.3m (converted to OSGM15)

For details on the survey of this hill

Myrddyn Phillips and Aled Williams (August 2019)