Mapping Mountains – Summit Relocations – 200m Twmpau

 

Llwynderw Hill (SJ 196 036) 

There has been a Summit Relocation 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 LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

LIDAR image of Llwynderw Hill (SJ 196 036)

The criteria for the list that this summit relocation 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 category entitled the 200m Sub-Twmpau consisting of all Welsh hills at or above 200m and below 300m 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 200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Llwynderw Hill and this was derived from local enquiry, 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 minor roads to its north, west and south and the A483 road to its south-east, and has the town of Y Trallwng (Welshpool) towards the north-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 main P30 list, and listed with a 238m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

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 analysis shows the ground on a raised field boundary to be the highest on the hill, however as this is deemed a relatively recent man-made construct such ground is discounted from the height of a hill in the listings I author.  LIDAR analysis also shows the position of the highest remaining natural ground and this has now been surveyed with the Trimble GeoXH 6000 and it is this result that is being prioritised for this hill. 

LIDAR summit image of Llwynderw Hill

The summit height and position produced by the Trimble GeoXH 6000 survey is 237.9m at SJ 19600 03662, and this position in relation to the raised field boundary shown by LIDAR analysis comes within the parameters of the Summit Relocations used within this page heading, these parameters are: 

The term Summit Relocations applies when the hill’s high point is found to be positioned; in a different field, to a different feature such as in a conifer plantation, placed within a different map contour, to a different point where a number of potential summit positions are within close proximity, or when natural ground or 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, 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. 

The Trimble GeoXH 6000 gathering data at the natural summit of Llwynderw Hill

Therefore, the new listed summit height for this hill is 237.9m and is positioned at SJ 19600 03662, this position is not given a spot height on contemporary Ordnance Survey maps and is in a different field to where the 238m spot height is positioned on the Ordnance Survey 1:25,000 Explorer map and is approximately seven metres east north-eastward from where LIDAR gives the high point of the raised field boundary. 

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Carnedd Wen

Name:  Llwynderw Hill

OS 1:50,000 map:  136

Summit Height:  237.9m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference (New Position):  SJ 19600 03662 (Trimble GeoXH 6000) 

Bwlch Height:  159.7m (LIDAR) 

Bwlch Grid Reference:  SJ 18827 03989 (LIDAR) 

Drop:  78.3m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (June 2021)