Mapping Mountains – Summit Relocations – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Lan Uchaf (SN 526 230) 

There has been a Summit Relocation to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Lan Uchaf (SN 526 230)

The criteria for the two listings that this summit relocation applies to are:

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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 30-99m Twmpau by Myrddyn Phillips

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Lan Uchaf and this was derived from the Tithe map, and it is adjoined to the Mynydd Mallaen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned encircled by minor roads with the A40 road farther to its south, and has the town of Llandeilo towards the east.

When the sub list was standardised and interpolated heights and drop values also included in the original Welsh 30-99m P30 list that was published on Geoff Crowder’s v-g.me website, this hill was listed with an estimated c 86m summit height positioned at SN 526 230, based on interpolation of its uppermost 80m ring contour. 

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 gives an 86.8m height positioned at SN 52695 23032 and SN 52697 23033 to ground on a raised field boundary as the highest on the hill, however protocols dictate that as this is deemed a relatively recent man-made construct such ground is discounted from the height of a hill. 

LIDAR summit image of Lan Uchaf (SN 526 230)

The height produced by LIDAR analysis to the natural summit of this hill is 86.7m positioned at SN 52675 23027, and this position in relation to the raised field boundary comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

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

Therefore, the summit height produced by LIDAR analysis is 86.7m and this is to the remaining natural summit of the hill which is positioned at SN 52675 23027, this position is not given a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 20 metres westward from where the raised field boundary is positioned. 

 

The full details for the hill are: 

Group:  Mynydd Mallaen 

Name:  Lan Uchaf 

OS 1:50,000 map:  159

Summit Height:  86.7m (LIDAR) 

Summit Grid Reference (New Position):  SN 52675 23027 (LIDAR) 

Bwlch Height:  39.0m (LIDAR) 

Bwlch Grid Reference:  SN 53053 23453 & SN 53053 23455 (LIDAR) 

Drop:  47.7m (LIDAR) 

Dominance:  55.02% (LIDAR) 

 

Myrddyn Phillips (May 2022)