Mapping Mountains – Summit Relocations – 30-99m Twmpau

 

Rhos Uchaf (SH 593 699) 

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

LIDAR image of Rhos Uchaf (SH 593 699)

The criteria for the list 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.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

30-99m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Rhos Uchaf, and this was derived from the Tithe map, and it is adjoined to the Glyder Fawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A5 road to its immediate west and the A55 road to its south-east, and has the city of Bangor towards the north-west.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

After 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 16m of drop, based on an estimated c 93m summit height which was listed at SH 59346 69940 and an estimated c 77m bwlch height, with both heights based on interpolation of 5m contouring. 

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 the highest ground on this hill as 93.0m positioned at SH 59327 69929.  However, this is a part of a raised boundary edge and 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 Rhos Uchaf (SH 593 699)

The height produced by LIDAR analysis to the highest remaining natural ground of this hill is 92.5m and this is positioned at SH 59332 69923, and this 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 either on Ordnance Survey maps or interactive mapping, 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 considered 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 height produced by LIDAR analysis to the summit of this hill is 92.5m and is positioned at SH 59332 69923, 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 six metres south-eastward from where the high point of the raised boundary edge is positioned. 

 

The full details for the hill are: 

Group:  Glyder Fawr 

Name:  Rhos Uchaf 

OS 1:50,000 map:  114, 115

Summit Height:  92.5m (LIDAR) 

Summit Grid Reference (New Position):  SH 59332 69923 (LIDAR)               

Bwlch Height:  71.8m (LIDAR) 

Bwlch Grid Reference:  SH 59236 69468 (LIDAR) 

Drop:  20.7m (LIDAR) 

 

Myrddyn Phillips (March 2025)