Mapping Mountains – Summit Relocations – 30-99m Twmpau

Ponciau (SH 468 609) 

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 Ponciau (SH 468 609)

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 Ponciau, and this was derived from the Tithe map, and it is adjoined to the Yr Wyddfa group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned encircled by minor roads, with the A487 road farther to its south-east, and has the town of Caernarfon towards the north-east.

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 24m of drop, based on an estimated c 47m twin summit height which was listed at SH 46498 60772 and SH 46820 60936 and an estimated c 23m 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 summit image of Ponciau (SH 468 609)

LIDAR analysis gives the highest ground on this hill as 46.7m positioned at SH 46811 60944 which corresponds to the position of one of the previously listed twin summits, with LIDAR also giving two other positions of note; 46.4m at SH 46492 60770, which corresponds to the other previously listed twin summit, and 46.65m at SH 46882 60943.  As LIDAR analysis has de-twinned the summit of this hill 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 the de-twinning of a summit, 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 46.7m and is positioned at SH 46811 60944, 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 320 metres east north-eastward from where the other previously listed twin summit of this hill is situated. 

 

The full details for the hill are: 

Group:  Yr Wyddfa 

Name:  Ponciau 

OS 1:50,000 map:  114, 115

Summit Height:  46.7m (LIDAR) 

Summit Grid Reference (New Position):  SH 46811 60944 (LIDAR)               

Bwlch Height:  22.7m (LIDAR) 

Bwlch Grid Reference:  SH 47971 61205 (LIDAR) 

Drop:  24.0m (LIDAR) 

 

Myrddyn Phillips (April 2025)