Mapping Mountains – Hill Reclassifications – 100m Twmpau / 30-99m Twmpau

Mynydd Carreg (SH 163 291) – 100m Twmpau reclassified to 30-99m Twmpau

There has been a reclassification of a 100m Twmpau to a 30-99m Twmpau (thirty welsh metre prominences and upward) initiated by a survey with the Trimble GeoXH 6000.  The details relating to this hill’s reclassification are retrospective as the survey with the Trimble took place on the 28^th December 2014.

The criteria for 100m Twmpau status is all Welsh hills at or above 100m and below 200m in height with 30m minimum drop, whilst the criteria for 30-99m Twmpau status is all Welsh hills at or above 30m and below 100m in height with 30m minimum drop.

When drop values were added to the original Welsh P30 lists that were published on Geoff Crowder’s v-g.me website, this hill was listed with an estimated c 95m summit height based on an uppermost 90m contour ring, and a bwlch height of c 54m, based on interpolation of bwlch contouring between 50m – 60m, with these values giving this hill c 41m of drop.  When the Ordnance Survey enlarged mapping hosted on the Geograph website became available online the whole of Wales was checked for new spot heights and as this hill was given a 100m summit spot height it was reclassified to the 100m P30 list, which was later renamed the 100m Twmpau.

Therefore, prior to the survey with the Trimble GeoXH 6000 this hill was listed with an estimated c 46m of drop based on its 100m summit spot height and an estimated c 53m bwlch height, based on interpolation of bwlch contouring between 50m – 55m.

The name of the hill is Mynydd Carreg and it is situated in the Pen Llŷn range of hills with its Cardinal Hill being Mynydd y Rhiw (SH 228 293) and is placed in the North Wales Region (Region A, Sub-Region A1), and is positioned with the small community of Aberdaron to the south south-east of the hill.

As the summit of the hill is not a part of designated open access land permission to visit its high point should be sought.  However, as the summit is a local viewpoint with a car park at the foot of the hill it is likely that an ascent will have no objection.

The full details for the hill are:

Cardinal Hill:  Mynydd y Rhiw

Summit Height:  99.2m (converted to OSGM15)

Name:  Mynydd Carreg

OS 1:50,000 map:  123

Summit Grid Reference:  SH 16371 29168

Drop:  c 46m

The Trimble GeoXH 6000 gathering data at the summit of Mynydd Carreg

For details on the survey that reclassified this hill from a 100m Twmpau to a 30-99m Twmpau

Myrddyn Phillips (September 2017)

Mapping Mountains – Trimble Surveys – Yr Eifl

28.12.14  Mynydd Carreg (SH 163 291)   

Having watched the sun set next to the dark outline of Ynys Enlli we descended Mynydd y Gwyddel and navigated our way to the track that leads to a parking area for Mynydd Carreg.

By now it was almost dark with the afterglow of colour lighting the sky up out to sea.  We left the car and walked up a field toward a gate which gave access toward the summit of Mynydd Carreg.

Up yonder somewhere in the dark is the summit of Mynydd Carreg

The outline of the stone structure placed on the summit became visible as we gained the little height that was needed to reach the top.  This structure, or the area beside it, is indicated as a viewpoint on Ordnance Survey maps.  As we reached it the coastal view gave a colour wash of deep blues merging with those of light blues and the oranges of a disappeared sun, all highlighted against the black of land.

The last light of day

We examined the ground immediately beside the stone structure and decided that the grass in the middle of it was higher.  As I set the Trimble up on the grass Mark headed toward a rocky outcrop and looked back, confirming in his estimation that the ground at the centre of the stone structure was higher.

The stone structure on the summit of Mynydd Carreg

The Trimble placement was not ideal as it was positioned in the centre of a stone walled dungeon with only a gap to walk through on its eastern side.  As it started struggling down toward its 0.1m accuracy when data can start to be logged, we considered if I should have placed it on top of the stone wall surrounding its present position and taken a measurement offset to the ground below.  I checked the Trimble and quickly pressed ‘Log’ and ran out of the structure and stood behind the wall.

Gathering data at the summit of Mynydd Carreg

The Trimble didn’t seem to enjoy being where it was as it occasionally screamed as the accuracy bounced back up from 0.1m.  I’ve found that when this happens the equipment still logs data and soon achieves its necessary accuracy once again.

By now Mark had descended to his car and left the headlights on for an indicator for our descent and once five minutes of data were collected I followed Aled down toward the gate and the parking area.

This proved an ideal hill to visit in the dark as it had very little ascent from where we parked the car, and although almost dark when we set off we had the last glimpse of light out to sea.

Survey Result:

Mynydd Carreg 

Summit Height:  99.2m (converted to OSGM15) (100m Twmpau reclassified to 30-99m Twmpau confirmed)

Summit Grid Reference:  SH 16371 29168

Drop:  c 46m 

Dominance:  46.58% (Lesser Dominant status confirmed)

For further details please consult the Trimble survey spreadsheet click {here}