Mapping Mountains – Significant Height Revisions – 200m Twmpau

Clip y Gylfinhir (SH 223 284)

There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, with the hill’s height, drop and status confirmed by a survey with the Trimble GeoXH 6000 which was conducted by Myrddyn Phillips on the 28^th December 2014 and 3^rd October 2018 in good, clear conditions with hardly a breath of breeze.

The criteria for the list that this significant height revision 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 list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 name of the hill is Clip y Gylfinhir, and it is adjoined to the Pen Llŷn range of hills, which are situated in the western part of North Wales (Region A, Sub-Region A1), and it is positioned toward the western end of Pen Llŷn, and is encircled by a series of minor roads and has the small community of Y Rhiw towards the south.

Clip y Gylfinhir (SH 223 284)

As the summit of the hill is a part of designated open access land it can be approached from most directions, with public footpaths leading in to the access land from a variety of directions, and for those wishing a quick ascent of the hill there is parking space at the immediate base of the hill to its north-east.

When the original Welsh 200m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed in the accompanying Hills to be surveyed sub list with a 270m summit height which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

The summit height produced by The Trimble GeoXH 6000 survey is 267.8m (converted to OSGM15), this is not a substantial height revision when compared to some revised heights, but it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

The Trimble GeoXH 6000 gathering data at the summit of Clip y Gylfinhir

Therefore, this hill’s new summit height is 267.8m (converted to OSGM15) and this was produced by surveying with the Trimble GeoXH 6000, this is 2.3m lower than its previously listed height of 270m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map, however this new height is in accordance with the 5m contouring on OS Maps.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Pen Llŷn

Name:  Clip y Gylfinhir

OS 1:50,000 map:  123

Summit Height (New height):  267.8m (converted to OSGM15)

Summit Grid Reference:  SH 22395 28472

  

Bwlch Height:  240.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 22514 28524

Drop:  27.7m (converted to OSGM15)

For details on the 1^st survey of this hill

For details on the 2nd survey that significantly revised the summit height of this hill

Myrddyn Phillips (December 2018)

Mapping Mountains – Trimble Surveys – Yr Eifl

03.10.18  Clip y Gylfinhir (SH 223 284)  

Clip y Gylfinhir (SH 223 284)

Clip y Gylfinhir was previously surveyed with the Trimble GeoXH 6000 on the 28^th December 2014, the resulting height proved substantially lower than the 270m spot height that appears on the Ordnance Survey 1:25,000 Explorer map, and as the Standard Deviation and Estimated Accuracies indicated the data set was not good, I wanted to re-visit and survey this hill’s summit ever since.  The opportunity to do so was now here as Aled and I had visited Mynydd Penarfynydd and Mynydd y Graig and it was only a short drive from these hills to the parking area at the base of Clip y Gylfinhir, and whilst surveying this hill Aled could visit the adjacent Mynydd y Rhiw, a hill he had not previously been up.

Clip y Gylfinhir thrusts upward and is coned in profile, it stands out even from afar as a pimple like addition to the southern slopes of its higher neighbour; Mynydd y Rhiw, and the hill has association with the Tylwyth Teg; the mythological fairy like creatures of Welsh folklore.

As Aled headed north toward the trig pillar atop Mynydd y Rhiw I left a friendly Shetland pony beside my car and as it nuzzled its head against the boot, and hoped that it would not attempt to eat my rear wiper blade whilst I was on the hill!

Aled on the summit of Mynydd y Rhiw

A paved road leads to the adjacent mast and radar station that now sits beside Clip y Gylfinhir, from this paved road the hill thrusts upward, with it being steep on all sides.  I followed the road to its end and slowly plodded up the steep rocky slopes toward its summit where an autumnal breeze met me as it blew in from the sea.

The previous time this hill was Trimbled I’d placed the equipment aligned with the highest rock, this time I wanted the Trimble to be elevated above its immediate surrounds and therefore placed it on top of my rucksack which acted as an improvised tripod, having measured the offset between its internal antenna and the ground below I pressed ‘Log’ and waited for ten minutes of data to be gathered and stored.

Gathering data at the summit of Clip y Gylfinhir

The Trimble set-up position at the summit of Clip y Gylfinhir

As the Trimble gathered data I sat below it watching Aled making good progress up the path to the summit of Mynydd y Rhiw, he was descending the hill as the Trimble continued beeping away gathering its 600 individual datum points, once data were stored I closed the equipment down, packed it away and headed down to the paved road and arrived back at the car no more than a minute or so before Aled did likewise.

Survey Result:

Clip y Gylfinhir

Summit Height:  267.8m (converted to OSGM15) (significant height revision)

Summit Grid Reference:  SH 22395 28472

Bwlch Height:  240.1m (converted to OSGM15) (from previous Trimble GeoXH 6000 survey)

Bwlch Grid Reference:  SH 22514 28524 (from previous Trimble GeoXH 6000 survey)

Drop:  27.7m

Dominance:  10.34%

For details on the first Trimble survey of Clip y Gylfinhir

For further details please consult the Trimble Survey Spreadsheet

Mapping Mountains – Trimble Surveys – Yr Eifl

28.12.14  Clip y Gylfinhir (SH 223 284)   

Clip y Gylfinhir (SH 223 284)

As Mark drove toward the western tip of Pen Llŷn (Lleyn Peninsula), Aled talked about a hill that stood a chance of becoming a P30, it was positioned just to the south of Mynydd y Rhiw.  Although our plans were not set firm we had discussed a route between three hills that was feasible this time of year, and as the skies were bathed in blue it would only be lack of daylight hours that curtailed this plan.  However, spontaneity can be fun and soon Aled was directing Mark through the narrow network of lanes toward the car park immediately below Clip y Gylfinhir.

Clip y Gylfinhir (one translation of the name is the Curlew’s crag) is listed as a 200m Sub-P30 with its bwlch contouring and 270m summit spot height implies that it has 28m – 29m of drop.  As Mark drove up the last icy road toward the car park the hill loomed up conically shaped and well proportioned.

Once parked, Aled headed into the heather to assess the direction of the bwlch as it swings down from Mynydd y Rhiw and ascends from the small community of Rhiw.  The small car park is also the hub for a meeting of tracks; these at least gave perspective towards the elevation of the bwlch and broke the heathery slopes.

As Aled assessed the bwlch Mark headed into the heather to photograph the hill without its adjacent wireless and radio transmitting mast in the scene, he then headed up the hill.

Mark on the summit of Clip y Gylfinhir

The point Aled had reached in the heather was good to pinpoint the critical bwlch but it would only be from the summit where we would have an aerial view of the scene.  Not wanting to keep Mark waiting, a spot close to the meeting of tracks adjacent to the earthen car park was chosen and I set the Trimble up.

Once five minutes of data were collected we headed up the hill to join Mark on the summit.  From the car park a small path veers away from the access road to the transmitting mast and heads straight up the hill, I left the path to take photos of Aled as he gained height on steepening broken ground.

Aled on the ascent of Clip y Gylfinhir

The summit area consists of a number of embedded rocks, the highest being easily identified, and as the Trimble had its internal antenna aligned to the high point of the hill and started to log data I stood back and took in the view.

Gathering data at the summit of Clip y Gylfinhir

To the north the continuation of the main track from the car park headed up to the summit of Mynydd y Rhiw, to the west the agricultural lowlands culminated in the last upthrusts of hills before the end of Wales and Ynys Enlli (Bardsey Island), the immediate eastern horizon was dominated by the great sweep of Porth Neigwl (Hell’s Mouth) and away inland stood a line of hills which were all snow-capped.

Mynydd y Rhiw (SH 228 293)

The lowlands of the western Llŷn leading to Ynys Enlli

Porth Neigwl (Hell's Mouth)

Yr Wyddfa dominating the central view

After packing the Trimble away we descended the hill’s southern slopes toward the transmitting mast where I walked into the heather and gorse next to the paved access road to try and get a photo of Mark and Aled against Clip y Gylfinhir.

Heading toward the bwlch

We had a good view from the summit of the lay of land around the area of the bwlch and could easier follow the southerly ridge of Mynydd y Rhiw as it heathered its way down toward the connecting land of Clip y Gylfinhir.  From the vantage point of the summit the hill to hill traverse looked as if it headed into an adjacent field toward a metal cattle feed which was conveniently placed and gave us a point to use for reference.

It was this hill to hill line that we now headed for, judging that the low point was on the heathery side of a double fenced and walled boundary and probably not in the closely cropped grassy adjacent field; the Trimble was set up and soon achieved its required 0.1m before it began to log data.

The set up position for the second data set

When I arrived back at Mark’s car we decided to head into Aberdaron for coffee and a bite to eat before revising our plans for further hills to the west.

Survey Result:

Clip y Gylfinhir

Summit Height:  267.8m (converted to OSGM15) (significant height revision) (height from subsequent Trimble GeoXH 6000 survey)

Summit Grid Reference:  SH 22395 28472 (from subsequent Trimble GeoXH 6000 survey)

Bwlch Height:  240.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 22514 28524

Drop:  27.7m

Dominance:  10.34%

 

For details on the second summit survey of Clip y Gylfinhir

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