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)