Sunday, 15 November 2015

Mapping Mountains – Significant Height Revisions – 100m Twmpau

The Pimple (SJ 299 472)

There has been a Significant Height Revision initiated by a survey with the Trimble GeoXH 6000 to the listing of the 100m Twmpau, with these details being retrospective as the survey that resulted in this height revision was conducted on the 2oth October 2015.

The criteria for the listing that this height revision affects are:

100m Twmpau - These are the Welsh hills at or above 100m and below 200m in height that have a minimum drop of 30m.  The word Twmpau is an acronym for thirty welsh metre prominences and upward.  The criteria for 100m Sub-Twmpau status are all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop. 

The name of the hill is The Pimple; this is a locally known name which was given me when I made enquiries during the survey for the bwlch position of the hill.  The hill remains unnamed on Ordnance Survey maps and its locally known name relates directly to the past industrial heritage of the hill, as its summit is made of mining waste that is now stabilised and grassed over.  The uppermost part of the summit is relatively small in area and squat in shape, and resembles a pimple as it sits upon another elevated part of the hill.

The hill is situated in the eastern part of the Moel y Gamelin range of hills, and is positioned above the small community of Pentre Bychan which is adjacent to the larger community of Rhosllanerchrugog.

The hill can be accessed from its west where a public footpath traverses the course of the valley where its bwlch is positioned.  However, permission should be sought for those wishing to visit as the summit is not on designated open access land.  When I visited I asked permission to do so and was kindly directed to the best ascent route.

Prior to the survey with the Trimble GeoXH 6000 this hill was not classified but its summit position was indicated to be at SJ 30075 47320, this is where a 154m spot height appears on Ordnance Survey 1:25,000 Explorer maps, with a 23m difference in height compared to the  131m summit spot height on the Ordnance Survey 1:50,000 Landranger map.   The latter spot height is positioned in a field which had been ploughed and probably seeded when I visited.  However there is also high ground to the south-west of where the 154m spot height appears on the ground and each respective point was surveyed, with the following results:

South-western high point (The Pimple):  153.5m (converted to OSGM15) at SJ 29984 47262

North-eastern high point (154m spot height in field):  153.1m (converted to OSGM15) at SJ 30071 47279

The 153.5m (converted to OSGM15) summit height is not a dramatic height revision when compared to the 154m spot height, but the new summit position does not possess any ring contours on current Ordnance Survey maps and therefore meets the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revision applies to any listed hill whose Ordnance Survey summit spot height has a 2m or more discrepancy when compared to the surveyed height produced by the Trimble GeoXH 6000, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 map is being prioritised for detailing these revisions.

Although this hill’s new summit height of 153.5m (converted to OSGM15) is only 0.5m different from the spot height that appears on the Ordnance Survey 1:25,000 Explorer map, the hill’s summit position has now been established and this is 8.5m higher than the 145m uppermost contour ring shown on the 1:25,000 map for where the summit of the hill is positioned.

The full details for the hill are:

Cardinal Hill:  Moel y Gamelin

Summit Height (New Height):  153.5m (converted to OSGM15)

Name:  The Pimple

OS 1:50,000 map:  117

Summit Grid Reference:  SJ 29984 47262 
Drop:  25.2m (converted to OSGM15)

Gathering data at the summit of The Pimple which resulted in this hill's significant height revision

Myrddyn Phillips (November 2015)

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