Thursday, 22 October 2015

Mapping Mountains – Significant Height Revisions – 300m Twmpau

Cefn Llwydallt (SO 074 409)

This is the twenty first post under the heading of Significant Height Revisions, and the following details are retrospective as the Trimble survey that resulted in this height revision was conducted on the 29th September 2015.

The twenty first major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted in the south-eastern lands of Mynydd Epynt, above the small communities of Erwyd (Erwood) to the north-east, Capel Uchaf (Upper Chapel) to the west and Llaneglwys to the south-west.

The hill was surveyed in the afternoon toward the end of a ridge walk that took in a number of hills from Cefn Clawdd (SO 043 403) eastward.

The hill is listed as a 300m Twmpau (thirty welsh metre prominences and upward) and prior to the survey with the Trimble it was listed as being 378m high based on the old 1,240ft (378.0m) imperial height given the hill on the Ordnance Survey Six Inch map.

Access to the hill is easy as the summit is situated on open access land and there is a minor road to the north-west next to where the bwlch for the hill is positioned.  Alternate routes to the summit can use public footpaths from the north-east, east and south that head toward the open access land.  The high point of the hill is positioned amongst small gorse bushes and is easily found.

The name of the hill is Cefn Llwydallt and prior to the survey with the Trimble GeoXH 6000 the height of the hill was listed as 378m.  It was surveyed with the Trimble GeoXH as being 380.7m (converted to OSGM15) high.  This is not a dramatic height revision when compared to its old listed height, but it does come within 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 Trimble height, 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.

Therefore this hill’s new summit height is 380.7m (converted to OSGM15) which is 2.7m higher than its previously listed height of 378m which was based on the old imperial Six Inch map height of 1,240ft.

The full details for the hill are:

Cardinal Hill:  Clepyn Melyn

Summit Height (New Height):  380.7m (converted to OSGM15)

Name:  Cefn Llwydallt

OS 1:50,000 map:  147, 160

Summit Grid Reference:  SO 07422 40950 
Drop:  41.0m

Gathering data from the summit of Cefn Llwydallt which resulted in this hill's significant height revision

For details on the survey that significantly revised the summit height of this hill please click {here}

Myrddyn Phillips (September 2015)

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