Mapping Mountains – Hill Reclassifications – 100m Twmpau

Tair Onnen (ST 036 739) – 100m Twmpau addition

There has been an addition to a hill that is now listed in the 100m Twmpau, with the summit height and its location confirmed by a Leica GS15 survey conducted by John Barnard, Graham Jackson and Myrddyn Phillips which took place on the 4^th November 2013.

LIDAR image of Tair Onnen (ST 036 739)

The criteria for the list that this addition applies to are:

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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 Tair Onnen, and it is adjoined to the Bro Morgannwg group of hills, which are situated in the southern part of South Wales (Region C, Sub-Region C2), and it is positioned with the A48 road to its north and minor roads to its south, and has the town of Y Bont-faen (Cowbridge) towards the west and the city of Caerdydd (Cardiff) towards the east.

Extract from the Ordnance Survey 1:25,000 Explorer map

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included in either the main P30 or accompanying Hills to be surveyed sub list as with a 137m summit height adjoined to its triangulation pillar it was deemed lower than Cae Cefn (ST 059 748) which has a 140m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map.  However, as the 140m spot height is to the top of a covered reservoir and as these structures are considered recent man-made constructs the two hills were subsequently surveyed using a Leica GS15 resulting in the following details.

Cae Cefn (ST 05910 74886):  136.2m (natural ground at base of covered reservoir)

Tair Onnen (ST 03660 73979):  137.3m

Gathering data with the Leica GS15 at the summit of Tair Onnen

When LIDAR became available the details for these two hills were re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.  The LIDAR summit details are given below:

Cae Cefn (ST 05915 74885):  136.4m (natural ground at base of covered reservoir)

Tair Onnen (ST 03661 73979):  137.4m

  

Therefore, the addition of Tair Onnen to 100m Twmpau status is due to a Leica GS15 survey, resulting in a 137.3m summit height as opposed to the 136.2m summit height for Cae Cefn which was taken to natural ground at the base of the man-made covered reservoir, and with an estimated bwlch height of c 67m, these values give Tair Onnen c 70m of drop which is sufficient for this hill to be classified as a 100m Twmpau.

The full details for the hill are:

Group:  Bro Morgannwg

Name:  Tair Onnen

OS 1:50,000 map:  170

Summit Height:  137.3m (Leica GS15)

Summit Grid Reference:  ST 03660 73979 (Leica GS15)

Bwlch Height:  c 67m

Bwlch Grid Reference:  ST 01235 77663

Drop:  c 70m

Myrddyn Phillips (September 2019)