Mapping Mountains – Summit Relocations – Y Trechol – The Dominant Hills of Wales

Tair Onnen (ST 036 739)

LIDAR image of Tair Onnen (ST 036 739)

There has been a Summit Relocation to a hill that is listed in the Y Trechol – The Dominant Hills of Wales, 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.

The criteria for the list that this summit relocation applies to are:

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015.

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 for Cae Cefn

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

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website for Cae Cefn

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, resulting in Cae Cefn being listed as the Dominant hill with 51.43% dominance.  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

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

LIDAR image of Cae Cefn (ST 059 748)

The summit height produced by the Leica GS15 survey and by LIDAR analysis gives the summit of Tair Onnen as higher than the natural ground at the immediate base of the covered reservoir on Cae Cefn, and as such this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to any listed hill whose summit meets the following criteria; where there are a number of potential summit positions within close proximity and the highest point is not where previously given, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist, or when the summit of the hill is in a different field compared to where previously given, or when the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct.

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

Gathering data with the Leica GS15 at the summit of Cae Cefn

Therefore, the summit height produced by the Leica GS15 survey is 137.3m and this is positioned at ST 03660 73979, this position is beside the triangulation pillar atop Tair Onnen and is approximately 2.3km west south-westward from where the originally listed summit of Cae Cefn is positioned.

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 (new position):  ST 03660 73979 (Leica GS15)

Bwlch Height:  c 67m (interpolation)

Bwlch Grid Reference:  ST 01235 77663 (interpolation)

Drop:  c 70m

Dominance:  51.20%

Myrddyn Phillips (September 2019)