Mapping Mountains – Significant Height Revisions – 100m Twmpau

Cae Cefn (ST 059 748)

There has been a Significant Height Revision to a hill that was 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, and the bwlch height and its location, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

The Leica GS15 gathering data at the summit of Cae Cefn

The criteria for the list that this height revision 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.

LIDAR image of Cae Cefn (ST 059 748)

The name the hill is now listed as is Cae Cefn and this derived from the Tithe map, 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 south-east and minor roads to its west, north and east, has the city of Caerdydd (Cardiff) towards the east and the town of Y Bont-faen (Cowbridge) to the west.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was listed with a 140m summit height based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.  However, as the 140m spot height is placed at the top of a covered reservoir and as these structures are considered recent man-made constructs and as Tair Onnen vied for being the higher hill, both hills were surveyed using a Leica GS15, resulting in:

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

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

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

The height produced by the Leica GS15 survey is 136.2m and is positioned at ST 05910 74886, this revised height comes 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 other GNSS equipment, 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.

Therefore, this hill’s new listed summit height is 136.2m and this is to natural ground at the base of a covered reservoir and positioned at ST 05910 74886,  this is 3.8m lower than the previously listed summit height of 140m which was based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

 ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Bro Morgannwg

Name:  Cae Cefn

OS 1:50,000 map:  170

Summit Height (New Height):  136.2m (Leica GS15)

Summit Grid Reference:  ST 05910 74886 (Leica GS15)

Bwlch Height:  116.25m (LIDAR)

Bwlch Grid Reference:  ST 04273 74542 (LIDAR)

 

Drop:  19.9m (Leica GS15 summit and LIDAR bwlch)

Myrddyn Phillips (October 2019)