Mapping Mountains – Hill Reclassifications – 200m Twmpau

Cae Top (SJ 061 016) – 200m Sub-Twmpau addition

There has been confirmation of an addition to the list of 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips.

Cae Top (SJ 061 016)

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 200m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Cae Top and this was derived from the Tithe map, and it is adjoined to the Carnedd Wen group of hills which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is encircled by minor roads, with also the B4389 and B4390 roads to its east, and has the village of Adfa towards the south.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category. 

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

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with 26m of drop, based on the 269m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and which is now presented on the mapping on the Magic Maps website, and the 243m bwlch spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

Extract from the Magic Maps website

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR image of Cae Top

The result produced by LIDAR analysis gives this hill 25.7m of drop, and as the summit has now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes.

The Trimble GeoXH 6000 gathering data at the summit of Cae Top

Therefore, the confirmation of the addition of this hill to 200m Sub-Twmpau status is due to LIDAR analysis and a Trimble GeoXH 6000 survey, resulting in a 268.8m summit height and a 243.0m bwlch height, with these values giving this hill 25.9m of drop, which is sufficient for it to be classified as a 200m Sub-Twmpau.

The full details for the hill are:

Group:  Carnedd Wen

Name:  Cae Top

OS 1:50,000 map:  136

Summit Height:  268.8m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SJ 06132 01671 (Trimble GeoXH 6000)

Bwlch Height:  243.0m (LIDAR)

Bwlch Grid Reference:  SJ 06613 01642 (LIDAR)

Drop:  25.9m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (November 2020)