Mapping Mountains – Summit Relocations – 100m Twmpau

Y Gaer (SJ 204 155) 

There has been a Summit Relocation to a hill that is listed in the 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

Y Gaer (SJ 204 155)

The criteria for the list that this summit relocation 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 100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Y Gaer, 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 positioned with minor roads encircling it, the A495 road farther to its north and the A490 road farther to its west, and has the village of Meifod towards the west south-west. 

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a c 125m summit height positioned at SJ 204 155 with an accompanying note stating; Two points of same height.  Height from Explorer 240. 

As this note implies the Ordnance Survey 1:25,000 Explorer map gives two points with uppermost 125m contours without any specific spot heights, the larger of these contours is positioned at SJ 204 155 and this is where the prioritised summit of this hill was given, with the smaller contour positioned at SJ 207 156. 

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

As the name of this hill implies it has the remains of an ancient hill fort on its summit area which takes in a part of the larger 125m uppermost contour.  Just to the east of the remains of the hill fort is a small covered reservoir which consists of steepened grassed sides leading to a breeze block construction.  The natural ground of the hill is at the base of the steepened grassed sides, with these forming a part of the covered reservoir. 

Gathering data during the first survey at the top of the steepened grass section

The summit of this hill has been surveyed with the Trimble GeoXH 6000 on two occasions.  The first when data were collected from the top of the steepened grass at the base of the breeze block section.  As this constitutes a part of the covered reservoir which is deemed a relatively recent man-made construct and therefore should not be taken as a part of this hill’s height, the summit was re-surveyed. 

Gathering data during the second survey at the highest remaining natural ground at the base of the steepened grass section

Gathering data during the third survey at the high point of the ancient hill fort

During the re-survey two points were surveyed, one at the highest remaining natural ground at the base of the steepened grass and the second at the high point of the remains of the ancient hill fort.  These and the result from the first survey are given below:

 

08.08.15 - 1^st survey, top of steepened grass section of covered reservoir:  130.450m at SJ 20413 15563 

02.07.21 - 2^nd survey, highest natural ground at base of steepened grass:  129.325m at SJ 20417 15566 

02.07.21 - 3^rd survey, highest part of ancient hill fort:  128.893m at SJ 20391 15555 

 

Between the 08.08.15 and 02.07.21 LIDAR became available and therefore 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 summit image of Y Gaer (SJ 204 155)

The result produced by LIDAR analysis confirmed the position of the high part of the ancient hill fort and the approximate position of the highest remaining natural ground of this hill, and as these two points have now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are: 

The term Summit Relocations applies to when the high point is positioned in a different field, to a different feature such as a conifer plantation, within a different map contour, a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, 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. 

Therefore, the summit height produced by the Trimble GeoXH 6000 survey is 129.3m and this is positioned at SJ 20417 15566, this is the remaining natural high point of the hill and not ground at the top of the steepened grass, which forms a part of the covered reservoir and which is considered a relatively recent man-made construct and therefore discounted from the height of the hill. 

 

The full details for the hill are: 

Group:  Carnedd Wen 

Name:  Y Gaer 

OS 1:50,000 map:  126

Summit Height:  129.3m (converted to OSGM15, Trimble GeoXH 6000)                                                          

Summit Grid Reference (New Position):  SJ 20417 15566 (Trimble GeoXH 6000) 

Bwlch Height:  91.2m (LIDAR) 

Bwlch Grid Reference:  SJ 21177 15977 (LIDAR) 

Drop:  38.2m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (September 2021)