Mapping Mountains – Significant Height Revisions – 200m Twmpau

 

Castle Hill (SO 314 945)

There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps produced from Ordnance Survey data and a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

The criteria for the list this height revision affects 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 listed by is Castle Hill and it is adjoined to the Stiperstones group of hills, which straddle the border between Wales and England with the Welsh part of this group situated in the north-eastern part of Mid and West Wales (Region B, Sub-Region B1), and it is positioned with the A490 road to its west, the A489 road to its south and the A488 road to its east, and has the small community of Hyssington towards the south-west. 

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.  

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 an estimated c 24m of drop, based on an estimated c 281m summit height and the 257m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was named the Interactive Coverage Map. 

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

Since the original compilation of this list there have been a number of Ordnance Survey maps made available online, some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website, whilst others are current and digitally updated such as the mapping on the WalkLakes website and the Magic Maps website. 

One of the mapping resources now available online is the Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  This map has many spot heights not on other publicly available Ordnance Survey maps and for this hill a 283m spot height is given on the area of its summit. 

Extract from the Magic Maps website

The summit of this hill has now been surveyed with the Trimble GeoXH 6000, resulting in 283.2m at SO 31430 94540, and this 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 summit 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 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. 

The Trimble GeoXH 6000 gathering data at the summit of Castle Hill

Therefore, this hill’s new listed summit height is 283.2m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SO 31430 94540 and is 2.2m higher than its originally listed height of c 281m which was estimated from the uppermost 280m contour on the contemporary Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Stiperstones 

Name:  Castle Hill 

OS 1:50,000 map:  137

Summit Height (New Height):  283.2m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SO 31430 94540 (Trimble GeoXH 6000) 

Bwlch Height:  c 261m (interpolation) 

Bwlch Grid Reference:  SO 31356 94661 (interpolation) 

Drop:  c 22m (Trimble GeoXH 6000 summit and interpolated bwlch)

 

Myrddyn Phillips (March 2021)