Mapping Mountains – Significant Height Revisions – 200m Twmpau

Castell Dolforwyn (SO 151 950) 

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 a Trimble GeoXH 6000 survey and LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Castell Dolforwyn (SO 151 950)

The criteria for the list that this height revision 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 Castell Dolforwyn, and it is adjoined to the Carnedd Wen group of hills, which are situated in the southern part of North Wales (Region A, Sub-Region A3), and it is positioned encircled by minor roads, with the B4389 road farther to its south-west and the A483 road farther to its south-east, and has the town of Y Drenewydd (Newtown) 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 listed with a summit height of c 23om, with this being the uppermost contour on the contemporary Ordnance Survey 1:25,000 Explorer map. 

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-evaluated and it was listed with an estimated c 43m of drop, based on the 231m 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 an estimated c 188m bwlch height, based on interpolation of 5m contouring between 185m – 190m that also appeared on this mapping. 

The Trimble GeoXH 6000 gathering data at the summit of Castell Dolforwyn

However, it was not until the Trimble GeoXH 6000 survey that the details for this hill could be accurately re-assessed.  The summit height produced by this survey is 228.5m and is positioned at SO 15189 95016, and this 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 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, the new listed summit height of this hill is 228.5m and this was derived from a Trimble GeoXH 6000 survey, this is 2.5m lower than the previously listed summit height of 231m, which was based on the spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Carnedd Wen 

Name:  Castell Dolforwyn 

OS 1:50,000 map:  136

Summit Height (New Height):  228.5m (Trimble GeoXH 6000) 

Summit Grid Reference:  SO 15189 95016 (Trimble GeoXH 6000) 

Bwlch Height:  188.1m (LIDAR) 

Bwlch Grid Reference:  SO 14988 95278 (LIDAR) 

Drop:  40.4m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (November 2023)