Mapping Mountains – Significant Height Revisions – 200m Twmpau

Parc Uchaf Gwydyr (SH 792 597) 

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 LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Parc Uchaf Gwydyr (SH 792 597)

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 listed by is Parc Uchaf Gwydyr, and it is adjoined to the Carnedd Llywelyn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with a minor road to its north-west, the A5 road to its south-west, and the B5106 road to its east, and has the town of Betws-y-coed 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 listed with a summit height of 272m, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map and which is positioned at SH 79181 60064. 

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

When the accompanying sub list to the 200m height band of Welsh P30 hills published on Geoff Crowder’s v-g.me website was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and its summit relocated to SH 79186 59729 and listed with an estimated height of c 274m.

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 summit image of Parc Uchaf Gwydyr (SH 792 597)

LIDAR analysis gives the highest ground on this hill as 276.7m positioned at SH 79204 59775, as opposed to its originally listed summit position which LIDAR analysis gives as 270.3m positioned at SH 79182 60067, 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 276.7m and this was derived from LIDAR analysis, this is 4.7m higher than the originally listed summit height of 272m, which was based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map, and 2.7m higher than the previously listed summit height of c 274m, which was based on interpolation of the uppermost 270m ring contour 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:  Carnedd Llywelyn 

Name:  Parc Uchaf Gwydyr 

OS 1:50,000 map:  115

Summit Height (New Height):  276.7m (LIDAR) 

Summit Grid Reference:  SH 79204 59775 (LIDAR) 

Bwlch Height:  225.2m (LIDAR) 

Bwlch Grid Reference:  SH 78923 59652 (LIDAR) 

Drop:  51.5m (LIDAR) 

 

Myrddyn Phillips (November 2023)