Mapping Mountains – Significant Height Revisions – 200m Twmpau

Mynydd y Dref (SH 759 777)

There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, with the summit height, its location and the drop of the hill confirmed by LIDAR analysis, and a subsequent summit and bwlch survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, with the latter taking place on the 15^th October 2018.

Mynydd y Dref (SH 759 777)

LIDAR image of Mynydd y Dref

The criteria for the list that this significant 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 name of the hill is Mynydd y Dref, and it is adjoined to the Carneddau range of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A55 road and the coast to its north, and has the town of Conwy towards its east south-east.

When the original Welsh 200m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 244m summit height based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

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

The summit height produced by the Trimble GeoXH 6000 survey is 246.1m, this is not a substantial height revision when compared to some revised heights, but it does come 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.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

The summit of Mynydd y Dref

Therefore, this hill’s new listed summit height is 246.1m and this was produced by surveying with the Trimble GeoXH 6000, this is 2.1m higher than its previously listed height of 244m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Carneddau

Name:  Mynydd y Dref

OS 1:50,000 map:  115

Summit Height (New height):  246.1m (converted to OSGM15)

Summit Grid Reference:  SH 75981 77799

  

Bwlch Height:  186.7m (converted to OSGM15)

Bwlch Grid Reference:  SH 75246 77548

Drop:  59.4m

For details on the survey of this hill

Myrddyn Phillips (April 2019)