Mapping Mountains – Significant Height Revisions – Y Pedwarau - The 400m Hills of Wales

Y Drum (SH 735 428)

There has been a Significant Height Revision to a hill that is listed in the Y Pedwarau – The 400m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips initiated by an on-site inspection conducted by Aled Williams.

Y Drum (SH 735 428)

The criteria for the list that this height revision applies to are:

Y Pedwarau – The 400m Hills of Wales.  Welsh hills at or above 400m and below 500m in height that have 30m minimum drop, accompanying the main list are five categories of sub hills; 500m Sub-Pedwarau, 500m Double Sub-Pedwarau, 400m Sub-Pedwarau, 390m Sub-Pedwarau and the 390m Double Sub-Pedwarau.  The list is co-authored by Myrddyn Phillips and Aled Williams and is published on Mapping Mountains in Google Doc format.

Y Pedwarau - The 400m Hills of Wales by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Y Drum, and this was derived from local and historical information with research conducted by Aled Williams, and it is adjoined to the Arenig Fach group of hills, which are situated in the central part of North Wales (Region C1, Sub-Region C1A), and it is positioned with the B4391 road to its south and the B4407 road to its east, and has the village of Ffestiniog towards the west south-west.

When the original 400m height band of Welsh P30 hills compiled by Myrddyn Phillips was published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list with a 479m summit height positioned at SH 733 428, with the caveat that the four small 480m ring contours just to the north-east and east had been missed.

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 35m of drop, based on an estimated c 482m summit height and an estimated c 447m bwlch height, with both heights based on interpolation of 10m contouring that appears on the Ordnance Survey 1:25,000 Explorer map.

When the 1^st edition of the Y Pedwarau was published by Europeaklist in May 2013 the details for this hill were re-evaluated and it was listed with a 486m summit height, based on the spot height that appears on the Harvey 1:40,000 British Mountain Map to Eryri/Snowdonia and which is positioned in the vicinity of the most northerly of the 480m ring contours that appear on the Ordnance Survey 1:25,000 Explorer map, with the high point of this contour ring positioned at SH 73542 42903.

Extract from the Harvey 1:40,000 British Mountain Map to Eryri/Snowdonia

The position of the listed summit of this hill was subsequently moved to SH 73541 42812 after an on-site inspection by Aled Williams.  However, it was not until the survey with the Trimble GeoXH 6000 that the details for this hill could be accurately re-assessed.

The new summit position of Y Drum is highlighted in yellow, with the old position of the summit highlighted in red.  Photo:  Aled Williams

This summit relocation was later confirmed by the Trimble GeoXH 6000 with the highest ground on this hill surveyed as 483.574m positioned at SH 73541 42812, whilst the position of the 486m Harvey spot height at SH 73542 42903 was surveyed as 482.686m, 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, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

The Trimble GeoXH 6000 gathering data at the new summit position of Y Drum (SH 735 428)

Therefore, the new listed summit height of this hill is 483.6m and this was derived from a Trimble GeoXH 6000 survey, this is 2.4m lower than the previously listed summit height, which was based on the 486m spot height that appears on the Harvey 1:40,000 British Mountain Map to Eryri/Snowdonia

The full details for the hill are:

Group:  Arenig Fach 

Name:  Y Drum

OS 1:50,000 map:  124

Summit Height (New Height):  483.6m (converted to OSGM15) 

Summit Grid Reference:  SH 73541 42812 

Bwlch Height:  449.2m (converted to OSGM15) 

Bwlch Grid Reference:  SH 73778 42825

Drop:  34.4m

Myrddyn Phillips and Aled Williams (March 2016)