Mapping Mountains – Significant Height Revisions – Y Trichant - The 300m Hills of Wales

Pen y Bigil (SH 576 621)

There has been a Significant Height Revision to a hill that is listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis and a Trimble GeoXH 6000 summit survey conducted by Myrddyn Phillips.

Pen y Bigil (SH 576 621)

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

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant, with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017, and the Introduction to the Mapping Mountains publication of the list appearing on the 1^st January 2022. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Pen y Bigil and it is adjoined to the Glyder Fawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A4244 road to its north-west and the A4086 road and Llyn Padarn to its south, and has the villages of Deiniolen towards the north and Llanberis towards the south.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list with a non-interpolated c 320m summit height as it was considered not to meet the criteria then used for the main P30 list.

When this list was standardised and interpolated heights and drop values also included, the details for this hill were reassessed and it was listed with an estimated c 33m of drop, based on an estimated c 326m summit height and an estimated c 293m bwlch height, with each based on interpolation of 5m contouring that appears on the contemporary Ordnance Survey 1:25,000 Explorer map.

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

The details for this hill were re-assessed when the Ordnance Survey 1:25,000 historical map and the series of Six-Inch maps became available online, the former has a 1081ft height and the latter has a 1080ft height given to the summit of this hill, these heights equate to 329m in metric, and when coupled with the c 293m estimated bwlch height gives this hill an estimated c 36m of drop.

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

Extract from the Ordnance Survey series of Six-Inch maps

However, it was not until the survey with the Timble GeoXH 6000 that the details for this hill could be accurately re-assessed  The 331.0m summit height produced by the Trimble GeoXH 6000 survey is not a dramatic increase in height compared to the 329m spot height, 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, 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 summit of Pen y Bigil

 

Therefore, the new listed summit height of this hill is 331.0m, and this was derived from a Trimble GeoXH 6000 survey.  This is 2.0m higher than its previously listed summit height of 329m, which was based on the imperial height that appeared on the Ordnance Survey 1:25,000 historical map and the series of Six-Inch maps, and 5.om higher than its previously estimated height of c 326m.

The full details for the hill are:

Group:  Glyder Fawr

Name:  Pen y Bigil

OS 1:50,000 map:  114, 115

Summit Height (New Height):  331.0m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 57600 62119 (Trimble GeoXH 6000)

Bwlch Height:  292.6m (LIDAR)

Bwlch Grid Reference:  SH 58014 62044 (LIDAR) 

Drop:  38.4m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (March 2019)