Mapping Mountains – Significant Height Revisions – Y Trichant

Pen y Bigil (SH 576 621)

There has been a Significant Height Revision to a hill that is listed in the Y Trichant, and which was initiated by LIDAR analysis and a subsequent survey with the Trimble GeoXH 6000 both of which were conducted by Myrddyn Phillips, with the latter taking place on the 06.10.18.

Pen y Bigil (SH 576 621)

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

Y Trichant – Welsh hills at or above 300m and below 400m in height with 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.

The name of the hill is Pen y Bigil and it is adjoined to the Glyderau group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it has 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 as it was thought not to meet the criteria then used for the main P30 list; however this sub list has now been standardised and interpolated heights and drop values also included in the main P30 and the accompanying sub list.

                                      

When this list was standardised and interpolated heights also included this hill 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 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 this hill’s summit, these heights equate to 329m in metric, and when coupled with the c 293m estimated bwlch height gives this hill 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 LIDAR became available that the details for this hill were next re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales, and soon after analysing this hill with LIDAR it was surveyed with the Trimble GeoXH 6000. 

LIDAR image of Pen y Bigil

The 331.0m summit height produced by the Trimble GeoXH 6000 is not a dramatic increase from the 329m spot height, but 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.

Therefore, this hill’s new summit height as surveyed with the Trimble GeoXH 6000 is 331.0m which is 2m higher than its updated height of 329m that appears as an imperial height on the Ordnance Survey 1:25,000 historical map and the series of Six-Inch maps, and 5m higher than its previously estimated height of c 326m.

The Trimble GeoXH 6000 gathering data at the summit of Pen y Bigil

 

The full details for the hill are:

Group:  Glyderau

Name:  Pen y Bigil

OS 1:50,000 map:  114, 115

Summit Height (new height):  331.0m (converted to OSGM15)

Summit Grid Reference:  SH 57600 62119

Bwlch Height:  292.6m (LIDAR)

Bwlch Grid Reference:  SH 58014 62043 (LIDAR) 

Drop:  38.4m (Trimble summit and LIDAR bwlch)

For details on the survey that significantly revised the summit height of this hill

Myrddyn Phillips (March 2019)