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

Glynllugwy (SH 754 592) 

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 conducted by Myrddyn Phillips. 

LIDAR image of Glynllugwy (SH 754 592)

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 now listed by is Glynllugwy, and this was derived from the Tithe map, 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 between Llyn Bychan to its west, Llyn Goddionduon to its south and Llyn Bodgynydd to its east, with the A5 road to its south and the B5106 road, the Afon Conwy and the A470 road to its east, and has the village of Capel Curig towards the west south-west and the village of Betws-y-coed towards the south-east.

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 main P30 list with a non interpolated summit height of c 310m, based on the uppermost contour on the contemporary 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 40m of drop, based on an estimated c 313m summit height and an estimated c 273m bwlch height, with both heights based on interpolation of 10m contouring. 

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

One of the resources recently available online is the mapping on the OS Maps website and the details for this hill were subsequently re-assessed against this mapping.  This is the replacement for OS Get-a-map and until recent times had contours at 5m intervals which were proving consistently more accurate compared to the 5m contours that sometimes appear on the Ordnance Survey 1:25,000 Explorer map and used to appear on the online Vector Map Local.  This mapping had an uppermost 305m contour ring and bwlch contouring between 270m – 275m, with interpolation placing the height of the summit as c 309m and the height of th e bwlch as c 271m, with these values giving this hill as estimated c 38m of drop.

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 Glynllugwy (SH 754 592)

The summit height produced by LIDAR analysis is 318.4m and is positioned at SH 75484 59240, 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.

Therefore, the new listed summit height of this hill is 318.4m and this was derived from LIDAR analysis, this is 5.4m higher than the estimated c 313m summit height, which was based on interpolation of the uppermost 310m contour that appears on the Ordnance Survey 1:25,000 Explorer map, and 9.4m higher than the estimated c 309m summit height, which was based on interpolation of the uppermost 305m contour that appeared on the interactive mapping hosted on the OS Maps website. 

 

The full details for the hill are: 

Group:  Carnedd Llywelyn 

Name:  Glynllugwy 

OS 1:50,000 map:  115

Summit Height (New Height):  318.4m (LIDAR)                                                           

Summit Grid Reference:  SH 75484 59240 (LIDAR) 

Bwlch Height:  270.4m (LIDAR) 

Bwlch Grid Reference:  SH 75296 59502 (LIDAR) 

Drop:  48.1m (LIDAR) 

 

Myrddyn Phillips (February 2025)