Mapping Mountains – Significant Height Revisions – Yr Uchafion and 800m Twmpau

Carnedd y Filiast (SH 620 627)

There has been a Significant Height Revision to a hill that appears in the listings of Yr Uchafion and 800m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and Aled Williams which took place on the 6^th July 2019.

Carnedd y Filast (SH 620 627)

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

Yr Uchafion – Welsh hills at or above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams, with the Introduction to the list appearing on Mapping Mountains on the 4^th November 2015 and an update relating to the list appearing on Mapping Mountains on the 19^th June 2019.

800m Twmpau - Welsh hills at or above 800m and below 900m in height with 30m minimum drop, with an accompanying sub list entitled the 800m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 800m and below 900m 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 list is authored by Myrddyn Phillips and the details to it were published on Mapping Mountains on the 15^th March 2015.

The name of the hill is Carnedd y Filiast 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 is positioned with the Afon Ogwen and the A5 road to its north-east and Llyn Padarn, Llyn Peris and the A4086 road towards its south-west, and has the town of Bethesda towards the north and the village of Llanberis towards the west south-west.

Prior to the Trimble GeoXH 6000 survey the height of the hill was listed as 821m which is based on the summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

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

The summit height produced by the Trimble GeoXH 6000 is 823.6m, this is not a dramatic height revision when compared to some survey results, 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. 

The Trimble GeoXH 6000 gathering data at the summit of Carnedd y Filiast

Therefore, this hill’s new summit height as surveyed with the Trimble GeoXH 6000 is 823.6m, this is positioned at SH 62040 62733 and is 2.6m higher than its previously listed height of 821m which appears as a spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Glyderau

Name:  Carnedd y Filiast

OS 1:50,000 map:  115

Summit Height (New Height):  823.6m (converted to OSGM15)

Summit Grid Reference:  SH 62040 62733

Bwlch Height:  744.4m (converted to OSGM15)

Bwlch Grid Reference:  SH 61995 61535

Drop:  79.1m

For details on the survey of this hill

Myrddyn Phillips and Aled Williams (October 2019)