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

 

Mynydd Tŷ’r Sais (SH 855 004) 

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 confirmed by a Trimble GeoXH 6000 survey, prompted by an enquiry from Ronnie Bowron based on detail in the summit analysis programme produced by Joe Nuttall. 

Mynydd Tŷ'r Sais (SH 855 004)

The criteria for the list this height revision affects 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. 

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

The name the hill is listed by is Mynydd Tŷ’r Sais and it is adjoined to the Pumlumon group of hills, which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with the A470 road to its north, minor roads to its west and south, and the B4518 road to its east, and has the village of Llanbrynmair towards the north-east. 

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

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 26m of drop, based on the 359m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and the 1:25,000 Explorer map and the 333m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

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

Since the original publication of the P30 lists of Geoff Crowder’s v-g.me website there have been a number of Ordnance Survey maps made available online, some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website, whilst others are current and digitally updated such as the mapping on the OS Maps website and the WalkLakes website. 

One of the resources now available online is the mapping on the OS Maps website.  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 shows an uppermost 360m ring contour and bwlch contouring much nearer the 330m height when compared to the contouring on the Ordnance Survey 1:25,000 Explorer map. 

Another resource now available online is the mapping on the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available Ordnance Survey maps and for this hill a 362m spot height is given on the area of its summit and again the bwlch contouring is much nearer the 330m height when compared to the contouring on the Ordnance Survey 1:25,000 Explorer map. 

Extract from the WalkLakes website

These details resulted in this hill being prioritised for a Trimble GeoXH 6000 survey.  This took place on the 30.03.21 resulting in a 362.2m summit height, 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 summit 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 Mynydd Tŷ'r Sais

Therefore, this hill’s new listed summit height is 362.2m and this was derived from a Trimble GeoXH 6000 survey, this is 3.2m higher than the previously listed summit height of 359m which appears as a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

 

The full details for the hill are: 

Group:  Pumlumon 

Name:  Mynydd Tŷ’r Sais 

OS 1:50,000 map:  135, 136

Summit Height (New Height):  362.2m (converted to OSGM15)                                                           

Summit Grid Reference:  SH 85514 00483 

Bwlch Height:  329.9m (converted to OSGM15) 

Bwlch Grid Reference:  SH 86220 00690 

Drop:  32.2m 

 

Myrddyn Phillips (July 2021)