Mapping Mountains – Hill Reclassifications – 800m Twmpau

800m Twmpau – Hill Reclassifications

The 800m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 800m and below 90om in height that have a minimum drop of 30m.  Accompanying the main P30 list is a sub list entitled the 800m Sub-Twmpau with the qualification to 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.  The Introduction to this list is linked in the above title.

The list is authored by Myrddyn Phillips and the posts that have appeared on Mapping Mountains detailing the additions, reclassifications and deletions to the main P30 list and the sub list appear below presented chronologically in receding order.

Mapping Mountains - Hill Reclassifications - 800m Twmpau

Mynydd Perfedd (SH 623 618) – 800m Sub-Twmpau addition (2nd reclassification)

There has been a reclassification to the listing of 800m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and Aled Williams which took place on the 6^th July 2019.

Mynydd Perfedd (SH 623 618)

The criteria for the list that this reclassification applies to are:

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 the hill is listed by is Mynydd Perfedd 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 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.

When the 800m height band of hills that later became known as the Twmpau were originally compiled this hill was listed with an estimated c 19m of drop, based on the 812m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 793m bwlch height, based on interpolation of 10m contouring between 790m – 800m.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping has additional spot heights not available on any other Ordnance Survey publicly available map and in the case of this hill it has an 813m summit and 793m bwlch spot height, with these values giving this hill 20m of drop.  However, as there is inconsistency between the summit heights on the various scaled Ordnance Survey maps the drop value was left as c 19m until an accurate survey could take place.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website

Therefore, the reclassification of this hill to 800m Sub-Twmpau status is due to a Trimble GeoXH 6000 survey, resulting in an 812.8m summit height and a 792.4m bwlch height, with these values giving this hill 20.4m of drop, which is sufficient for 800m Sub-Twmpau status.

The Trimble GeoXH 6000 gathering data at the summit of Mynydd Perfedd

The Trimble GeoXH 6000 gathering data at the bwlch of Mynydd Perfedd

The full details for the hill are:

Group:  Glyderau

Name:  Mynydd Perfedd

OS 1:50,000 map:  115

Summit Height:  812.8m (converted to OSGM15)

Summit Grid Reference:  SH 62319 61877

Bwlch Height:  792.4m (converted to OSGM15)

Bwlch Grid Reference:  SH 62267 62382

Drop:  20.4m

For details on the survey of this hill

Myrddyn Phillips (October 2019)

Mapping Mountains - Hill Reclassifications - 800m Twmpau

Pen y Gadair Fawr (SO 229 287) – 800m Twmpau reclassified to 700m Twmpau (1st reclassification)

There has been a reclassification to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams.  The hill was listed in the 800m height band of P30 Twmpau hills and it has now been reclassified to a 700m Twmpau.

The criteria for 700m Twmpau status is all Welsh hills at or above 700m and below 800m in height with 30m minimum drop, whilst the criteria for 800m Twmpau status is all Welsh hills at or above 800m and below 900m in height with 30m minimum drop.  Details to the 700m Twmpau were published on Mapping Mountains on 15^th April 2015, and details to the 800m Twmpau were published on Mapping Mountains on the 15^th March 2015.

The name of the hill is Pen y Gadair Fawr and it had been listed in the 800m height band of Twmpau hills, based on the 800m summit spot height that appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.  With the 800m height on Ordnance Survey maps originating from the 2624ft (799.8m) imperial bench mark height on the series of old Six-Inch maps.

The hill is situated on one of the central ridges that forms the Mynyddoedd Duon (Black Mountains) group of hills with its Cardinal Hill being Waun Fach (SO 215 299) and is placed in the Region of South Wales (Region C, Sub-Region C3).  It is situated between the steam valleys of the Grwyne Fawr to its north-east and the Grwyne Fechan to its south, with the town of Talgarth towards the north-west and Y Gelli Gandryll (Hay-on-Wye) towards the north.

As the hill is a part of designated open access land it can be accessed from most directions, however an extended circular walk taking in the ridge upon which it is positioned in conjunction with the ridge to its west is a good way to accustom oneself with this hill range.

The reclassification of Pen y Gadair Fawr to 700m Twmpau status is due to the analysis of LIDAR data by Aled Williams.  LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Aled’s analysis of LIDAR data gives the hill the following details:

Pen y Gadair Fawr

Summit Height:  799.8m

Summit Grid Reference:  SO 22944 28758

Bwlch Height:  753.2m

Bwlch Grid Reference:  SO 22657 29359

Drop:  46.6m

Therefore, the 799.8m LIDAR data produced for the summit position at SO 22944 28758 and the 753.2m LIDAR data produced for the bwlch position at SO 22657 29359 gives this hill 46.6m of drop, and as the summit height is below 800m it is reclassified to a 700m Twmpau with a 799.8m summit height.

This now revises the total in the 800m and 700m Twmpau which will be updated accordingly.

The full details for the hill are:

Cardinal Hill:  Waun Fach

Summit Height:  799.8m (LIDAR data)

Name:  Pen y Gadair Fawr

OS 1:50,000 map:  161

Summit Grid Reference:  SO 22944 28758  

Drop:  46.6m (LIDAR data)

Pen y Gadair Fawr (SO 229 287) now reclassified from an 800m Twmpau to a 700m Twmpau

My thanks to Aled Williams for sending the details of this hill to me

Myrddyn Phillips (October 2017)