Mapping Mountains – Hill Reclassifications – 600m Twmpau

600m Twmpau – Hill Reclassifications

The 600m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 600m and below 70om in height that have a minimum drop of 30m.  Accompanying the main P30 list is a sub list entitled the 600m Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 600m and below 700m in height with 20m or more and below 30m of drop.

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 - 600m Twmpau

Chwarel Graig Ddu (SH 725 455) – 600m Sub-Twmpau addition (3rd reclassification)

Survey post for Chwarel Graig Ddu

There has been an addition to the listing of the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps. 

Chwarel Graig Ddu (SH 725 455)

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

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 600m Twmpau by Myrddyn Phillips

The name the hill is listed by is Chwarel Graig Ddu, and it is adjoined to the Arenig Fach group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A470 road to its west and the B4391 road to its south, and has the town of Blaenau Ffestiniog towards the west.

When the list that later became known as the 600m Twmpau was first compiled this hill was not included in the accompanying sub list, as because it is a part of a working quarry the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map do not show any spot heights or contours, therefore the hill seemed not to exist. 

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

Since the original compilation of this list there have been a number of 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 were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

One of the resources recently 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.  Although the mapping on the OS Maps website no longer has contours at 5m intervals, such contours are represented on other mapping available online and it was Aled Williams who scrutinised this mapping which has an uppermost 610m ring contour and bwlch contouring between 590m – 595m, with these contours giving this hill a drop value exceeding the minimum 15m to qualify for this list. 

Extract from the online map showing 5m contouring

The hill has subsequently been inspected from adjacent hillsides and its validity confirmed and as a quarry road passes over its summit area it is considered solid and stable.

Therefore, the addition of this hill to 600m Sub-Twmpau status is due to detail on contemporary online mapping with a subsequent inspection from adjacent hillsides undertaken to confirm its validity, resulting in a 613m summit height and a 592m bwlch height, with these values giving this hill 21m of drop, which is sufficient for it to be classified as a 600m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Arenig Fach

Name:  Chwarel Graig Ddu

OS 1:50,000 map:  115

Summit Height:  613m

Summit Grid Reference:  SH 72505 45533

Bwlch Height:  592m

Bwlch Grid Reference:  SH 72456 45744   

Drop:  21m 

 

Myrddyn Phillips (June 2022)

 

Mapping Mountains - Hill Reclassifications - 600m Twmpau

Drum yr Eira (SN 851 589) – 600m Sub-Twmpau addition (2nd reclassification)

There has been an addition to the listing of the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps produced from Ordnance Survey data.

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

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The name the hill is listed by is Drum yr Eira and it is adjoined to the Elenydd group of hills, which are situated in the northern part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with the B4343 road to its north-west, the A483 road to its south-east and the A470 road to its east, and has the town of Rhaeadr Gwy (Rhayader) towards the north-east.

When the list that later became known as the 600m Twmpau was first compiled this hill was not included but it was listed with an estimated c 19m of drop, based on a c 601m summit height and a c 582m bwlch height, with both heights based on interpolation of 10m contouring that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

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

Since the original compilation of this list there are now a number of interactive maps originated from Ordnance Survey data that are available online.  One of these was the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.  This map had many spot heights not on other publicly available Ordnance Survey maps and showed a 582m spot height at the area of this hill’s bwlch; substantiating the previously listed interpolated bwlch height.

Another resource now available online is 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 it  shows a 602m spot height on the summit area of this hill.

Extract from the WalkLakes website

The details for this hill were also re-examined against the Ordnance Survey non-contour Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.  This is another map that has many spot heights that do not appear on other publicly available Ordnance Survey maps and it also gives a 602m summit spot height.

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

The positions of these two spot heights were then compared to the contouring on the OS Maps website.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and used to appear on the online Vector Map Local.

Therefore, the addition of this hill to 600m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data, resulting in a 602m summit height and a 582m bwlch height, with these values giving this hill 20m of drop, which is sufficient for it to be classified as a 600m Sub-Twmpau.

The full details for the hill are:

Group:  Elenydd

Name:  Drum yr Eira

OS 1:50,000 map:  147

Summit Height:  602m (spot height)

Summit Grid Reference:  SN 85120 58920 (spot height)

Bwlch Height:  582m (spot height)

Bwlch Grid Reference:  SN 85606 58495 (spot height)

Drop:  20m (spot height summit and bwlch)

Myrddyn Phillips (August 2020)

Mapping Mountains - Hill Reclassifications - 600m Twmpau

Foel Penolau (SH 661 347 and SH 661 348) – 600m Sub-Twmpau reclassified to 600m Twmpau (1st reclassification)

Survey post for Foel Penolau

There has been a reclassification to a hill that is listed in the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by a survey with the Trimble GeoXH 6000 which was conducted over two days by Myrddyn Phillips and Aled Williams on the 17^th November 2018 and by Myrddyn Phillips on the 26^th November 2018.

Foel Penolau (SH 661 347 and SH 661 348)

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

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The name of the hill is Foel Penolau and it is adjoined to the Rhinogydd group of hills, which are situated in the western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A496 road to the west and the A470 road and the village of Trawsfynydd towards the east.

When the list that later became known as the 600m Twmpau was first compiled this hill was listed with c 26m of drop based on the 614m summit spot height that appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and an estimated bwlch height of c 588m based on interpolation of 10m contouring between 580m – 590m.

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

The details for this hill were re-evaluated when the Ordnance Survey Vector Map Local became available online; this map is hosted on the Geograph website and is entitled the Interactive Coverage Map.  As well as showing twin 614m map heighted summits it also gave a 588m spot height in the area of this hill’s bwlch, but as lower ground was known to exist on the hill to hill traverse compared to where this spot height is positioned the drop value was amended to an estimated c 27m.

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

This hill’s details were again re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals and these showed that the 588m spot height that appears on the Ordnance Survey Vector Map Local is not positioned at the critical bwlch of this hill.  These contours placed the position of the critical bwlch in the boulder field below this hill’s eastern face and resulted in an estimated c 30m of drop, based on an estimated c 584m bwlch height based on interpolation of contours between 580m – 585m and the 614m spot height given to the two twin map heighted summits.

Extract from the OS Maps website

The confirmation of the reclassification of Foel Penolau to 600m Twmpau status is due to a survey with the Trimble GeoXH 6000 resulting in a 614.4m summit height (with 1mm difference between the northern and southern summits the Trimble was unable to split these tops) and a 582.5m bwlch height, with these values giving this hill 31.9m of drop which is sufficient for it to be classified as a 600m Twmpau.

The Trimble set-up position at the northerly summit of Foel Penolau

The Trimble set-up position at the critical bwlch of Foel Penolau

The full details for the hill are:

Group:  Rhinogydd

Name:  Foel Penolau

OS 1:50,000 map:  124

Summit Height:  614.4m (converted to OSGM15)

Summit Grid Reference:  SH 66184 34832 and SH 66145 34774

Bwlch Height:  582.5m (converted to OSGM15)

Bwlch Grid Reference:  SH 66099 34735

Drop:  31.9m

For details on the survey of Foel Penolau

Myrddyn Phillips (May 2019)