Thursday, 9 November 2017

Mapping Mountains – Significant Height Revisions – 30-99m Twmpau


30-99m Twmpau – Significant Height Revisions

The 30-99m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 30m and below 100m in height that have a minimum drop of 30m.  Accompanying the main P30 list are two sub lists entitled the 30-99m Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, and the Double Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 20m and below 30m 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 significant height revisions to the main P30 list and the sub lists appear below presented chronologically in receding order.







Mapping Mountains - Significant Height Revisions - 30-99m Twmpau

Allt Fawr (SH 383 356) - 8th significant height revision

Hill Reclassifications post for Allt Fawr

Summit Relocations post for Allt Fawr


There has been a Significant Height Revision to a hill that now appears in the 30-99m Twmpau, with the summit height, drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

The criteria for this list are:

30-99m Twmpau – Welsh hills at and above 30m and below 100m in height that have a minimum 30m of drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at and above 30m and below 100m in height with 20m and 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 Allt Fawr and it is adjoined to the Pen Llŷn range of hills, which is situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A 499 road to its west and the A 497 road to its east, and has the town of Pwllheli towards its south-west.

As the hill is not a part of designated open access land permission to visit should be sought, for those wishing to do so the proximity of the roads will give relatively easy access.

Prior to LIDAR analysis this hill was listed with c 18m of drop based on an estimated c 46m summit height with the hill given three uppermost 45m ring contours on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website and an estimated bwlch height of c 28m based on bwlch contouring at 5m intervals and between 25m – 30m on this same map.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

The summit height produced by LIDAR analysis is 48.9m, this is not a dramatic height revision when compared to some revised heights, but it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

LIDAR image of Allt Fawr

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.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new summit height is 48.9m and this was produced by LIDAR analysis, this is 2.9m higher than its previously listed height of c 46m which was estimated from the Ordnance Survey Interactive Coverage map hosted on the Geograph website.


The full details for the hill are:

Group:  Pen Llŷn

Summit Height (New Height):  48.9m (LIDAR)

Name:  Allt Fawr

OS 1:50,000 map:  123

Summit Grid Reference:  SH 38347 35695 (LIDAR)  
Drop:  23.1m (LIDAR)


Myrddyn Phillips (April 2018)






Mapping Mountains - Significant Height Revisions - 30-99m Twmpau

Bryn y Bont (SH 596 459) - 7th significant height revision

Hill Relassifications post for Bryn y Bont

Summit Relocations post for Bryn y Bont


There has been a Significant Height Revision to a hill that now appears in the Double Sub-Twmpau list that accompanies the 30-99m Twmpau list, with the hill’s height and drop confirmed by analysis by Aled Williams of data produced by LIDAR.  With the title of Twmpau being an acronym for thirty welsh metre prominences and upward.

Accompanying each 100m height band within the main Twmpau list is a sub list entitled the Sub-Twmpau, with the Double Sub-Twmpau attached to the lower tier height band of Welsh hills.

The criteria for the lower tier of Twmpau hills are:

30-99m Twmpau – All Welsh hills at and above 30m and below 100m in height with 30m minimum drop.

30-99m Sub-Twmpau – All Welsh hills at and above 30m and below 100m in height with 20m or more and below 30m of drop.

Double Sub-Twmpau – All Welsh hills at and above 20m and below 30m in height with 20m or more and below 30m of drop.

The name of the hill is Bryn y Bont and it is situated in the Moelwynion range of hills and is placed in the Region of North Wales (Region A, Sub-Region A1).  The hill is positioned with the A 4085 road to its north and its east and the A 498 road to its west, and has the Afon Glaslyn also to its immediate west, with the hamlet of Nantmor positioned to the east of the hill and the village of Beddgelert towards its north.

As the hill is not a part of designated open access land permission to visit should be sought, for those wishing to do so an ascent from the convenience of the road to its north or east is likely to be the easiest.

Prior to analysis of data produced via LIDAR this hill was not listed in the Hills to be surveyed sub list that accompanied the Welsh P30 lists published on Geoff Crowder’s v-g.me website.  When these lists were re-evaluated after they were standardised and also drop values added, this hill was listed with c 16m of drop based on an estimated c 20m summit height which was based on this hill’s small uppermost 20m ring contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps, and an estimated bwlch height of c 4m based on bwlch contouring at 10m intervals and between 0 – 10m, with the caveat that the 4m spot height positioned at SH 59756 45814 and shown on the Ordnance Survey 1:25,000 Explorer map is probably not at the position of the critical bwlch.  However, since these lists were re-evaluated OS Maps (the recent replacement for OS Get-a-map) are now available and show 5m contour intervals with an uppermost 25m ring contour for this hill. 

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

Extract from the OS Maps website

The summit height produced by data via LIDAR is 25.2m, this is a dramatic height revision in relation to its previously estimated height of c 20m, and therefore 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 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, and also hill’s that do not possess a summit spot height and whose estimated summit height has a 2m or more discrepancy when compared to data produced either by the Trimble or by LIDAR.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new summit height is 25.2m and this was produced from data via LIDAR, this is 5.2m higher than its previously listed estimated height of c 20m which was based on the small uppermost 20m ring contour on the Ordnance Survey 1:25,000 Explorer map.


The full details for the hill are:

Group:  Moelwynion

Summit Height (New Height):  25.2m (data via LIDAR)

Name:  Bryn y Bont

OS 1:50,000 map:  115

Summit Grid Reference:  SH 59641 45973
   
Drop:  20.9m (data via LIDAR)


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

Myrddyn Phillips (February 2018)






Mapping Mountains - Significant Height Revisions - 30-99m Twmpau

Bryn y Gwynt (SH 599 449) - 6th significant height revision

Hill Reclassifications post for Bryn y Gwynt

Significant Name Changes post for Bryn y Gwynt

Summit Relocations post for Bryn y Gwynt


There has been a Significant Height Revision to a hill that now appears in the following lists; 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the hill’s height, drop, dominance and status confirmed by analysis conducted by Aled Williams of data produced via LIDAR.

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

30-99m Twmpau – All Welsh hills at and above 30m and below 100m in height that have a minimum 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  Accompanying the main Twmpau list is a sub list entitled Sub-Twmpau with this hill being reclassified from the 30-99m Sub-Twmpau category.  The criteria for 30-99m Sub-Twmpau status are all Welsh hills at and above 30m and below 100m in height that have 20m or more and below 30m of drop.

Y Trechol – The Dominant Hills of Wales – All Welsh P30 hills whose prominence equal or exceed half that of their absolute height.

The name of the hill is Bryn y Gwynt and this was derived from local enquiry and historical mapping (see Significant Name Changes) and it is situated in the Moelwynion range of hills and is placed in the Region of North Wales (Region A, Sub-Region A1), and is positioned with the A 4085 road to its north-east and has the Welsh Highland Railway to its immediate west and overlooks the Afon Glaslyn also to its west, and has the village of Beddgelert to its north north-west and the town of Porthmadog to its south south-west.

The hill is a part of Coed Hafod y Llyn, and as this name implies its summit is placed in woodland which is deciduous, and as the summit is not a part of designated open access land permission to visit should be sought, however paths are indicated on the map to the north, east and south of this hill’s summit and therefore gaining access to its lower slopes has probably been accepted for many years.

Prior to analysis of data produced via LIDAR this hill was listed in the Hills to be surveyed sub list that accompanied the Welsh P30 lists published on Geoff Crowder’s v-g.me website.  When these sub lists were standardised and also drop values added, this hill was listed with c 29m of drop based on the 56m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 27m based on contouring at 10m intervals and between 20m – 30m, with the caveat that the 27m spot height that appears on a road at SH 60168 45293 on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website is not positioned at the critical bwlch.

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

The summit height produced by data via LIDAR analysis is 58.2m, this is not a dramatic height revision when compared to some revised heights, 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 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.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new summit height is 58.2m and this was produced from data via LIDAR, this is 2.2m higher than its previously listed height of 56m which appears on the Ordnance Survey 1:25,000 Explorer map.


The full details for the hill are:

Group:  Moelwynion

Summit Height (New Height):  58.2m (data via LIDAR)

Name:  Bryn y Gwynt

OS 1:50,000 map:  124

Summit Grid Reference:  SH 59949 44915 
  
Drop:  30.2m (data via LIDAR)

Dominance:  51.89%

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

Myrddyn Phillips (February 2018)






Mapping Mountains - Significant Height Revisions - 30-99m Twmpau

Bryn Tŵr (SH 604 441) - 5th significant height revision

Survey post for Bryn Tŵr

Summit Relocation post for Bryn Tŵr


There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, and which was initiated by a survey with the Trimble GeoXH 6000 which took place on the 23rd December 2017 in wet conditions but with clear visibility.

The criteria for the two lists this height revision affects are:

30-99m Twmpau – Welsh hills at and above 30m and below 100m in height that have 30m minimum drop.  With the word Twmpau being an acronym standing for ‘thirty welsh metre prominences and upward’.

Y Trechol – The Dominant Hills of Wales - Welsh P30s whose prominence equal or exceed half that of their absolute height.

The name of the hill is Bryn Tŵr and it is placed in the Moelwynion range of hills which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and the hill is positioned overlooking the A 4085 road and the Nanmor to its east, with it being positioned between the village of Beddgelert to its north north-west and Porthmadog to its south south-west. 

As the summit of the hill is not on designated open access land permission to visit should be sought, however the hill has a well-used path leading to its summit indicating that people have visited this hill for many years.

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with c 40m of drop and 76.92% Dominance based on the 52m spot height that appears on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website and an estimated bwlch height of c 12m based on contouring between 10m – 20m.  The summit height produced by the Trimble survey is 54.5m (converted to OSGM15) with its position to the west of where the 52m spot height appears and outside of the uppermost 50m ring contour on Ordnance Survey maps, with this summit height coming 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 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 a previous estimated summit height based on interpolation of contours.  Also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new summit height is 54.5m (converted to OSGM15) which is 2.5m higher than its previously listed height of 52m and 14.5m higher than the uppermost 40m ring contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.


The full details for the hill are:

Group:  Moelwynion

Summit Height (New Height):  54.5m (converted to OSGM15)

Name:  Bryn Tŵr

OS 1:50,000 map:  124

Summit Grid Reference:  SH 60453 44187

Drop:  c 43m

Dominance:  77.99%


Gathering data from the summit of Bryn Tŵr which resulted in this hill's significant height revision


Myrddyn Phillips (January 2018)






Mapping Mountains - Significant Height Revisions - 30-99m Twmpau

Gallt y Gog (SH 751 012) - 4th significant height revision

Survey post for Gallt y Gog


There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, and which was initiated by a survey with the Trimble GeoXH 6000 which took place on the 29th October 2017 in good, clear conditions.

The criteria for the two lists this height revision affects are:

30-99m Twmpau – Welsh hills at and above 30m and below 100m in height that have 30m minimum drop.  With the word Twmpau being an acronym standing for ‘thirty welsh metre prominences and upward’.

Y Trechol – The Dominant Hills of Wales - Welsh P30s whose prominence equal or exceed half that of their absolute height.

The name of the hill is Gallt y Gog and it is placed in the Pumlumon range of hills which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and the hill is positioned overlooking the A 489 road to its south and the Afon Dyfi to its north, with the town of Machynlleth at the base of the hill to the south. 

As the summit of the hill is not on designated open access land permission to visit should be sought, for those wishing to do so the nearest open access land is to the south-west of the hill, however there are paths in the conifer plantation that covers this hill’s summit and these indicate that people have visited this hill for many years.

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with c 66m of drop and 72.53% Dominance based on an estimated summit height of c 91m which was based on a 90m uppermost contour, and an estimated bwlch height of c 25m, with the summit height produced by the Trimble being 87.5m (converted to OSGM15), with this summit height coming 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 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 a previous estimated summit height based on interpolation of contours.  Also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

However, although the 5-15cm: 43.58% Estimated Accuracy recorded by the Trimble GeoXH 6000 is reasonable considering that the survey took place within a wood; the Standard Deviation of 0.5m signifies that the accuracy of the data set is questionable.  To compound this uncertainty there is question whether the uppermost 90m contour on Ordnance Survey maps is erroneous as it is placed a significant distance from where the accepted summit position of this hill is situated, according to the map the accepted summit position on the ground is above 80m but below the 90m contour.  However, this 90m uppermost contour could in fact be a depression and this ring contour signifies a 70m height, with the uppermost contour on Ordnance Survey maps being 80m, whatever the uncertainty, the result produced by the Trimble GeoXH 6000 for the summit of this hill does come within the above specified parameters. 

Therefore, this hill’s new summit height is 87.5m (converted to OSGM15) which is 3.5m lower than its previously estimated height of c 91m and 2.5m lower that what is thought to be the uppermost ring contour on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.


The full details for the hill are:

Group:  Pumlumon

Summit Height (New Height):  87.5m (converted to OSGM15)

Name:  Gallt y Gog

OS 1:50,000 map:  135

Summit Grid Reference:  SH 75169 01223

Drop:  62.7m (converted to OSGM15)

Dominance:  71.68%


The Trimble GeoXH 6000 gathering data at the summit of Gallt y Gog (SH 751 012) which resulted in this hill's significant height revision


Myrddyn Phillips (January 2018)






Mapping Mountains - Significant Height Revisions - 30-99m Twmpau

Emsger (SM 651 226) - 3rd significant height revision

Survey post for Emsger


There has been a Significant Height Revision to a hill that is listed in the Y Pellennig30-99m Twmpau and Y Trechol - The Dominant Hills of Wales, and which was initiated by a survey with the Trimble GeoXH 6000, with the survey that resulted in this height revision being conducted on the 14th May 2016 in good, bright conditions, perched beside a lighthouse on one of the most remote places in the whole of Wales.

The criteria for the three listings that this height revision affects are:

Y Pellennig –The Remotest Hills of Wales comprise all Welsh hills whose summit is 2.5km or more from the nearest paved public road and which have a minimum 15m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams.

30-99m Twmpau - These are the Welsh hills at or above 30m and below 100m in height that have a minimum drop of 30m.  The word Twmpau is an acronym for thirty welsh metre prominences and upward, and the list is authored by Myrddyn Phillips. 

Y Trechol - The Dominant Hills of Wales - These are the Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.  The list is authored by Myrddyn Phillips.

The hill is positioned on the island known as Emsger, which is also commonly known as South Bishop.  This island is positioned an approximate 8km (5 miles) west of St David’s Head in western Pembrokeshire, and unless being a competent and enthusiastic seal, it can only be approached by boat.  The island forms a part of the Bishops and Clerks chain of islands which form a compact groups of islets; all are rocky and are aligned northward to southward with North Bishop, Carreg Rhoson, Daufraich and Emsger being the main named island in each small group. 

The island is predominantly rock and if not for a series of steps leading up toward its summit its ascent would no doubt prove more problematic.  The upper part of Emsger has a lighthouse on it; this was built in 1839, and was converted to operating by electric in 1959, and demanned and automated in 1983.

The upper part of the island now has the lighthouse and associated buildings and platforms built on it, and therefore what once constituted the natural summit of the island is either buried under the lighthouse / platforms or it was destroyed during construction of the lighthouse.

I visited Emsger with Adrian Rayner, Ayako, Rob Woodall, Jon Glew, Sheila Glew, Sarah Kerr and Bob Kerr, and a quick assessment of what is left of natural ground pinpointed that this is situated just to the south-west of the main southerly corner of the lighthouse and its grounds.  This position was adjacent to steps leading down to the island’s helipad.

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with a 37m summit height, which originates from the spot height on the Ordnance Survey 1:25,000 Explorer map, with the spot of this spot height positioned on the south-east part of the land immediately adjacent to the lighthouse.  This is on a part of the construction and is man-made; however natural ground exists close to this point but it is significantly lower.

Therefore this island’s new summit height is 33.8m (converted to OSGM15) which is 3.2m lower than its previously listed height of 37m which appears on Ordnance Survey 1:25,000 Explorer maps.


The full details for the hill are:

Cardinal Island:  Bishops and Clerks

Summit Height:  33.8m (converted to OSGM15)

Name:  Emsger

OS 1:50,000 map:  157

Summit Grid Reference:  SM 65116 22619

Drop:  33.8m (converted to OSGM15)

Dominance:  100.00%



The Trimble GeoXH 6000 gathering data 0.99m above the highest remaining natural ground at the summit of Emsger, otherwise known as South Bishop (SM 651 226) which resulted in this island's significant height revision


Myrddyn Phillips (June 2016)






Mapping Mountains - Significant Height Revisions - 30-99m Twmpau

Pt. 36.9, Ynys Hir (SH 566 398) - 2nd significant height revision

Summit survey post for Pt. 36.9, Ynys Hir

Bwlch survey post for Pt. 36.9, Ynys Hir

Hill Reclassifications post for Pt. 36.9, Ynys Hir 


There has been a Significant Height Revision in the 30-99m Sub-Twmpau (thirty welsh metre prominences and upward) list initiated by a survey with the Trimble GeoXH 6000, with the following details being retrospective as the  survey that resulted in this height revision was conducted on 1st February 2014.

The criteria for the listing that this height revision affects are:


30-99m Twmpau - these are the Welsh hills at or above 30m and below 100m in height that have a minimum drop of 30m. 

30-99m Sub-Twmpau - these are the Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop


The listed name of the hill is Pt. 36.9m, Ynys Hir with its adjacent and higher hill taking on the full name without the point (Pt.) notation, and it is situated in the lowlands outside of Porthmadog in north-west Wales, and the survey was conducted in the company of Aled Williams who had suggested the walk, as he considered at least one of the hills he proposed for us to visit would become a new P30 – he was right!  With the new P30 being Ynys Hir (SH 56695 39393) which is adjacent to the hill this blog post relates to; Pt. 36.9m, Ynys Hir (SH 56649 39840).

The hill is situated to the north of Porthmadog in the area known as Traeth Mawr, and it can be accessed from a lay-by on the A487 to the south of the hill, however its ascent can be problematic due to the undergrowth.

The summit height of this hill has been dramatically increased from its current Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map height, as prior to the survey with the Trimble GeoXH 6000 the hill was not listed as it only had an uppermost ring contour of 20m on both of these maps, with an estimated summit height of c 24m.  Its new summit height is 36.9m (converted to OSGM15), which is 12.9m higher than its previously estimated height and 16.9m higher than the 20m uppermost ring contour on current Ordnance Survey maps.

Although the new summit height of 36.9m (converted to OSGM15) is dramatically different to current Ordnance Survey map detail, the Ordnance Survey Historical 1:25,000 map has an uppermost ring contour of 100ft for this hill which is correct when compared to the surveyed height produced by the Trimble GeoXH 6000. 


The full details for the hill are:


Cardinal Hill:  Hir Ynys

Summit Height (New Height):  36.9m (converted to OSGM15)

Name:  Pt. 36.9m, Ynys Hir

OS 1:50,000 map:  124

Summit Grid Reference:  SH 56649 39840
  
Drop:  25.8m (converted to OSGM15)


Setting up the Trimble geoXH 6000 at the summit of Pt. 36.9m, Ynys Hir.  Photo: Aled Williams


Myrddyn Phillips (August 2015)






Mapping Mountains - Significant Height Revisions - 30-99m Twmpau

Ynys Hir (SH 566 396) - 1st significant height revision

Survey post for Ynys Hir

Hill Reclassifications post for Ynys Hir - 30-99m Twmpau

Hill Reclassifications post for Ynys Hir - Y Trechol - the Dominant Hills of Wales


There has been a Significant Height Revision to the 30-99m Twmpau and Y Trechol - The Dominant Hills of Wales listings initiated by a survey with the Trimble GeoXH 6000, with the following details being retrospective as the survey that resulted in this height revision was conducted on 1st February 2014.

The two listings that this height revision affects are:


30-99m Twmpau - these are the Welsh hills at or above 30m and below 100m in height that have a minimum drop of 30m. 

Y Trechol - The Dominant Hills of Wales - these are the Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.


The name of the hill is Ynys Hir and it is situated in the lowlands outside of Porthmadog in north-west Wales, and the survey was conducted in the company of Aled Williams who had suggested the walk as he considered at least one of the hills he proposed for us to visit would become a new P30 – he was right!

The hill is situated to the north of Porthmadog in the area known as Traeth Mawr, and it can be easily accessed from a lay-by on the A487 to the south-west.  The term easily accessed is only applicable to where the road is situated in relation to the hill as it can be a bit of a battle to reach its top.

The summit height of Ynys Hir has been dramatically increased from its current Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map height, as prior to the survey with the Trimble GeoXH 6000 the hill was not listed as it only had an uppermost ring contour of 20m on both of these maps, with an estimated summit height of c 23m.  Its new summit height is 37.3m (converted to OSGM15), which is 14.3m higher than its previously estimated height and 17.3m higher than the 20m uppermost ring contour on the current Ordnance Survey maps.

Although the new summit height of 37.3m (converted to OSGM15) is dramatically different to current Ordnance Survey map detail, the Ordnance Survey Historical 1:25,000 map has an uppermost ring contour of 100ft for this hill which is correct when compared to the surveyed height produced by the Trimble GeoXH 6000.  


The full details for the hill are:


Cardinal Hill:  Hir Ynys

Summit Height (New Height):  37.3m (converted to OSGM15)

Name:  Ynys Hir

OS 1:50,000 map:  124

Summit Grid Reference:  SH 56695 39693 

Drop:  c 34m

Dominance:  91.21%


Beyond the trees, vertical cliffs and undergrowth is the summit of Ynys Hir, a new P30 which has a significant height revision



Myrddyn Phillips (August 2015)

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