Thursday, 2 November 2017

Mapping Mountains – Significant Height Revisions – 200m Twmpau


200m Twmpau – Significant Height Revisions

The 200m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 200m and below 300m in height that have a minimum drop of 30m.  Accompanying the main P30 list is a sub list entitled the 200m Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 200m and below 300m 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 list appear below presented chronologically in receding order.








Mapping Mountains - Significant Height Revisions - 200m Twmpau

Allt Wen (SH 745 772) - 6th significant height revision

Survey post for Allt Wen


There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height and its location, the drop and status of the hill confirmed by LIDAR analysis, and a subsequent survey with the Trimble GeoXH 6000, both conducted by Myrddyn Phillips with the latter taking place on the 15th October 2018.

Allt Wen (SH 745 772)

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

200m Twmpau - Welsh hills at or above 200m and below 300m in height with 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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.

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3rd December 2015.

The name of the hill is Allt Wen, and it is adjoined to the Carneddau 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 A55 road and the coast to its north, and has the town of Conwy towards its east.

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 255m summit height based on the 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

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.  LIDAR analysis was closely followed by the survey of this hill with the Trimble GeoXH 6000. 

LIDAR image of Allt Wen

The summit height produced by the Trimble GeoXH 6000 survey is 252.3m,  this is not a substantial revision when compared to some revised heights, and 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.  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.

The Trimble GeoXH 6000 gathering data at the summit of Allt Wen

Therefore, this hill’s new listed summit height is 252.3m and this was produced by surveying with the Trimble GeoXH 6000, this is 2.7m lower than its previously listed height of 255m which was based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.


The full details for the hill are:

Group:  Carneddau

Name:  Allt Wen

OS 1:50,000 map:  115

Summit Height (New height):  252.3m (converted to OSGM15)

Summit Grid Reference:  SH 74546 77264  

Bwlch Height:  160.2m (converted to OSGM15)

Bwlch Grid Reference:  SH 74855 77154

Drop:  92.1m

Dominance:  36.51%



Myrddyn Phillips (April 2019)






Mapping Mountains - Significant Height Revisions - 200m Twmpau

Mynydd y Dref (SH 759 777) - 5th significant height revision

Survey post for Mynydd y Dref


There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, with the summit height, its location and the drop of the hill confirmed by LIDAR analysis, and a subsequent summit and bwlch survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, with the latter taking place on the 15th October 2018.

Mynydd y Dref (SH 759 777)

LIDAR image of Mynydd y Dref

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 name of the hill is Mynydd y Dref, and it is adjoined to the Carneddau range of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A55 road and the coast to its north, and has the town of Conwy towards its east south-east.

When the original Welsh 200m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 244m summit height based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

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

The summit height produced by the Trimble GeoXH 6000 survey is 246.1m, this is not a substantial 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 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.

The summit of Mynydd y Dref

Therefore, this hill’s new listed summit height is 246.1m and this was produced by surveying with the Trimble GeoXH 6000, this is 2.1m higher than its previously listed height of 244m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.


The full details for the hill are:

Group:  Carneddau

Name:  Mynydd y Dref

OS 1:50,000 map:  115

Summit Height (New height):  246.1m (converted to OSGM15)

Summit Grid Reference:  SH 75981 77799
  
Bwlch Height:  186.7m (converted to OSGM15)

Bwlch Grid Reference:  SH 75246 77548

Drop:  59.4m



Myrddyn Phillips (April 2019)






Mapping Mountains - Significant Height Revisions - 200m Twmpau

Coed Eryr (SH 636 508) - 4th significant height revision

Survey post for Coed Eryr


There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, with the summit height, its location and the drop of the hill confirmed by LIDAR analysis, and a subsequent summit survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, with the latter taking place on the 6th October 2018.

LIDAR image of Coed Eryr

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 name of the hill is Coed Eryr, and it is adjoined to the Moelwynion range of hills, which are situated in the western part of North Wales (Region A, Sub-Region A1), and it is positioned between the lakes of Llyn Dinas to ts south-west and Llyn Gwynant to its north-east, with the A498 road to its north-west, and has the village of Beddgelert towards the south-west.

When the original Welsh 200m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 241m summit height based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger map.

Extract from the Ordnance Survey 1:50,000 Landranger map

LIDAR summit image of Coed Eryr

The summit height produced by the Trimble GeoXH 6000 survey is 243.7m (converted to OSGM15), this is not a substantial 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 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 listed summit height is 243.7m (converted to OSGM15) and this was produced by surveying with the Trimble GeoXH 6000, this is 2.7m higher than its previously listed height of 241m which appears as a spot height on the Ordnance Survey 1:50,000 Landranger map.


The Trimble GeoXH 6000 gathering data at the summit of Coed Eryr

The full details for the hill are:

Group:  Moelwynion

Name:  Coed Eryr

OS 1:50,000 map:  115

Summit Height (New height):  243.7m (converted to OSGM15)

Summit Grid Reference:  SH 63612 50890
  
Bwlch Height:  181.6m (LIDAR)

Bwlch Grid Reference:  SH 64230 51021 (LIDAR)

Drop:  62.1m (Trimble summit and LIDAR bwlch)



Myrddyn Phillips (March 2019)






Mapping Mountains - Significant Height Revisions - 200m Twmpau

Clip y Gylfinhir (SH 223 284) - 3rd significant height revision

1st survey of Clip y Gylfinhir

2nd summit survey of Clip y Gylfinhir


There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, with the hill’s height, drop and status confirmed by a survey with the Trimble GeoXH 6000 which was conducted by Myrddyn Phillips on the 28th December 2014 and 3rd October 2018 in good, clear conditions with hardly a breath of breeze.

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 name of the hill is Clip y Gylfinhir, and it is adjoined to the Pen Llŷn range of hills, which are situated in the western part of North Wales (Region A, Sub-Region A1), and it is positioned toward the western end of Pen Llŷn, and is encircled by a series of minor roads and has the small community of Y Rhiw towards the south.

Clip y Gylfinhir (SH 223 284)

As the summit of the hill is a part of designated open access land it can be approached from most directions, with public footpaths leading in to the access land from a variety of directions, and for those wishing a quick ascent of the hill there is parking space at the immediate base of the hill to its north-east.

When the original Welsh 200m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed in the accompanying Hills to be surveyed sub list with a 270m summit height which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.

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

The summit height produced by The Trimble GeoXH 6000 survey is 267.8m (converted to OSGM15), this is not a substantial 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 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.

The Trimble GeoXH 6000 gathering data at the summit of Clip y Gylfinhir

Therefore, this hill’s new summit height is 267.8m (converted to OSGM15) and this was produced by surveying with the Trimble GeoXH 6000, this is 2.3m lower than its previously listed height of 270m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map, however this new height is in accordance with the 5m contouring on OS Maps.


The full details for the hill are:

Group:  Pen Llŷn

Name:  Clip y Gylfinhir

OS 1:50,000 map:  123

Summit Height (New height):  267.8m (converted to OSGM15)

Summit Grid Reference:  SH 22395 28472
  
Bwlch Height:  240.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 22514 28524

Drop:  27.7m (converted to OSGM15)




Myrddyn Phillips (December 2018)






Mapping Mountains - Significant Height Revisions - 200m Twmpau

Winllan Hill (SJ 217 214) - 2nd significant height revision 

1st summit survey post for Winllan Hill

2nd summit survey post for Winllan Hill

Summit Relocations post for Winllan Hill


There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau initiated by a survey with the Trimble GeoXH 6000, with these details being retrospective as the survey that resulted in this height revision was conducted over two days; 28th / 29th August 2015.

The listing that this height revision affects is:

200m Twmpau - These are the Welsh hills at or above 200m and below 300m in height that have a minimum drop of 30m.  The word Twmpau is an acronym for thirty welsh metre prominences and upward.

The name of the hill is Winllan Hill and it is situated in the lower easterly hills of Y Berwyn, and is positioned above Llansanffraid-ym-Mechain which is situated beside where the Afon Cain flows in to the Afon Efyrnwy (River Vyrnwy).  The hill was initially surveyed in the late evening in the company of Mark Trengove after nourishing our appetite in the Bradford Arms in Llanymynech.

Access to Winllan Hill is good as public footpaths from the south, west, north and east head towards its summit and parking is easily found when approaching the hill from the west and north, this is also likely to be the case if approaching the hill from the east and south.  The high point of the hill is beside a fence and under a large overhanging tree, and is only a few metres from where a public footpath crosses the field that the summit is situated in.

Prior to the survey with the Trimble GeoXH 6000 the hill was listed as 216m high based on the spot height on the area of the summit that appears on the Ordnance Survey 1:25,000 Explorer map, and an estimated drop value of c 48m, based on bwlch contour interpolation which gives the height of the bwlch as c 168m.  However, there is significantly higher land to the north of where the 216m spot height is positioned on the ground.  With the height resulting from the survey of the summit with the Trimble GeoXH 6000 being 220.3m (converted to OSGM15).  

Therefore this hill’s new summit height is 220.3m (converted to OSGM15) which is 4.3m higher than the previously listed height of 216m which was based on the Ordnance Survey 1:25,000 Explorer map spot height.


The full details for the hill are:

Cardinal Hill:  Rhialgwm

Summit Height (New Height):  220.3m (converted to OSGM15)

Name:  Winllan Hill

OS 1:50,000 map:  126

Summit Grid Reference:  SJ 21709 21460 

Drop:  c 52m


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


Myrddyn Phillips (October 2015)






Mapping Mountains - Significant Height Revisions - 200m Twmpau

Pen y Berth (SJ 081 127) - 1st significant height revision 

Summit survey post for Pen y Berth

Bwlch survey post for Pen y Berth 

Summit Relocations post for Pen y Berth


There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau (thirty welsh metre prominences and upward) as well as Y Trechol - the Dominant Hills of Wales, and which was initiated by a survey with the Trimble GeoXH 6000, with these details being retrospective as the survey that resulted in this height revision was conducted on 19th October 2014.

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


200m Twmpau - these are the Welsh hills at or above 200m and below 300m 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.  With the criteria for Lesser Dominant status being those addition Welsh P30 hills whose prominence is 33% or more and below 50% of their absolute height.


The name of the hill is Pen y Berth and it is situated in the Y Berwyn range of hills and the survey with the Trimble GeoXH 6000 was conducted in the company of Mark Trengove on a colourful autumnal day in October.

The hill can be accessed from 1km east of the small community of Dolanog on a track that heads north-west and then east to sneak up to the summit from the opposing side of the hill.  The summit area of the hill is given two 280m uppermost ring contours aligned east and west of one another on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps, with each contour ring given a 282m spot height on the Ordnance Survey enlarged mapping hosted on the Geograph website.


The following results are those obtained with the Trimble GeoXH 6000 for the two points surveyed:

Eastern 282m spot height:  283.7m (converted to OSGM15) at SJ 08322 12748

Western 282m spot height at SJ 08012 12780 not surveyed

Easterly high point of western 280m ring contour:  287.5m (converted to OSGM15) at SJ 08126 12730


The land at SJ 08126 12730 was visually judged to be higher than the land at SJ 08012 12780, therefore the latter position was not surveyed.  The height given this hill on the Ordnance Survey enlarged mapping hosted on the Geograh website is 282m, with the height resulting from the survey with the Trimble GeoXH 6000 being 287.5m (converted to OSGM15).

Therefore this hill’s new summit height is 5.5m higher than its previously listed height which came from the 282m spot height on the Ordnance Survey enlarged mapping hosted on the Geograph website.


The full details for the hill are:

Cardinal Hill:  Carreg y Big

Summit Height (New Height):  287.5m (converted to OSGM15)

Name:  Pen y Berth

OS 1:50,000 map:  125

Summit Grid Reference:  SJ 08126 12730 

Drop:  134.2m (converted to OSGM15)

Dominance:  46.68%


Pen y Berth (SJ 081 127) a hill whose height has been significantly revised


Myrddyn Phillips (August 2015)


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