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Friday 13 October 2017

Mapping Mountains – Significant Height Revisions – 700m Twmpau


700m Twmpau – Significant Height Revisions

The 700m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 700m and below 800m in height that have a minimum drop of 30m.  Accompanying the main P30 list is a sub list entitled the 700m Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 700m and below 800m 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 - 700m Twmpau

Pt. 714.3m (SH 627 359) - 4th significant height revision

Hill Reclassifications post for Pt. 714.3m

 

There has been a Significant Height Revision to a hill that is listed in the Y Pellennig – The Remotest Hills of Wales, Welsh Highlands – Uchafion Cymru and 700m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Aled Williams. 

LIDAR image of Pt. 714.3m (SH 627 359)

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

Y Pellennig –The Remotest Hills of Wales - Welsh hills whose summit is at least 2.5km from the nearest paved public road and the hill has a minimum 15m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available as a downloadable e-booklet or print-booklet version on Mapping Mountains Publications with the up-to-date master list available on Mapping Mountains to download in Google Doc format. 

Y Pellennig - The Remotest Hills of Wales by Myrddyn Phillips and Aled Williams

Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

Welsh Highlands - Uchafion Cymru by Myrddyn Phillips and Aled Williams

700m Twmpau - Welsh hills at or above 700m and below 800m in height with 30m minimum drop, with an accompanying sub list entitled the 700m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 700m and below 800m 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 is available to download in Google Doc format from Mapping Mountains. 

The 700m Twmpau by Myrddyn Phillips

The hill is being listed by the point (Pt. 714.3m) notation and it is adjoined to the Arenig Fawr group of hills, which are situated in the central part of North Wales (Region A, Sub-Region A2), and it is positioned with a minor roads to the north, south-west and east, and the A4212 road farther to its north, and has the town of Y Bala towards the east.

When the original list that later became known as the Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with 18m of drop, based on an eye and spirit level survey conducted by John and Anne Nuttall that resulted in 60ft (18.3m) of drop, and it was originally listed with a 712m summit height based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and a 694m bwlch height. 

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 summit image of Pt. 714.3m (SH 627 359)

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.

Therefore, the new listed summit height of this hill is 714.3m and this was derived from LIDAR analysis, this is 2.3m higher than the previously listed height of 712m 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:  Arenig Fawr

Name:  Pt. 714.3m

OS 1:50,000 map:  124, 125

Summit Height (New Height):  714.3m (LIDAR) 

Summit Grid Reference:  SH 82741 35938 (LIDAR) 

Bwlch Height:  693.6m (LIDAR) 

Bwlch Grid Reference:  SH 82725 36128 (LIDAR) 

Drop:  20.7m (LIDAR) 

Remoteness:  2.925km

 

Myrddyn Phillips (August 2023)






Mapping Mountains - Significant Height Revisions - 700m Twmpau

 Fan y Big (SO 036 206) - 3rd significant height revision

 1st survey post for Fan y Big

 2nd survey post for Fan y Big

 Hill Reclassifications post for Fan y Big


There has been a Significant Height Revision to a hill that appears in the following lists; Yr Uchafion, 700m Twmpau and Y Pellennig – The Remotest Hills of Wales, and which was initiated by LIDAR analysis conducted by Aled Williams followed by a survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, with the latter conducted over two days on the 30th June 2018 and the 21st August 2018 in clear conditions turning to mist on the first survey and misty conditions clearing on the second survey.

LIDAR image of Fan y Big

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

Yr Uchafion – Welsh hills at or above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams, with the Introduction to the list appearing on Mapping Mountains on the 4th November 2015.

700m Twmpau - Welsh hills at or above 700m and below 800m in height with 30m minimum drop, with an accompanying sub list entitled the 700m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 700m and below 800m 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.

Y Pellennig –The Remotest Hills of Wales - Welsh hills whose summit is at least 2.5km from the nearest paved public road and the hill has a minimum 15m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available as a downloadable e-booklet or print-booklet version on Mapping Mountains Publications with the up-to-date master list available on Mapping Mountains in Google Doc format.

The name of the hill is Fan y Big and it is placed in the central part of the main ridge that forms the Bannau Brycheiniog (Brecon Beacons) group of hills, which are situated in the South Wales Region (Region C, Sub-Region C2), and it is positioned between the towns of Aberhonddu (Brecon) to its north and Merthyr Tudful to its south.

The hill can be accessed from public footpaths originating from its north and from its south; or it can be combined as a longer expedition taking in the main Bannau Brycheiniog ridge either from its west or from its east.

Prior to LIDAR analysis and the subsequent survey with the Trimble GeoXH 6000 the height of the hill was listed as 719m based on the summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

Extract from the Ordnance Survey 1:125,000 Explorer map

The summit of this hill was surveyed three times over two days, each 53 days apart resulting in the following:


1st survey 30.06.18:  716.800m

2nd survey 30.06.18:  716.676m

3rd survey 21.08.18:  716.754m

Average of three summit surveys:  716.746m


Therefore, the average of three summit surveys produced by the Trimble GeoXH 6000 is 716.7m (converted to OSGM15), this is not a dramatic height revision when compared to some survey results, 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 summit height as surveyed with the Trimble GeoXH 6000 is 716.7m (converted to OSGM15) which is 2.3m lower than its previously listed height of 719m which appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The Trimble GeoXH 6000 gathering data at the summit of Fan y Big which resulted in this hill's significant height revision


The full details for the hill are:

Group:  Bannau Brycheiniog

Summit Height (New height):  716.7m (converted to OSGM15)

Name:  Fan y Big

OS 1:50,000 map:  16o

Summit Grid Reference:  SO 03652 20658
  
Drop:  28.5m (converted to OSGM15)


For details on the 1st survey that significantly revised the summit height of this hill

For details on the 2nd survey that significantly revised the summit height of this hill


Myrddyn Phillips (November 2018)




Mapping Mountains - Significant Height Revisions - 700m Twmpau

 Twyn Llech (SO 255 353) - 2nd significant height revision

 Survey post for Twyn Llech

Significant Name Changes post for Twyn Llech

Summit Relocation post for Twyn Llech


There has been a Significant Height Revision to a hill that appears in the Yr Uchafion and 700m Twmpau lists, 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 28th September 2017 in good, clear conditions with just a breath of breeze.

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

Yr Uchafion – All Welsh hills at and above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams, with the Introduction to the list appearing on Mapping Mountains on the 4th November 2015.

700m Twmpau – All Welsh hills at and above 700m and below 800m in height that have 30m minimum drop, with the word Twmpau being an acronym standing for ‘thirty welsh metre prominences and upward’.  The list is authored by Myrddyn Phillips.

The name of the hill is Twyn Llech and it is a part of the Mynyddoedd Duon (Black Mountains) range, which is an extensive group of hills in the south-eastern part of South Wales, and it is situated above and between the upper part of the Olchon Valley to its south-east and the Dyffryn Ewias (Vale of Ewyas) to its south.  The hill is also situated between the small communities of Craswall towards its east and Capel-y-ffin towards its south, and the hill can be accessed from a car park just below the high point of Bwlch yr Efengyl (Gospel Pass) via the summit of Hay Bluff, or it can be approached from the south-east via paths leading to its southerly ridge from either the Olchon Valley and the Dyffryn Ewias (Vale of Ewyas).

Prior to the survey with the Trimble GeoXH 6000 the height of the hill was listed as 706m based on the summit spot height which is positioned at approximately SO 25545 35372 (taken by using a ruler) on the Harvey 1:40,000 Offa’s Dyke South map.  Whilst Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps give a summit spot height of 703m positioned at SO 25586 35010.

The Harvey maps 706m summit spot height
The Ordnance Survey 703m summit spot height

Two points were surveyed for summit position using the Trimble GeoXH 6000, one on the southerly point where the Ordnance Survey give their 703m spot height and one on the northerly point where Harvey maps give their 706m spot height.  The summit area of this hill has two rises which complement these spot height positions and which have a slight dip in between.

The results produced by the Trimble GeoXH 6000 appear below:

Northerly high point:  703.639m (converted to OSGM15) at SO 25520 35383

Southerly high point:  702.721m (converted to OSGM15) at SO 25572 35032

The above result 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, 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.

As the Harvey summit position was the one used for listing, it is the 706m spot height that this Significant Height Revisions post applies to.

The summit of this hill consists of tussock grass close to the main and now well maintained paved path that crosses this hill on a south – north direction, and therefore this hill’s new summit height as surveyed with the Trimble GeoXH 6000 is 703. 6m (converted to OSGM15) which is 2.4m lower than its previously listed height of 706m which appears on the Harvey 1:40,000 Offa’s Dyke South map.


The full details for the hill are:


Cardinal Hill:  Twyn Llech

Summit Height (New height):  703.6m (converted to OSGM15)

Name:  Twyn Llech

OS 1:50,000 map:  161

Summit Grid Reference:  SO 25520 35383

Drop:  154.5m (converted to OSGM15)


The Trimble GeoXH 6000 gathering data at the summit of Twyn Llech (SO 255 353) which resulted in this hill's significant height revision

For details on the survey that significantly revised the summit height of this hill

Myrddyn Phillips (November 2017)



Mapping Mountains - Significant Height Revisions - 700m Twmpau

 Llechog (SH 606 567) - 1st significant height revision

 Survey post for Llechog


There has been a Significant Height Revision to a hill that is listed in the Welsh Highlands – Uchafion Cymru and 700m 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.

Llechog (SH 606 567)

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

Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

700m Twmpau - Welsh hills at or above 700m and below 800m in height that have 30m minimum drop, with an accompanying sub list entitled the 700m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 700m and below 800m 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 15th April 2015.

700m Twmpau by Myrddyn Phillips

The name the hill is listed by is Llechog and it is adjoined to the Yr Wyddfa 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 A4086 road to its north-east and the A4085 road to its south-west, and has the village of Llanberis towards the north-west.

When the listing that later became known as the Welsh Highlands - Uchafion Cymru was first compiled this hill was listed with an estimated c 26m of drop, based on the 718m summit spot height that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 692m bwlch height, based on interpolation of 10m contouring between 690m - 700m. 

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

However, it was not until the Trimble GeoXH 6000 survey that the details for this hill could be accurately re-assessed.  The summit height produced by this survey is 720.0m and is positioned at SH 60613 56751, 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 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 from the summit of Llechog 

Therefore, the new listed summit height of this hill is 720.0mm and this was derived from a Trimble GeoXH 6000 survey, this is 2.0m higher than the previously listed summit height of 718m, which was based on the spot height that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.


The full details for the hill are:

Group:  Yr Wyddfa

Name:  Llechog

OS 1:50,000 map:  115

Summit Height (New Height):  720.0m (converted to OSGM15)

Summit Grid Reference:  SH 60613 56751
  
Bwlch Height:  692.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 60557 56634

Drop:  28.0m


Myrddyn Phillips and Aled Williams (August 2015)



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