Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Pellennig – The Remotest Hills of Wales

Carreg Rhoson (SM 672 256) 

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and the Y Pellennig – The Remotest Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill initially derived from detail on the DataMapWales and subsequently confirmed by LIDAR analysis conducted by Myrddyn Phillips.  With detail on the DataMapWales meeting the Significant Height Revisions protocols and subsequent LIDAR analysis negating the 2m discrepancy used within these same protocols.  

Carreg Rhoson (SM 672 256 - the island on the right)

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is entitled the 30-99m Sub-Twmpau with the criteria for 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 second sub category is entitled the Double Sub-Twmpau with the criteria for 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, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

30-99m Twmpau 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 to download in Google Doc format. 

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

The name the hill is listed by is Carreg Rhoson, and it is adjoined to the Garn Fawr group of hills which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it forms a part of the islet chain known as Carreg Rhoson, and is positioned to the west of the Pembrokeshire coast, with the small city of Tyddewi (St David’s) towards the east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the main P30 list as the small uppermost 30m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map had been missed. 

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

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 30m summit height, based on interpolation of its uppermost 30m ring contour.  As this hill is the high point of an island it was also listed with an estimated c 30m drop. 

Extract from the DataMapWales

However, it was not until the DataMapWales became available online that the details of this hill could be checked against a product viewed as being more accurate than the contouring on the contemporary Ordnance Survey 1:25,000 Explorer map.  This mapping has a 28m spot height for this hill and this is similar to the 27.42m summit height produced by the surface analysis programme developed by Joe Nuttall (JNSA) that uses the OS Terrain product where LIDAR coverage is not available, 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.

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 Carreg Rhoson (SM 672 256)

Therefore, the new listed summit height of this hill is 29.8m and this was derived from LIDAR analysis, this is 0.2m lower than the previously listed summit height of c 30m, which was based on interpolation of its uppermost 30m ring contour that appears on the Ordnance Survey 1:25,000 Explorer.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Garn Fawr

Name:  Carreg Rhoson

OS 1:50,000 map:  157

Summit Height:  29.8m (LIDAR)

Summit Grid Reference:  SM 67227 25662 (LIDAR)

Bwlch Height:  N/A (island)

Bwlch Grid Reference:  N/A (island)

Drop:  29.8m (LIDAR)

Remoteness:  5.175km (LIDAR)

 

Myrddyn Phillips and Aled Williams (July 2024)

  

Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Carreg Rhoson (SM 672 256) – 30-99m Twmpau reclassified to Double Sub-Twmpau 

There has been a reclassification to the list of 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill initially derived from detail on the DataMapWales and its status subsequently confirmed from LIDAR analysis conducted by Myrddyn Phillips. 

Carreg Rhoson (SM 672 256) 

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is entitled the 30-99m Sub-Twmpau with the criteria for 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 second sub category is entitled the Double Sub-Twmpau with the criteria for 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, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Carreg Rhoson, and it is adjoined to the Garn Fawr group of hills which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it forms a part of the islet chain known as Carreg Rhoson, and it is positioned to the west of the Pembrokeshire coast, with the small city of Tyddewi (St David’s) towards the east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the main P30 list as the small uppermost 30m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map had been missed.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 30m summit height, based on interpolation of its uppermost 30m ring contour.  As this hill is the high point of an island it was also listed with an estimated c 30m drop. 

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

However, it was not until the DataMapWales became available online that the details of this hill could be checked against a product viewed as being more accurate than the contouring on the contemporary Ordnance Survey 1:25,000 Explorer map.  This mapping has a 28m spot height for this hill and this is similar to the 27.42m summit height produced by the surface analysis programme developed by Joe Nuttall (JNSA) that uses the OS Terrain product where LIDAR coverage is not available. 

Extract from the DataMapWales

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 image of Carreg Rhoson (SM 672 256)

Therefore, the confirmation of the reclassification of this hill from 30-99m Twmpau status is due to LIDAR analysis, resulting in a 29.8m summit height, and as this summit is the high point of an island the drop value is also 29.8m, which is sufficient for Double Sub-Twmpau status. 

 

The full details for the hill are:

Group:  Garn Fawr

Name:  Carreg Rhoson

OS 1:50,000 map:  157

Summit Height:  29.8m (LIDAR)

Summit Grid Reference:  SM 67227 25662 (LIDAR)

Bwlch Height:  N/A (island)

Bwlch Grid Reference:  N/A (island)

Drop:  29.8m (LIDAR)

 

Myrddyn Phillips (July 2024) 

Mapping Mountains – Hill Reclassifications – Y Trechol – The Dominant Hills of Wales

Carreg Rhoson (SM 672 256) – Dominant deletion

There has been a deletion from the list of the Y Trechol – The Dominant Hills of Wales, with the summit height and its location, the drop, dominance and status of the hill initially derived from detail on the DataMapWales and its status subsequently confirmed from LIDAR analysis conducted by Myrddyn Phillips. 

Carreg Rhoson (SM 672 256) is positioned on the right of this photograph

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

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 3^rd December 2015, and the list is now available inits entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Carreg Rhoson, and it is adjoined to the Garn Fawr group of hills which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it forms a part of the islet chain known as Carreg Rhoson, and it is positioned to the west of the Pembrokeshire coast, with the small city of Tyddewi (St David’s) towards the east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the main P30 list as the small uppermost 30m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map had been missed. 

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

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 30m summit height, based on interpolation of its uppermost 30m ring contour.  As this hill is the high point of an island it was also listed with an estimated c 30m drop, which was sufficient for it to be classified as a Dominant hill with 100.00% dominance.

However, it was not until the DataMapWales became available online that the details of this hill could be checked against a product viewed as being more accurate than the contouring on the contemporary Ordnance Survey 1:25,000 map.  This mapping has a 28m spot height for this hill and this is similar to the 27.42m summit height produced by the surface analysis programme developed by Joe Nuttall (JNSA) that uses the OS Terrain product where LIDAR coverage is not available. 

Extract from the DataMapWales

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 image of Carreg Rhoson (SM 672 256)

Therefore, the confirmation of the deletion of this hill from Dominant status is due to LIDAR analysis, resulting in a 29.8m summit height, and as this summit is the high point of an island the drop value is also 29.8m, which is insufficient for Dominant status. 

 

The full details for the hill are:

Group:  Garn Fawr

Name:  Carreg Rhoson

OS 1:50,000 map:  157

Summit Height:  29.8m (LIDAR)

Summit Grid Reference:  SM 67227 25662 (LIDAR)

Bwlch Height:  N/A (island)

Bwlch Grid Reference:  N/A (island)

Drop:  29.8m (LIDAR)

 

Myrddyn Phillips (June 2024)

 

  

Mapping Mountains – Hill Reclassifications – Y Trechol – The Dominant Hills of Wales

THIS HILL HAS SUBSEQUENTLY BEEN DELETED FROM DOMINANT STATUS

Carreg Rhoson (SM 672 256) - Dominant addition

There has been an addition to the listing of Y Trechol – The Dominant Hills of Wales due to detail included on the Ordnance Survey enlarged mapping hosted on the Geograph website.  This has resulted in the hill being added to the Dominant list.  With the criteria for inclusion to this list being those Welsh P30 hills whose prominence equal or exceed half that of their absolute height.

The details relating to this hill’s inclusion as a Dominant hill are retrospective as it was added to the list shortly after its initial compilation was completed with the date of 31^st October 2012 given against its details.

Prior to this hill being listed as a P30 and its inclusion as a Dominant hill it had remained unclassified, as the uppermost 30m ring contour on the Ordnance Survey 1:25,000 Explorer map had not been spotted.

The hill is in the Pembrokeshire Islands group of hills and is placed in the Region of Mid and West Wales (Region B, Sub-Region B4) and is situated 5.175km from the nearest paved public road off the Pembrokeshire coast, and is positioned in the island chain of the Bishops and Clerks.

The name of the hill is Carreg Rhoson and this is also the name given to a number of adjacent islets, it is included as a Dominant hill as the Ordnance Survey enlarged mapping hosted on the Geograph website has greater scale and definition when compared to the smaller scaled and publicly available Ordnance Survey maps, and therefore its uppermost 30m ring contour is easier to see, and as this hill is also an island its c 30m summit height also gives it 100.00% Dominance.

The full details for the hill are:

Group:  Pembrokeshire Islands

Name:  Carreg Rhoson

Dominance:  100.00%

OS 1:50,000 map:  157

Summit Grid Reference:  SM 67235 25678

Summit Height:  c 30m

Drop Summit to Bwlch:  N/A

Drop Bwlch to ODN:  N/A 

The island chain of Carreg Rhoson with the new Dominant hill (SM 672 256) being the highest point on the right of this photograph

Myrddyn Phillips (March 2017)