Mapping Mountains – Summit Relocations – The Fours – The 400m Hills of England

 

Disgwylfa (SO 172 841) 

There has been a Summit Relocation to a hill listed in the The Fours – The 400m Hills of England, with the summit height, col height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Disgwylfa (SO 172 841)

The criteria for the list that this summit relocation applies to are:

The Fours – The 400m Hills of England - English hills at or above 400m and below 500m in height that have 30m minimum drop, accompanying the main list is a sub list entitled the Sub-Fours, the criteria for which are all English hills at or above 400m and below 500m in height that have 15m or more and below 30m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available to download in Google Doc format from the Mapping Mountains site.

The Fours - The 400m Hills of England by Myrddyn Phillips and Aled Williams

The name the hill is now listed by is Disgwylfa, and this was derived from the Ordnance Survey One-Inch ‘Old Series’ map, and it is adjoined to the Cilfaesty group of hills, which are situated adjoined to the Welsh border, and it is positioned with the B4368 road to its north, and minor roads to its west and east, and has the town of Ceri towards the north north-west.

When the Introduction to the first group of hills for the updated and revised listing of the The Fours – The 400m Hills of England was published by Mapping Mountains on the 10^th September 2022, it was announced that the accompanying sub lists were being revised with the two 390m categories dispensed with and the criteria and name of the 400m Sub-Fours revised.  The one accompanying sub list is now named the Sub-Fours with its criteria being all English hills 400m and above and below 500m in height that have 15m and more and below 30m of drop.

Prior to this revision this hill was listed with an estimated c 15m of drop, based on the 427m summit spot height positioned at SO 17503 84053 that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 412m col height, based on interpolation of 10m contouring between 410m – 420m. 

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 gives the highest ground on this hill as 429.2m positioned at SO 17249 84185.  However, this is to the top of a raised track that forms a field boundary and protocols dictate that as this is deemed a relatively recent man-made construct such ground is discounted from the height of a hill. 

LIDAR summit image of Disgwylfa (SO 172 841)

The height produced by LIDAR analysis to the natural summit of this hill is 428.9m and this is positioned at SO 17246 84195, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour either on Ordnance Survey maps or interactive mapping, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, or the de-twinning of a summit, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist.

Therefore, the summit height produced by LIDAR analysis is 428.9m and this is positioned at SO 17246 84195.  This position is not given a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and is approximately 10 metres north north-westward from where the high point of the raised track that forms a field boundary is positioned and approximately 263 metres west north-westward from where the 427m spot height appears. 

 

The full details for the hill are:

Group:  Cilfaesty

Name:  Disgwylfa

OS 1:50,000 map:  136

Summit Height:  428.9m (LIDAR)

Summit Grid Reference (New Position):  SO 17246 84195 (LIDAR)

Col Height:  410.9m (LIDAR)

Col Grid Reference:  SO 17775 83941 (LIDAR)

Drop:  18.0m (LIDAR)

 

Myrddyn Phillips and Aled Williams (March 2026)

Mapping Mountains – Significant Height Revisions – 100m Twmpau

 

Coed Penllyn (SH 786 696) 

There has been a Significant Height Revision to a hill that is listed in the 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Coed Penllyn (SH 786 696)

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

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

100m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Coed Penllyn, and this was derived from the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, and it is adjoined to the Mynydd Hiraethog group of hills, which are situated in the northern part of North Wales (Region A, Sub-Region A2), and it is positioned with the A470 road to its west and a minor road to its east, and has the town of Llanrwst towards the south.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 list, and listed with a 193m summit height positioned at SH 786 697 with an accompanying note stating; Height from 1989 1:50000 map. 

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

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed as a twin top with an estimated c 25m of drop, based on the 193m spot height that appears on the 1:50,000 Landranger map and a twin 193m map heighted top positioned at SH 78713 69823 that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and an estimated c 168m bwlch height, based on interpolation of 10m contouring between 160m – 170m. 

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 Coed Penllyn (SH 786 696)

The summit height produced by LIDAR analysis is 198.0m and is positioned at SN 78630 69665, 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, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 198.0m and this was derived from LIDAR analysis, this is 5.0m higher than the previously listed 193m summit height which appeared as a twin top with a spot height on the Ordnance Survey 1:50,000 Landranger map and the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.

 

ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Mynydd Hiraethog 

Name:  Coed Penllyn 

OS 1:50,000 map:  115

Summit Height (New Height):  198.0m (LIDAR) 

Summit Grid Reference:  SH 78630 69665 (LIDAR) 

Bwlch Height:  170.9m (LIDAR) 

Bwlch Grid Reference:  SH 78892 70134 (LIDAR) 

Drop:  27.1m (LIDAR) 

 

Myrddyn Phillips (March 2026)

Mapping Mountains – Significant Height Revisions – The Welsh P15s

Grinllwm (SH 772 625) 

There has been a Significant Height Revision to a hill that was listed in The Welsh P15s, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Grinllwm (SH 772 625)

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

The Welsh P15s – Welsh hills with 15m minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s, with the criteria for this sub category being all Welsh hills with 14m or more and below 15m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list appearing on Mapping Mountains on the 10^th May 2019. 

The Welsh P15s by Myrddyn Phillips

The name the hill is now listed by is Grinllwm, and its name placement was derived from the Ordnance Survey series of Six-Inch maps, and it is adjoined to the Carnedd Llywelyn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with minor roads to its north, south and east, with the B5106 road and the A470 road farther to its east, and has the town of Llanrwst towards the east south-east.

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the main list with an estimated c 15m of drop, based on an estimated c 264m summit height and the 249m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage 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 summit image of Grinllwm (SH 772 625)

The summit height produced by LIDAR analysis is 267.7m and when compared to the previously listed summit height, this comes within the parameters of the Significant Height Revisions used within this page heading, and these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 267.7m and this was derived from LIDAR analysis.  This is 3.7m higher than the previous listed summit height of c 264m, which was based on interpolation of the uppermost 260m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

 

The full details for the hill are: 

Group:  Carnedd Llywelyn 

Name:  Grinllwm 

OS 1:50,000 map:  115

Summit Height (New Height):  267.7m (LIDAR)                                                           

Summit Grid Reference:  SH 77258 62516 (LIDAR)                                                  

Bwlch Height:  249.1m (LIDAR) 

Bwlch Grid Reference:  SH 77295 62465 (LIDAR) 

Drop:  18.6m (LIDAR) 

 

Myrddyn Phillips (March 2026)

 

Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Carreg Rhoson (SM 665 254) – Double Sub-Twmpau deletion 

There has been a deletion to the list of 30-99m Twmpau, with the summit height and its location, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

Carreg Rhoson (SM 665 254) - second right of the highest island

The criteria for the list that this deletion 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.  The list is authored by Myrddyn Phillips, 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 part of an islet chain positioned to the west of mainland Wales with a minor road on the mainland to its east, and has the city of Tyddewi (St David’s) on the mainland also 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 Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

After the sub list was standardised, and interpolated heights and drop values also included the details for this island were re-evaluated and it was listed with an estimated c 20m of drop, based on interpolation of the uppermost 20m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website 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 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 mapping resources now available online is on the Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  This mapping only has an uppermost 15m ring contour and therefore compliments the LIDAR result.

Extract from the Magic Maps website

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 665 254)

Therefore, the deletion of this hill from Double Sub-Twmpau status is due to LIDAR analysis, resulting in a 19.1m summit height and as this hill is an island, this value also gives it 19.1m of drop, which is insufficient for it to be classified as a Double Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Garn Fawr 

Name:  Carreg Rhoson 

OS 1:50,000 map:  157

Summit Height:  19.1m (LIDAR) 

Summit Grid Reference:  SM 66538 25421 (LIDAR)               

Bwlch Height:  N/A (sea level) 

Bwlch Grid Reference:  N/A (sea level) 

Drop:  19.1m (LIDAR) 

 

Myrddyn Phillips (March 2026)

 

Mapping Mountains – Summit Relocations – The Fours – The 400m Hills of England

 

Llanfair Hill (SO 255 785) 

There has been a Summit Relocation to a hill listed in the The Fours – The 400m Hills of England, with the summit height, col height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 summit survey and LIDAR col analysis conducted by Myrddyn Phillips. 

LIDAR image of Llanfair Hill (SO 255 785)

The criteria for the list that this summit relocation applies to are:

The Fours – The 400m Hills of England - English hills at or above 400m and below 500m in height that have 30m minimum drop, accompanying the main list is a sub list entitled the Sub-Fours, the criteria for which are all English hills at or above 400m and below 500m in height that have 15m or more and below 30m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available to download in Google Doc format from the Mapping Mountains site.

The Fours - The 400m Hills of England by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Llanfair Hill, and it is adjoined to the Cilfaesty group of hills, which are situated adoined to the Welsh border, and it is positioned encircled by minor roads, with the B4368 road farther to its north, the B4355 road farther to its south-west and the A488 road farther to its east, and has the town of Clun towards the east north-east.

When the Introduction to the first group of hills for the updated and revised listing of the The Fours – The 400m Hills of England was published by Mapping Mountains on the 10^th September 2022, it was announced that the accompanying sub lists were being revised with the two 390m categories dispensed with and the criteria and name of the 400m Sub-Fours revised.  The one accompanying sub list is now named the Sub-Fours with its criteria being all English hills 400m and above and below 500m in height that have 15m and more and below 30m of drop. 

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

Prior to this revision this hill was listed with an estimated c 14m of drop, based on twin 431m and c 431m summits with the 431m summit spot height positioned at SO 25592 78509 and an estimated c 431m twin summit height positioned at SO 25493 78438 that appear on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 417m col height, based on interpolation of 5m contouring between 415m – 420m.

The Trimble GeoXH 6000 gathering data at the summit of Llanfair Hill (SO 255 785)

However, it was not until the survey with the Trimble GeoXH 6000 and when 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 Llanfair Hill (SO 255 785)

The Trimble GeoXH 6000 survey gives the summit of this hill as 430.6m positioned at SO 25594 78509 and LIDAR analysis confirms this is the highest ground on this hill and therefore its de-twinning, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour either on Ordnance Survey maps or interactive mapping, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, or the listing of a new twin summit or de-twinning of a summit, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist.

Therefore, the summit height produced by the Trimble GeoXH 6000 survey is 430.6m and this is positioned at SO 25594 78509.  This position matches that of the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and LIDAR analysis confirms the de-twinning of this summit.

 

The full details for the hill are:

Group:  Cilfaesty

Name:  Llanfair Hill

OS 1:50,000 map:  137, 148

Summit Height:  430.6m (Trimble GeoXH 6000)

Summit Grid Reference (New Position):  SO 25594 78509 (Trimble GeoXH 6000)

Col Height:  415.3m (LIDAR)

Col Grid Reference:  SO 25209 78802 (LIDAR)

Drop:  15.3m (Trimble GeoXH 6000 summit and LIDAR col)

 

Myrddyn Phillips and Aled Williams (March 2026)