Mapping Mountains – Significant Height Revisions – The 500-Metre Tops of England and Wales – Deweys

Rhobell Ganol (SH 785 274) 

There has been a Significant Height Revision to a hill that is listed in the The 500-Metre Tops of England and Wales, 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 Rhobell Ganol (SH 785 274)

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

The 500-Metre Tops of England and Wales are affectionately known after their hill list compiler; Michael Dewey.  This list mixes metric and imperial height in its criteria to bookend up to the 2000ft height band and takes in all hills in England, Isle of Man and Wales that are 500m and above and below 2000ft (609.6m) in height that have 30m minimum drop. 

Mountain tables by Michael Dewey

Michael Dewey died on the 5^th November 2022.  His wife; Gillian wished for this list to be carried on and made provision for it to be co-authored by Myrddyn Phillips.  Michael will always be the compiler and originator, it’s just that he’s now got a co-author to safeguard and carry this list on.

The name the hill is listed by is Rhobell Ganol and it is adjoined to the Arenig Fawr group of hills, which are situated in the central part of North Wales, and it is positioned with a minor road to its north, the A470 road to its west and the A494 road to its east, and has the small community of Rhyd-y-main towards the south south-east.

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

When the original list of Deweys appeared in the Mountain tables book published by Constable in 1995, this hill was listed with a non-interpolated summit height of 520+m, based on the uppermost 520m ring contour 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

Since the original publication of this list 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 the Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  This mapping has a 522m summit spot height applied to the southerly top that matches the summit position given the hill by LIDAR analysis. 

Extract from the interactive mapping hosted on the Magic Maps website

Another of the mapping resources now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available maps and gives a 522m spot height to a northerly top positioned at SH 78603 27533. 

Extract from the interactive mapping hosted on the WalkLakes 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 summit image of Rhobell Ganol (SH 785 274)

LIDAR analysis gives the summit of this hill as 524.8m positioned at SH 78578 27406, 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 other GNSS equipment 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 524.8m and this was derived from LIDAR analysis, this is 4.8m higher than the originally listed summit height of 520+m, which was based on the non-interpolated uppermost 520m ring contour that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and 2.8m higher than the spot heights that appear on the Magic Maps interactive map and the interactive mapping hosted on the WalkLakes website. 

 

The full details for the hill are:

Group:  Arenig Fawr

Name:  Rhobell Ganol

OS 1:50,000 map:  124

Summit Height (New Height):  524.8m (LIDAR)

Summit Grid Reference:  SH 78578 27406 (LIDAR)  

Bwlch Height:  485.7m (LIDAR)

Bwlch Grid Reference:  SH 78549 26977 (LIDAR)

Drop:  39.1m (LIDAR)

 

Myrddyn Phillips (July 2026)

  

Mapping Mountains – Significant Height Revisions – 500m Twmpau and The Welsh Highlands – Uchafion Cymru

 

Mynydd Nodol (SH 865 393) 

There has been a Significant Height Revision that is retrospective to a hill that is listed in the 500m Twmpau and The Welsh Highlands – Uchafion Cymru, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 summit survey conducted by Myrddyn Phillips and LIDAR bwlch analysis initially conducted by Aled Williams and subsequently by Myrddyn Phillips. 

Mynydd Nodol (SH 865 393)

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

500m Twmpau – Welsh hills at or above 500m and below 600m in height that have 30m minimum drop, with an accompanying sub category entitled the 500m Sub-Twmpau consisting of all Welsh hills at or above 500m and below 600m in height that have 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 published on Mapping Mountains in Google Doc format.

500m Twmpau by Myrddyn Phillips

The 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.

The Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

The name the hill is listed by is Mynydd Nodol 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 Llyn Celyn and the A4212 road to its north and a minor road to its south, and has the town of Y Bala towards the south-east.

When the listings of the 500m Twmpau and The Welsh Highlands – Uchafion Cymru were first compiled, this hill was listed with a summit height of 539m, 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 the survey with the Trimble GeoXH 6000 and 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. 

The Trimble GeoXH 6000 gathering data at the summit of Mynydd Nodol

The survey with the Trimble GeoXH 6000 gives the summit of this hill as 540.0m positioned at SH 86514 39339, 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 other GNSS equipment 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 540.0m and this was derived from a Trimble GeoXH 6000 survey, this is 1.0m higher than the originally listed summit height of 539m, which was based on the spot height that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and importantly this height signifies that an uppermost 540m ring contour is missing from this map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Arenig Fawr

Name:  Mynydd Nodol

OS 1:50,000 map:  124, 125

Summit Height (New Height):  540.0m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 86514 39339 (Trimble GeoXH 6000)  

Bwlch Height:  370.8m (LIDAR)

Bwlch Grid Reference:  SH 85660 39014 (LIDAR)

Drop:  169.2m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips and Aled Williams (July 2026)

Mapping Mountains – Significant Height Revisions – The Welsh Highlands – Uchafion Cymru

 

Craig Nyth y Gigfran (SH 687 461) 

There has been a Significant Height Revision to a hill that is listed in The Welsh Highlands – Uchafion Cymru, 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 Craig Nyth y Gigfran (SH 687 461)

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

The 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.

The Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

The name the hill is listed by is Craig Nyth y Gigfran and it is adjoined to the Moelwyn Mawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A470 road to its east and the A496 road to its south-east, and has the town of Blaenau Ffestiniog towards the east.

A basic levelling survey conducted by Myrddyn Phillips on the 30.08.04 gave this hill 43½ft (13.3m) of drop and the hill was therefore listed with an estimated c 529m summit height and an estimated c 516m bwlch height, with both heights based on the basic levelling survey and interpolation of 10m contouring that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

Since the original compilation of this list 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 the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available maps and for this hill is has a 530m summit spot height. 

Extract from the interactive mapping hosted on the WalkLakes 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 summit image of Craig Nyth y Gigfran (SH 687 461)

LIDAR analysis gives the summit of this hill as 530.1m positioned at SH 68732 46178, 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 530.1m and this was derived from LIDAR analysis, this is 1.1m higher than the original estimated c 529m summit height and 10.1m higher than the uppermost 520m contour that appears on the contemporary Ordnance Survey 1:25,000 Explorer map and importantly this height signifies that an uppermost 530m ring contour is missing from this map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Moelwyn Mawr

Name:  Craig Nyth y Gigfran

OS 1:50,000 map:  115

Summit Height (New Height):  530.1m (LIDAR)

Summit Grid Reference:  SH 68732 46178 (LIDAR)  

Bwlch Height:  516.9m (LIDAR)

Bwlch Grid Reference:  SH 68649 46174 (LIDAR)

Drop:  13.2m (LIDAR)

 

Myrddyn Phillips and Aled Williams (June 2026)

Mapping Mountains – Significant Height Revisions – Y Trichant – The 300m Hills of Wales and Y Trechol – The Dominant Hills of Wales

 

Cefn y Coed (SO 211 934) 

There has been a Significant Height Revision to a hill that is listed in the Y Trichant – The 300m Hills of Wales and the Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, the dominance and status of the hill derived from a Leica RX1250 survey conducted by Alan Dawson and a subsequent Trimble GeoXH 6000 summit survey and LIDAR bwlch analysis conducted by Myrddyn Phillips. 

Cefn y Coed (SO 211 934)

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

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant, with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017, and the Introduction to the Mapping Mountains publication of the list appearing on the 1^st January 2022. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

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 in its 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 Cefn y Coed and this was derived from the Tithe map, the Ordnance Survey One-Inch ‘Old Series’ map and local enquiry, and it is adjoined to the Cilfaesty group of hills, which are situated in the north-eastern part of South Wales (Region B, Sub-Region B2), and it is positioned with a minor road to its north and the A489 road to its south, and has the town of Trefaldwyn (Montgomery) to its north north-east.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was listed with a non interpolated summit height of c 355m, based on the uppermost ring contour that appears on the Ordnance Survey 1:25,000 Explorer map. 

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 173m drop, based on an estimated c 356m summit height and an estimated c 183m bwlch height, with both heights based on interpolation of 5m contouring that appears on the Ordnance Survey 1:25,000 Explorer map. 

The Leica RX1250 set-up position at the summit of Cefn y Coed

However, it was not until the survey with the Leica RX1250 and subsequent LIDAR analysis and the survey with the Trimble GeoXH 6000 that the details for this hill could be accurately re-assessed. 

The Trimble GeoXH 6000 gathering data at the summit of Cefn y Coed

The summit height produced by the Leica RX1250 survey is 353.5m positioned at SO 21163 93413 and the summit height produced by the Trimble GeoXH 6000 survey is 353.6m positioned at SO 21164 93411, 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 353.6m and this was derived from a Trimble GeoXH 6000 survey, this is 2.4m lower than the previously listed summit height of c 356m, which was based on interpolation of the uppermost ring contour that appears on the contemporary Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Cilfaesty

Name:  Cefn y Coed

OS 1:50,000 map:  137

Summit Height (New Height):  353.6m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SO 21164 93411 (Trimble GeoXH 6000)  

Bwlch Height:  184.2m (LIDAR)

Bwlch Grid Reference:  SO 20667 90868 (LIDAR)

Drop:  169.3m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Dominance:  47.89% (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (May 2026)