600m
Twmpau – Significant Height Revisions
The 600m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 600m
and below 700m in height that have a minimum drop of 30m. Accompanying the main P30 list is a sub list
entitled the 600m Sub-Twmpau with the
qualification to this sub category being all Welsh hills at or above 600m and
below 700m 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 - 600m Twmpau
Pt. 631.7m (SH 733 622) - 7th significant height revision
There has been a Significant Height Revision to a hill that is listed in the 600m 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 LIDAR analysis conducted by Aled Williams.
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| Pt. 631.7m (SH 733 622) |
The criteria for the two listings that this height revision applies to are:
600m Twmpau - Welsh hills at or above 600m and below 700m in height with 30m minimum drop, with an accompanying sub list entitled the 600m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 600m and below 700m 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.
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| 600m Twmpau by Myrddyn Phillips |
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.
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| Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips |
The hill is being listed by the point (Pt. 631.7m) notation 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 the A5 road to its south-west and the B5106 and A470 roads to its east, and has the village of Capel Curig towards the south south-west.
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| Extract from the Ordnance Survey 1:25,000 Explorer map |
When the original list that later became known as the Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with an estimated c 36m of drop based on the 634m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 598m bwlch height based on interpolation of 10m contouring between 590m – 600m. The drop value for this hill was later amended to 36m, based on the 598m 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.
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| LIDAR image of Pt. 631.7m (SH 733 622) |
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.
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| LIDAR summit image of Pt. 631.7m (SH 733 622) |
The summit height produced by LIDAR analysis is 631.7m and is positioned at SH 73366 62247 and SH 73365 62246, 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.
Therefore, the new listed summit height of this hill is 631.7m and this was derived from LIDAR analysis, this is 2.3m lower than the previously listed summit height of 634m, 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: Carnedd Llywelyn
Name: Pt. 631.7m
OS 1:50,000 map: 115
Summit Height (New Height): 631.7m (LIDAR)
Summit Grid Reference: SH 73366 62247 & SH 73365 62246 (LIDAR)
Bwlch Height: 595.9m (LIDAR)
Bwlch Grid Reference: SH 73256 62104 (LIDAR)
Drop: 35.8m (LIDAR)
Myrddyn Phillips (September 2024)
Mapping Mountains - Significant Height Revisions - 600m Twmpau
Cefn Dylif (SJ 089 369) - 6th significant height revision
There has been a Significant Height Revision to a hill that is listed in the 600m Twmpau, The Welsh Highlands – Uchafion Cymru and Y Pellennig – The Remotest Hills of Wales, 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 and Aled Williams with assistance from Mark Trengove.
The criteria for the three listings that this height revision applies to are:
600m Twmpau - Welsh hills at or above 600m and below 700m in height with 30m minimum drop, with an accompanying sub list entitled the 600m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 600m and below 700m 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.
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| The 600m Twmpau by Myrddyn Phillips |
The Welsh Highlands – Uchafion Cymru – This is the revised draft title for the Welsh 500m P15s list that takes in all hills in Wales at or above 500m in height with 15m minimum drop. Accompanying the main list are three sub lists, these are; 500m Sub-Uchaf, 490m Sub-Uchaf and the Double Sub category. The list is co-authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and an update relating to the list appearing on Mapping Mountains on the 19th June 2019.
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| The Welsh Highlands - Uchafion Cymru by Myrddyn Phillipa and Aled Williams |
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.
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| Y Pellennig - The Remotest Hills of Wales by Myrddyn Phillips and Aled Williams |
The name the hill is listed by is Cefn Dylif and it is adjoined to the Y Berwyn group of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with the A5 road to its north, the B4401 road to its west and the B4391 road to its south-west, and has the village of Llandrillo towards the west.
When the original list of Welsh 500m P15s that later became known as Yr Uchafion and latterly as The Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with an estimated c 36m of drop based on the 621m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 585m bwlch height based on interpolation of 10m contouring between 580m – 590m. The drop value for this hill was later amended to an estimated c 38m, based on an estimated c 583m bwlch height based on interpolation of 5m contouring between 580m – 585m that appeared on the OS Maps website.
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| Extract from the Ordnance Survey 1:25,000 Explorer map |
The summit of this hill was now been surveyed with the Trimble GeoXH 6000, resulting in a height of 618.3m. 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 summit 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.
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| The Trimble GeoXH 6000 gathering data at the summit of Cefn Dylif |
Therefore, this hill’s new listed summit height is 618.3m and this was derived from a Trimble GeoXH 6000 survey, this is 2.7m lower than the previously listed height of 621m which was based on the spot height that appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.
The full details for the hill are:
Group: Y Berwyn
Name: Cefn Dylif
OS 1:50,000 map: 125
Summit Height (New Height): 618.3m (converted to OSGM15)
Summit Grid Reference: SJ 08943 36942
Bwlch Height: 581.3m (converted to OSGM15)
Bwlch Grid Reference: SJ 09068 36590
Drop: 37.0m
Myrddyn Phillips (June 2021)
Mapping Mountains - Significant Height Revisions - 600m Twmpau
Cnicht (SH 645 466) - 5th significant height revision
Survey post for Cnicht
Cnicht (SH 645 466) - 5th significant height revision
Survey post for Cnicht
There has been a Significant Height Revision to a hill that is listed in the 600m Twmpau and Yr Uchafion, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips on the 23rd March 2019.
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| Cnicht (SH 645 466) |
The criteria for the two listings that this significant height revision applies to are:
600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m 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.
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 this list appearing on Mapping Mountains on the 4th November 2015.
The name of the hill is Cnicht, and it is adjoined to the Moelwynion 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 A4085 road to its south-west, the A498 road to its north-west and the A470 road towards the east, and has the small community of Croesor towards the south-west and the town of Blaenau Ffestiniog towards the east.
Prior to the survey with the Trimble GeoXH 6000 this hill was listed with a 689m summit height that appears as a spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.
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| Extract from the Ordnance Survey 1:25,000 Explorer map |
The summit height produced by the Trimble GeoXH 6000 survey is 690.7m, 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 any other GNSS receiver, 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.
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| The Trimble GeoXH 6000 gathering data at the summit of Cnicht |
Therefore, this hill’s new summit height is 690.7m and this was produced by surveying with the Trimble GeoXH 6000, this is 1.7m higher than its previously listed height of 689m which appears as a spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, however this new height is in accordance with the 5m contouring on OS Maps.
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| Extract from the OS Maps website |
The full details for the hill are:
Group: Moelwynion
Name: Cnicht
OS 1:50,000 map: 115
Summit Height (New height): 690.7m (converted to OSGM15)
Summit Grid Reference: SH 64545 46613
Bwlch Height: 585.7m (converted to OSGM15)
Bwlch Grid Reference: SH 65589 47707
Drop: 105.0m
Myrddyn Phillips (July 2019)
Foel Lwyd (SH 720 723) - 4th significant height revision
Survey post for Foel Lwyd
There has been a Significant Height Revision to a hill that is listed in the Yr Uchafion and 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Leica GS15 summit survey conducted by John Barnard, Graham Jackson and Myrddyn Phillips and LIDAR bwlch analysis conducted by Aled Williams.
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| Foel Lwyd (SH 720 723) |
The criteria for the two listings that this significant 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 this list appearing on Mapping Mountains on the 4th November 2015.
600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m 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.
The name the hill is listed as is Foel Lwyd, 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 A 55 road to its north and the Afon Conwy and the A 470 road to its east, and has the town of Penmaenmawr towards the north.
Prior to the survey with the Leica GS15 this hill was listed with an estimated c 54m of drop, based on the 603m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map, and an estimated c 549m bwlch height based on interpolation of 10m contouring between 540m - 550m.
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| Extract from the Ordnance Survey Vector Map local hosted on the Geograph website and which is entitled the Interactive Coverage Map |
The summit height produced by the Leica GS15 survey is 600.2m, 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 any other GNSS receiver, 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.
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| The Leica GS15 set-up position at the summit of Foel Lwyd |
Therefore, this new listed summit height of this hill is 600.2m and this was derived by a Leica GS15 survey, this is 2.8m lower than its previously listed height of 603m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map and the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.
The full details for the hill are:
Group: Carneddau
Name: Foel Lwyd
OS 1:50,000 map: 115
Summit Height (New height): 600.2m (converted to OSGM15, Leica GS15)
Summit Grid Reference: SH 72036 72323 (Leica GS15)
Bwlch Height: 547.2m (LIDAR)
Bwlch Grid Reference: SH 72647 72325 (LIDAR)
Drop: 53.0m (Leica GS15 summit and LIDAR bwlch)
Myrddyn Phillips (January 2019)
Mapping Mountains - Significant Height Revisions - 600m Twmpau
Pen y Boncyn Trefeilw (SH 962 283) - 3rd significant height revision
Survey post for Pen y Boncyn Trefeilw
There has been a Significant Height Revision to a hill that is listed in the Yr Uchafion and 600m Twmpau, with the summit height and drop of the hill being confirmed by a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips on the 16th August 2018.
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| Pen y Boncyn Trefeilw (SH 962 283) |
The criteria for the two listings that this name change 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 this list appearing on Mapping Mountains on the 4th November 2015.
600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m 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.
The name of the hill is Pen y Boncyn Trefeilw, and it is adjoined to the Y Berwyn range of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned to the north of Llyn Efyrnwy (Lake Vyrnwy) and has the town of Y Bala towards the north north-west.
As the summit of the hill is not a part of designated open access land permission to visit should be sought, however the upper part of this hill is encircled by open access land and the summit of the hill is only a few metres from the boundary fence between this land and the grazing field where it is situated. Therefore common sense should prevail for those who want to visit.
Prior to the survey with the Trimble GeoXH 6000 this hill was listed with a 646m summit height, which was based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.
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| Extract from the Ordnance Survey 1:25,000 Explorer map |
The summit height produced by The Trimble GeoXH 6000 survey is 644.0m (643.953m to three decimal places) (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.
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| The Trimble GeoXH 6000 gathering data at the summit of Pen y Boncyn Trefeilw |
Therefore, this hill’s new summit height is 644.0m (converted to OSGM15) and this was produced by surveying with the Trimble GeoXH 6000, this is 2.0m lower than its previously listed height of 646m which appears as a spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps. However, this new height is in accordance with the 5m contouring on OS Maps.
The above details have now been superseded by the Leica RX1250 survey conducted by Alan Dawson, and it is this summit result that is given below.
The above details have now been superseded by the Leica RX1250 survey conducted by Alan Dawson, and it is this summit result that is given below.
The full details for the hill are:
Group: Y Berwyn
Name: Pen y Boncyn Trefeilw
OS 1:50,000 map: 125
Summit Height (New height): 644.2m (converted to OSGM15)
Summit Grid Reference: SH 96250 28328
Bwlch Height: 588.4m (converted to OSGM15)
Bwlch Grid Reference: SH 97406 27854
Drop: 55.8m (Leica RX1250 summit and Trimble GeoXH 6000 bwlch)
Myrddyn Phillips (January 2019)
Mapping Mountains - Significant Height Revisions - 600m Twmpau
Mynydd yr Hewyrch (SH 816 150) - 2nd significant height revision
Summit survey post for Mynydd yr Hewyrch
Bwlch survey post for Mynydd yr Hewyrch
There has been a Significant Height Revision to a hill that is listed in the Yr Uchafion, 600m Twmpau and Y Trechol - The Dominant Hills of Wales, and which was initiated by a survey with the Trimble GeoXH 6000 and a survey with the Leica RX1250, with the survey that resulted in this height revision being conducted on the 2oth October 2016 in good, clear and calm conditions in the company of Alan Dawson.
The criteria for the three listings that this height revision affects are:
Yr Uchafion - All Welsh hills at or above 500m in height that have 15m minimum drop. The list is co-authored by Myrddyn Phillips and Aled Williams.
600 Twmpau - These are the Welsh hills at or above 600m and below 700m in height that have a minimum drop of 30m. The word Twmpau is an acronym for thirty welsh metre prominences and upward, and the list is authored by Myrddyn Phillips.
Y Trechol - The Dominant Hills of Wales - These are the Welsh P30 hills whose prominence equal or exceed half that of their absolute height. The list is authored by Myrddyn Phillips.
The name that the hill is currently listed by is Mynydd yr Hewyrch and it is situated in the hills of the Rhwng Dyfi a Dulas range which have been referred to as the Bryniau Dyfi and which are situated in the south-western part of north Wales, with the survey of this hill conducted during the late afternoon in the company of Alan Dawson, who had suggested the walk and also surveyed the hill with his Leica RX1250, and it is the result from the Leica equipment that is being used for this hill's data.
The criteria for the three listings that this height revision affects are:
Yr Uchafion - All Welsh hills at or above 500m in height that have 15m minimum drop. The list is co-authored by Myrddyn Phillips and Aled Williams.
600 Twmpau - These are the Welsh hills at or above 600m and below 700m in height that have a minimum drop of 30m. The word Twmpau is an acronym for thirty welsh metre prominences and upward, and the list is authored by Myrddyn Phillips.
Y Trechol - The Dominant Hills of Wales - These are the Welsh P30 hills whose prominence equal or exceed half that of their absolute height. The list is authored by Myrddyn Phillips.
The name that the hill is currently listed by is Mynydd yr Hewyrch and it is situated in the hills of the Rhwng Dyfi a Dulas range which have been referred to as the Bryniau Dyfi and which are situated in the south-western part of north Wales, with the survey of this hill conducted during the late afternoon in the company of Alan Dawson, who had suggested the walk and also surveyed the hill with his Leica RX1250, and it is the result from the Leica equipment that is being used for this hill's data.
The hill is situated between the Afon Cerist and the A 470 road to its north and the conifer plantation of the Dyfi Fforest (Dovey Forest) to its south and is positioned above the small community of Dinas Mawddwy to its east, and the hill can be easily accessed from a lane leading to a path that heads for the summit via the escarpment edge above Craig Maesglase, or via the north-eastern ridge that heads straight for the summit of its adjacent top; Maen Du.
Prior to the survey with the Leica RX1250 and the Trimble GeoXH 6000 the height of the hill was listed as 676m based on the summit spot height on the Harvey 1:40,000 Britain Mountain Map. However, the summit of Mynydd yr Hewyrch only has a 670m uppermost contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps with the adjacent summit of Maen Du at SH 822 151 being given a 674m spot height, because of this the summit of Maen Du was previously listed as that for the hill, and the higher summit of Mynydd yr Hewyrch is therefore a recent addition and first noted by Tim Jones who reported that he believed this summit to be higher to John and Anne Nuttall in 1996.
Three points were surveyed for summit position using the Trimble GeoXH 6000 and the Leica RX1250, each position being close in height and taken to ground consisting of moorland peat hags rising above a shapely large pool which is positioned on the summit area.
The 678.5m (converted to OSGM15) height produced by the Leica RX1250 is 2.5m higher than the previously listed height of 676m, which was based on the Harvey British Mountain Map spot height, and 8.5m higher than the uppermost contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and therefore comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:
The term Significant Height Revision 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 map is being prioritised in favour of the 1:50,000 for detailing these revisions.
The summit of this hill consists of ground at the top of a moorland peat hag, and therefore this hill’s new summit height as surveyed by Alan Dawson with the Leica RX1250 is 678.5m (converted to OSGM15) which is 2.5m higher than its previously listed height of 676m which appears on the Harvey 1:40,000 British Mountain Map and 8.5m higher than the uppermost 670m contour ring on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.
The full details for the hill are:
Cardinal Hill: Mynydd yr Hewyrch
Summit Height (New Height): 678.5m (converted to OSGM15, Leica RX1250)
Name: Mynydd yr Hewyrch
OS 1:50,000 map: 124, 125
Summit Grid Reference: SH 81697 15003
Drop: 318.0m (converted to OSGM15, Trimble GeoXH 6000 bwlch)
Dominance: 46.87%
Dominance: 46.87%
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| The Trimble GeoXH 6000 gathering data at one of the three positions surveyed atop Mynydd yr Hewyrch which when coupled with the Leica RX1250 survey resulted in this hill's significant height revision |
Myrddyn Phillips (November 2016)
Mapping Mountains - Significant Height Revisions - 600m Twmpau
Crib y Rhiw (SH 663 248) - 1st significant height revision
Summit survey post for Crib y Rhiw
Bwlch survey post for Crib y Rhiw
There has been a Significant Height Revision to a hill that is listed in the Y Pellennig, Welsh Highlands and the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from separate Trimble GeoXH 6000 surveys conducted by Myrddyn Phillips and Aled Williams respectively.
The criteria for the listings this height revision affects appear below:
Y Pellennig –The Remotest Hills of Wales comprise all Welsh hills whose summit is 2.5km or more from the nearest paved public road and which have a minimum 15m of drop. The list is co-authored by Myrddyn Phillips and Aled Williams.
Welsh Highlands - All Welsh hills at or above 500m in height that have 15m minimum drop. The list is co-authored by Myrddyn Phillips and Aled Williams.
600m Twmpau - All Welsh hills at or above 600m and below 700m in height with 30m minimum drop. With the 600m Sub-Twmpau being all Welsh hills at or above 600m and below 700m in height with 20m or more and below 30m of drop. The word Twmpau is an acronym for thirty welsh metre prominences and upward and the list is authored by Myrddyn Phillips.
Y Pellennig –The Remotest Hills of Wales comprise all Welsh hills whose summit is 2.5km or more from the nearest paved public road and which have a minimum 15m of drop. The list is co-authored by Myrddyn Phillips and Aled Williams.
Welsh Highlands - All Welsh hills at or above 500m in height that have 15m minimum drop. The list is co-authored by Myrddyn Phillips and Aled Williams.
600m Twmpau - All Welsh hills at or above 600m and below 700m in height with 30m minimum drop. With the 600m Sub-Twmpau being all Welsh hills at or above 600m and below 700m in height with 20m or more and below 30m of drop. The word Twmpau is an acronym for thirty welsh metre prominences and upward and the list is authored by Myrddyn Phillips.
The name the hill is listed by is Crib y Rhiw and it is situated in the hills of the Rhinogydd which are positioned in the western part of North Wales. The summit survey was conducted on a warm and sunny day in the company of John Barnard and Graham Jackson, although they were not present during the survey, as John was inspecting the northerly top of this hill and Graham remained at the southerly connecting bwlch with our Leica GS15 that was gathering data to determine the drop value of Diffwys (SH 661 234). Whilst the bwlch was surveyed in January 2023 by Aled Williams with Rebecca Williams in attendance.
Crib y Rhiw is a relatively remote hill for Wales and it can be accessed on a traverse of the southern ridge of the Rhinogydd, either from Cwm Nantcol to the west or from the end of the paved road to the south which is situated above the small community of Bontddu.
The hill does not possess a summit spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps, and is given an uppermost contour of 670m on both of these maps, with the height resulting from the survey with the Trimble GeoXH 6000 being 681.3m (converted to OSGM15). Crib y Rhiw has a northerly top that has a 2,228ft (679.1m) height given it on the Ordnance Survey Six-Inch Map, and its previously listed height in the Y Pellennig, Yr Uchafion and 600m Twmpau was c 680m, which was based on the estimated height difference between the position of the Six-Inch Map height and the high point of the hill which is positioned on the southerly top.
The 681.3m (converted to OSGM15) height produced by the Trimble GeoXH 6000 is not a dramatic height revision when compared to the previously listed height of this hill of c 680m, but it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:
The term Significant Height Revision applies to any listed hill whose Ordnance Survey summit spot height has a 2m or more discrepancy when compared to the surveyed height 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 map is being prioritised for detailing these revisions.
Therefore, the new listed summit height of this hill is 681.3m (converted to OSGM15), and this is 1.3m higher than its previous c 680m estimated height and 10.3m higher than its uppermost ring contour on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.
The full details for the hill are:
Cardinal Hill: Y Llethr
Name: Crib y Rhiw
OS 1:50,000 map: 124
Summit Height (New Height): 681.3m (converted to OSGM15)
Summit Grid Reference: SH 66368 24868
Bwlch Height: 652.1m (converted to OSGM15)
Bwlch Grid Reference: SH 66080 25302
Drop: 29.2m
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| Gathering data at the summit of Crib y Rhiw which resulted in the significant height revision of this hill |
Myrddyn Phillips (September 2015)
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