800m
Twmpau – Significant Height Revisions
The 800m Twmpau (thirty welsh metre prominences and upward) are the Welsh
hills at or above 800m and below 900m in height that have a minimum drop of
30m. Accompanying the main P30 list is a
sub list entitled the 800m Sub-Twmpau
with the qualification to this sub category being all Welsh hills at or above
800m and below 900m 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 - 800m Twmpau
Y Foel Goch (SH 677 582) - 2nd significant height revision
There has been a Significant Height Revision to a hill that is listed in the Welsh Highlands – Uchafion Cymru and 800m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis initially conducted by Aled Williams and subsequently by Myrddyn Phillips.
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| LIDAR image of Y Foel Goch (SH 677 582) |
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.
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| Welsh Highlands - Uchafion Cymru by Myrddyn Phillips and Aled Williams |
800m Twmpau - Welsh hills at or above 800m and below 900m in height that have 30m minimum drop, with an accompanying sub list entitled the 800m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 800m and below 900m 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 March 2015.
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| 800m Twmpau by Myrddyn Phillips |
The name the hill is listed by is Y Foel Goch and it is adjoined to the Glyder Fawr 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 north and the A4086 road to its south, and has the village of Capel Curig towards the east.
Prior to LIDAR analysis this hill was listed with 63m of drop, based on the 805m summit spot height and the 742m bwlch spot height that appear on the contemporary Ordnance Survey 1:25,000 Explorer map.
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| 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.
%20-%20LIDAR%20summit%201.jpg) |
| LIDAR summit image of Y Foel Goch (SH 677 582) |
The summit height produced by LIDAR analysis is 807.2m and is positioned at SH 67783 58216, 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 807.2m and this was derived from LIDAR analysis, this is 2.2m higher than the previously listed summit height of 805m, 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: Glyder Fawr
Name: Y Foel Goch
OS 1:50,000 map: 115
Summit Height (New Height): 807.2m (LIDAR)
Summit Grid Reference: SH 67783 58216 (LIDAR)
Bwlch Height: 742.5m (LIDAR)
Bwlch Grid Reference: SH 67252 58277 (LIDAR)
Drop: 64.8m (LIDAR)
Myrddyn Phillips (July 2024)
Mapping Mountains - Significant Height Revisions - 800m Twmpau
There has been a Significant Height Revision to a hill that appears in the listings of Yr Uchafion and 800m Twmpau, 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 and Aled Williams which took place on the 6th July 2019.
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| Carnedd y Filast (SH 620 627) |
The criteria for the two 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 and an update relating to the list appearing on Mapping Mountains on the 19th June 2019.
Prior to the Trimble GeoXH 6000 survey the height of the hill was listed as 821m which is based on the summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.
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 and an update relating to the list appearing on Mapping Mountains on the 19th June 2019.
800m Twmpau - Welsh hills at or above 800m and below 900m in height with 30m minimum drop, with an accompanying sub list entitled the 800m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 800m and below 900m 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 March 2015.
The name of the hill is Carnedd y Filiast and it is adjoined to the Glyderau 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 Afon Ogwen and the A5 road to its north-east and Llyn Padarn, Llyn Peris and the A4086 road towards its south-west, and has the town of Bethesda towards the north and the village of Llanberis towards the west south-west.
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| Extract from the Ordnance Survey 1:25,000 Explorer map |
The summit height produced by the Trimble GeoXH 6000 is 823.6m, 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.
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| The Trimble GeoXH 6000 gathering data at the summit of Carnedd y Filiast |
Therefore, this hill’s new summit height as surveyed with the Trimble GeoXH 6000 is 823.6m, this is positioned at SH 62040 62733 and is 2.6m higher than its previously listed height of 821m which appears as a spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.
The full details for the hill are:
Group: Glyderau
Name: Carnedd y Filiast
OS 1:50,000 map: 115
Summit Height (New Height): 823.6m (converted to OSGM15)
Summit Grid Reference: SH 62040 62733
Bwlch Height: 744.4m (converted to OSGM15)
Bwlch Grid Reference: SH 61995 61535
Drop: 79.1m
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