Myrddyn Phillips - Mapping Mountains: Mapping Mountains – Significant Height Revisions

Thursday 19 October 2017

Mapping Mountains – Significant Height Revisions – 500m Twmpau

500m Twmpau – Significant Height Revisions

The 500m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 500m and below 600m in height that have a minimum drop of 30m. Accompanying the main P30 list is a sub list entitled the 500m Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 500m and below 600m 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 - 500m Twmpau

Blorens (SO 269 118) - 6th significant height revision

Survey post for Blorens

There has been a Significant Height Revision 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 survey and LIDAR analysis conducted by Aled Williams and Myrddyn Phillips.

Blorens (SO 269 118)

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.

The 500m 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.

Welsh Highlands - Uchafion Cymru by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Blorens and it is adjoined to the Cefn yr Ystrad group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with the A465 road to its north-east, the B4246 road to its north-west and a minor road to its south, and has the town of Y Fenni (Abergavenny) towards the north-east.

When the list of Welsh 500m P15s that later became known as the Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with an estimated c 133m of drop, based on the 561m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 428m bwlch height, based on interpolation of 10m contouring between 420m – 430m.

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

The summit of this hill has now been surveyed with the Trimble GeoXH 6000, resulting in a 558.9m summit height positioned at SO 26974 11830, and this height comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The Trimble GeoXH 6000 gathering data at the summit of Blorens

Therefore, the new listed summit height of this hill is 558.9m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SO 26974 11830 and is 2.1m lower than its originally listed height of 561m, which was based on the summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The full details for the hill are:

Group: Cefn yr Ystrad

Name: Blorens

OS 1:50,000 map: 161

Summit Height (New Height): 558.9m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference: SO 26974 11830 (Trimble GeoXH 6000)

Bwlch Height: 426.9m (LIDAR)

Bwlch Grid Reference: SO 21949 10939 (LIDAR)

Drop: 132.0m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Aled Williams and Myrddyn Phillips (March 2023)

Mapping Mountains - Significant Height Revisions - 500m Twmpau

Twyn Crug yr Afan (SS 920 954) - 5th significant height revision

Survey post for Twyn Crug yr Afan

There has been a Significant Height Revision 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 survey conducted by Myrddyn Phillips and Aled Williams.

Twyn Crug yr Afan (SS 920 954)

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

The 500m 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. The list is co-authored by Myrddyn Phillips and Aled Williams with the introduction to the list being published on Mapping Mountains in November 2015 and an update relating to the list appearing on Mapping Mountains on the 19^th June 2019.

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

The name the hill is listed by is Twyn Crug yr Afan and it is adjoined to the Cymoedd Morgannwg group of hills, which are situated in the central part of South Wales (Region C, Sub-Region C2), and it is positioned with the A4058 road to its north-east and the A4107 road to its immediate south, and has the connecting villages of Blaengwynfi and Abergwynfi towards the west north-west.

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

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 31m of drop, based on the 558m spot height that appears on the Ordnance Survey 1:50,000 Landranger map and the 527m bwlch spot height 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 Ordnance Survey 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 are current and digitally updated such as the old Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and the mapping on the OS Maps, Magic Maps and WalkLakes websites.

The details for this hill were reassessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online. This mapping had many spot heights not on other publicly available maps and for this hill it had a 559m summit spot height adjoined to the ancient tumulus and positioned at SS 92037 95445.

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 also has many spot heights not on other publicly available Ordnance Survey maps and for this hill the 559m summit spot height is also shown.

Extract from the Magic Maps website

The summit of this hill has now been surveyed with the Trimble GeoXH 6000, resulting in 560.2m positioned at SS 92035 95448 and this height comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The Trimble GeoXH 6000 gathering data at the summit of Twyn Crug yr Afan

Therefore, this hill’s new listed summit height is 560.2m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SS 92035 95448 and is 2.2m higher than its originally listed height of 558m which appears as a spot height on contemporary Ordnance Survey 1:50,000 Landranger maps, and 1.2m higher than the 559m spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and which now appears on the interactive mapping on the Magic Maps website.

The full details for the hill are:

Group: Cymoedd Morgannwg

Name: Twyn Crug yr Afan

OS 1:50,000 map: 170

Summit Height (New Height): 560.2m (converted to OSGM15)

Summit Grid Reference: SS 92035 95448

Bwlch Height: 526.0m (converted to OSGM15)

Bwlch Grid Reference: SS 92019 95054

Drop: 34.2m

Myrddyn Phillips (August 2021)

Mapping Mountains - Significant Height Revisions - 500m Twmpau

Carreg Croes Ifor (SO 248 106) - 4th significant height revision

Summit Relocations post for Carreg Croes Ifor

There has been a Significant Height Revision 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 confirmed by LIDAR analysis and a subsequent Trimble GeoXH 6000 summit survey conducted by Aled Williams.

LIDAR image of Carreg Croes Ifor (SO 248 106)

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

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. The list is co-authored by Myrddyn Phillips and Aled Williams with the Introduction to the list being published on Mapping Mountains in November 2015 and an update relating to the list appearing on Mapping Mountains on the 19th June 2019.

The name the hill is listed by is Carreg Croes Ifor and it is adjoined to the Cymoedd Gwent group of hills, which are situated in the eastern part of South Wales (Region C, Sub-Region C2), and it is positioned with the A465 road to its north, the B4248 road to its south-west and the B4246 road to its east, and has the town of Blaenafon towards the south south-east.

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 18m of drop based on a non-interpolated c 500m summit height and a 482m bwlch height taken from the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

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

Since first compilation of the Welsh 500m P15 list there are a number of maps now available online, and the details for this hill were re-assessed when the Ordnance Survey 1:25,000 Historical map became available online, and this map gives a 1657ft (505.1m) summit spot height for this hill.

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

The imperial height on the Ordnance Survey 1:25,000 Historical map is derived from a 1657.4m (505.2m) Surface Height that appears on the Ordnance Survey series of Six-Inch maps. Therefore, the summit height for this hill was revised and listed as 505m with the caveat that since this height was derived this hill has undergone mining activity which has substantially altered the landscape.

Extract from the Ordnance Survey series of Six-Inch maps

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 for Carreg Croes Ifor

The summit height and position produced by LIDAR analysis is 509.8m at SO 24848 10663, this is a dramatic height revision when compared to some revised heights, and it 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 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.

During analysis of this hill, Aled used overlays from old maps with contemporary maps and the details produced via LIDAR. This shows that the natural 505.2m summit positioned at SO 24810 10620 is now under a substantial amount of mine spoil and this has raised the height of the summit.

The summit of this hill has now been surveyed with the Trimble GeoXH 6000 and as the natural summit is now under a spoil tip and an on-site visit confirms this artificially raised summit to be solid and stable, it is the Trimble summit height and position that is being prioritised for this hill.

Therefore, the new listed summit height of this hill is 509.7m and this was derived from a Trimble GeoXH 6000 survey, this is 4.7m higher than the 505m summit height that was previously given for this hill.

The full details for the hill are:

Group: Cymoedd Gwent

Name: Carreg Croes Ifor

OS 1:50,000 map: 161

Summit Height (New Height): 509.7m (converted to OSGM15, Trimble GeoXH 6000, artificially raised)

Summit Grid Reference: SO 24848 10663 (Trimble GeoXH 6000)

Bwlch Height: 482.1m (LIDAR)

Bwlch Grid Reference: SO 25485 10565 (LIDAR)

Drop: 27.6m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Aled Williams and Myrddyn Phillips (May 2020)

Mapping Mountains - Significant Height Revisions - 500m Twmpau

Banc y Foel (SN 725 186) - 3rd significant height revision

Survey post for Banc y Foel

Significant Name Changes post for Banc y Foel

There has been a Significant Height Revision to a hill that appears in the Yr Uchafion and 500m Twmpau lists, and which was initiated by a survey with the Trimble GeoXH 6000 which was conducted on the 6^th May 2018 in good, clear conditions with hardly a breath of breeze.

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 4^th November 2015.

500m Twmpau – Welsh hills at or above 500m and below 600m in height that have 30m minimum 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 Banc y Foel and it is adjoined to the Mynydd Du range of hills, which are positioned in the western part of South Wales (Region C, Sub-Region C1). The hill is positioned with the A 4069 road to its east, and has the village of Llangadog towards the north and Brynaman towards the south.

Banc y Foel (SN 725 186)

Prior to the survey with the Trimble GeoXH 6000 the height of the hill was estimated as c 536m based on a relatively large 530m uppermost contour ring on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

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

The summit height produced by The Trimble GeoXH 6000 survey is 531.6m (converted to OSGM15), this is a substantial height revision when compared to some revised heights, and 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 by LIDAR analysis, 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 analysis of LIDAR. 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.

Therefore, this hill’s new summit height is 531.6m (converted to OSGM15) and this was produced by surveying with the Trimble GeoXH 6000, this is 4.4m lower than its previously listed height of c 536m which was based on interpolation of the hill’s uppermost 530m ring contour on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The full details for the hill are:

Group: Mynydd Du

Name: Banc y Foel

OS 1:50,000 map: 160

Summit Grid Reference: SN 72536 18670

Summit Height (New height): 531.6m (converted to OSGM15)

Drop: 38.9m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data at the summit of Banc y Foel (SN 725 186) which resulted in this hill's significant height revision

For details on the survey that significantly revised the summit height of this hill

Myrddyn Phillips (July 2018)

Mapping Mountains - Significant Height Revisions - 500m Twmpau

Mynydd Ton (SS 949 940) - 2nd significant height revision

2nd survey post for Mynydd Ton

1st survey post for Mynydd Ton

Summit Relocations post for Mynydd Ton

There has been a Significant Height Revision initiated by a survey conducted with the Trimble GeoXH 6000 to a hill listed in the Yr Uchafion and the 500m Twmpau (thirty welsh metre prominences and upward), with the survey that resulted in this height revision being conducted on the 23^rd July 2016.

The criteria for the 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

500m Twmpau - All Welsh hills at or above 500m and below 600m in height with 30m minimum drop. The list is authored by Myrddyn Phillips.

The name of the hill is Mynydd Ton and it is situated in the Y Cymoedd range of hills in south Wales, and the survey was conducted on a warm and sunny afternoon in good, clear conditions. The Y Cymoedd are an extensive group of hills and comprise the area of the south Wales valleys.

Mynydd Ton is situated between Cwm Ogwr Fawr to its west and the Rhondda Fawr valley to its east and is positioned above the towns of Treorci (Treorchy) and Treherbert to its north and Nant-y-moel, Price Town and Ogmore Vale to its south-west, and the hill can be easily accessed from a track that leaves the A 4061 near to its high point and continues south-eastward to the north-western edge of a large conifer plantation, a path continues gaining the access point to a wide forest break which is relatively near the summit of the hill.

Prior to the survey with the Trimble GeoXH 6000 the height of the hill was listed as 539m at SS 94796 93958 with this position being beside an ancient cairn. This height was based on a 1,769.3ft surface height given on the Ordnance Survey Six-Inch map published in 1885. The 539m height was a relatively new addition to this hill’s listed height as its summit position was originally listed as ground beside its map heighted 535m triangulation pillar situated at SS 94931 94058.

The highest ground at the base of and beside the ancient cairn was surveyed with the Trimble GeoXH 6000 as being 534.2m (converted to OSGM15) high. This data set gave Estimated Accuracies of 5-15cm: 99.41% and a Standard Deviation of 0.0m with a margin of uncertainty for the height placement of the Trimble of 0.1m.

On my previous visit to this hill in February 2003 I judged ground approximately one minute's walk from the trig pillar to be slightly higher than ground at its base, and as the flush bracket adjoined to the trig pillar is given a height of 535.335m in the OS Trig Database it means that ground at its base is approximately 535m high. This figure is dependent upon the accuracy of the flush bracket height which is given as 3^rd order. Therefore the ground at the base of or near to the triangulation pillar is likely to be higher than that beside the ancient cairn; this is dependent upon the accuracy of the flush bracket height and the accuracy of the Trimble GeoXH 6000 survey.

The 534.2m (converted to OSGM15) height produced by the Trimble GeoXH 6000 is 4.8m lower than the previously listed height of 539m, which was based on the 1,769.3ft surface height on the 1885 Ordnance Survey Six-Inch map 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 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 is easily identified and consists of ground at or near to the triangulation pillar, and therefore this hill’s new summit height is based on the 535m map height adjoined to the trig pillar, as the 534.2m (converted to OSGM15) result produced by the Trimble GeoXH 6000 for ground beside the ancient cairn at SS 94791 93954 is 4.8m lower than the hill’s previously listed height of 539m which appears as a 1,769.3ft imperial height on the Ordnance Survey Six-Inch map.

The full details for the hill are:

Cardinal Hill: Werfa

Summit Height (New Height): 534.4m (converted to OSGM15)

Name: Mynydd Ton

OS 1:50,000 map: 170

Summit Grid Reference: SS 94931 94059

Drop: 91.3m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data beside the ancient cairn at the old summit position of Mynydd Ton which resulted in this hill's significant height revision

For details on the survey that significantly revised the summit height of this hill

For details on the second survey of this hill

Myrddyn Phillips (August 2016)

Mapping Mountains - Significant Height Revisions - 500m Twmpau

Twyn Walter (SN 828 175) - 1st significant height revision

Survey post for Twyn Walter

Hill Reclassifications post for Twyn Walter

There has been a Significant Height Revision initiated by a survey conducted with the Trimble GeoXH 6000 to the listing of the Yr Uchafion and the 500m Twmpau (thirty welsh metre prominences and upward), with these details being retrospective as the survey that resulted in this height revision was conducted on 10^th July 2014.

The criteria for these listings appear below:

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

500m Twmpau - All Welsh hills at or above 500m and below 600m in height with 30m minimum drop. With the 500m Sub-Twmpau being all Welsh hills at or above 500m and below 600m in height with 20m or more and below 30m of drop. The list is authored by Myrddyn Phillips.

The name of the hill is Twyn Walter and it is situated in the Mynydd Du range of hills in south Wales, and the survey was conducted in the company of Mark Trengove on a lovely summer’s day when we visited the remotest point of mainland Wales as well as the remotest hill of mainland Wales.

The hill can be accessed from Glyntawe on a good path and track that ascends above the Dan-yr-Ogof Showcaves. Heading west beyond the hill is some of the remotest land in the whole of Wales with the southern land of Y Mynydd Du comprising hills with limestone outcrops and a great wilderness of open grassland, a visit is highly recommended.

The summit height of this hill has been dramatically decreased from its current Ordnance Survey 1:25,000 Explorer map height of 509m, with the survey conducted with the Trimble GeoXH 6000 giving the height of the hill as 502.1m (converted to OSGM15). Based on the 509m summit map height this hill used to be listed as a 500m Twmpau with over 30m of drop, however its height has now dramatically decreased and its drop value also re-evaluated giving this hill c 26m of drop and therefore it has been reclassified to a 500m Sub-Twmpau.

Therefore this hill’s new summit height is 6.9m lower than its previously listed height which came from the 509m spot height on current Ordnance Survey 1:25,000 Explorer maps and the Ordnance Survey enlarged mapping hosted on the Geograph website.

Since this height revision was announced the height and position of this hill's bwlch has been determined via LIDAR analysis conducted by Aled Williams, the details for which are given below.

The full details for the hill are:

Group: Mynydd Du

Name: Twyn Walter

OS 1:50,000 map: 160

Summit Height (New Height): 502.1m (converted to OSGM15)

Summit Grid Reference: SN 82837 17506

Bwlch Height: 475.9m (LIDAR)

Bwlch Grid Reference: SN 82597 17917 (LIDAR)

Drop: 26.25m (Trimble summit and LIDAR bwlch)

Gathering data from the summit of Twyn Walter which resulted in this hill's significant height revision

For details on the survey that significantly revised the summit height of this hill

Myrddyn Phillips (August 2015)

Thursday 19 October 2017

Mapping Mountains – Significant Height Revisions – 500m Twmpau

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