Mapping Mountains – Summit Relocations – Y Trichant – The 300m Hills of Wales

Yr Allt (SO 196 358)

There has been a Summit Relocation to a hill that is listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Yr Allt (SO 196 358)

The criteria for the list this summit relocation affects 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 13th May 2017.

The name the hill is listed by is Yr Allt and it is adjoined to the Mynyddoedd Duon group of hills which are situated in the eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with minor roads to its west, north and east, with the A4078 road further to its west and the A438 road further to its north-west, and has the village of Aberllynfi (Three Cocks) towards the north-west.

When the original 300m 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 that accompanied the main P30 list, as it was considered not to meet the criteria then used for the main P30 list. 

When 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 31m of drop, based on an estimated c 335m summit height and an estimated c 304m bwlch height.

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

The details for this hill were re-examined when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping has many spot heights not on any other publicly available Ordnance Survey map and gives a 302m spot height on the area of this hill’s bwlch.  This spot height is also shown on Ordnance Survey data that appears on the Magic Maps website.

During the re-assessment of this hill’s details the summit height was amended to 334m based on the 1096ft (334.06m) imperial height positioned at SO 19663 35913 that appears on the summit area of this hill on the series of Ordnance Survey Six-Inch maps.  These re-assessments resulted in the hill being listed with 32m of drop.

Extract from the series of Ordnance Survey 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 Yr Allt

The summit height produced by LIDAR analysis is 334.0m positioned at SO 19677 35879, with LIDAR also giving a 334.1m height positioned at SO 19663 25913 to a raised field boundary that is excluded from the height of this hill as it is considered a recent man-made construct and with the natural summit of the hill positioned in a different field compared to where the summit was originally listed.  The above detail comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to any listed hill whose summit meets the following criteria; where there are a number of potential summit positions within close proximity and the highest point is not where previously given, 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, or when the summit of the hill is in a different field compared to where previously given, or when it is positioned to a different feature such as in a conifer plantation, or when the high point of the hill is placed within a different map contour compared to its previous listed position, or when the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct.

Close up LIDAR summit image for Yr Allt

Therefore, the summit height produced by LIDAR analysis is 334.0m to ground meeting the criteria used within this list and this is positioned at SO 19677 35879, this is in a different field to the previously listed summit position which matches the LIDAR position of the highest part of the raised field boundary and is approximately 35 metres southward from it, and as the raised field boundary is considered a relatively recent man-made construct it is not taken as a part of the height of this hill.

The full details for the hill are:

Group:  Mynyddoedd Duon

Name:  Yr Allt

OS 1:50,000 map:  161

Summit Height:  334.0m (LIDAR, natural summit)

Summit Grid Reference (New Position):  SO 19677 35879 (LIDAR, natural summit)

Bwlch Height:  301.7m (LIDAR)

Bwlch Grid Reference:  SO 20120 35549 (LIDAR)

Drop:  32.3m (LIDAR)

Myrddyn Phillips (January 2020)