Tuesday, 9 June 2020

Mapping Mountains – Hill Reclassifications – Y Trichant – The 300m Hills of Wales

Boncyn Nadroedd (SJ 027 540) – Sub-Trichant addition

There has been confirmation of an addition to the list of Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps produced from Ordnance Survey data.

The criteria for the list that this addition 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 13th May 2017.

The name the hill is listed by is Boncyn Nadroedd and it is adjoined to the Mynydd Hiraethog group of hills which are situated in the north-eastern part of North Wales (Region A, Sub-Region A2), and it is positioned with minor roads to its north, west and east, and has the town of Rhuthun (Ruthin) towards the east 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 not 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 this sub category. 

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 23m of drop, based on interpolated values of c 390m and c 367m for its summit and bwlch respectively, with each based on interpolation of 10m contouring on the Ordnance Survey 1:25,000 Explorer map.

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

The details for this hill were re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and used to appear on the online Vector Map Local.  This re-assessment resulted in the estimated bwlch height for this hill being amended to c 368m, with this based on interpolation of 5m contouring between 365m – 370m.

Extract from the Magic Maps website

The details for this hill were also re-assessed against Ordnance Survey data hosted on the Magic Maps website as well as the WalkLakes website, and what was thought to be a tiny uppermost 390m summit ring contour is given a 389m spot height on these two interactive maps, implying that what was taken as a small ring contour on the Ordnance Survey 1:25,000 Explorer map, is in fact a spot height missing its 389m height.

Extract from the WalkLakes website

Therefore, the confirmation of the addition of this hill to Sub-Trichant status is due to detail derived from contemporary maps produced from Ordnance Survey data, resulting in a 389m summit height and a c 368m bwlch height, with these values giving this hill c 21m of drop, which is sufficient for it to be classified as a Sub-Trichant.

The full details for the hill are:

Group:  Mynydd Hiraethog

Name:  Boncyn Nadroedd

OS 1:50,000 map:  116

Summit Height:  389m (spot height)

Summit Grid Reference:  SJ 02740 54035 (spot height)

Bwlch Height:  c 368m (interpolation)

Bwlch Grid Reference:  SJ 02565 54068 (interpolation)

Drop:  c 21m (spot height summit and interpolated bwlch)

Myrddyn Phillips (June 2020)

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