Mapping Mountains – Hill Reclassifications – 500m Twmpau

Trawsallt (SN 778 704) – 500m Sub-Twmpau addition

There has been an addition to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by detail on contemporary maps produced from Ordnance Survey data

The criteria for the list that this addition applies to are:

500m Twmpau - Welsh hills at or above 500m and below 600m in height with 30m minimum drop, with an accompanying sub list entitled the 500m Sub-Twmpau with the criteria for 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, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

The name the hill is listed by is Trawsallt, and it is adjoined to the Elenydd group of hills, which are situated in the northern part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with the B4343 road to its west, and has the village of Ysbyty Ystwyth towards the west north-west.

When the list that later became known as the 500m Twmpau was first compiled this hill was not included as a basic levelling survey conducted by Myrddyn Phillips on the 8^th June 2005 resulted in 63ft / 19.3m of drop.  With the hill’s details listed as having a 572m summit height that appears as a spot height on the Ordnance Survey 1:50,000 Landranger map and with 19m of drop.

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

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

However, although notes made at the time did not give detail from where the survey took place, it is likely that it was conducted from the bwlch to an intermediary 564m point positioned at SN 78299 70191, with 8m then added to the survey result to compensate between the 564m and 572m spot heights. 

The accuracy of this survey is dependent upon the survey method and two spot heights ascertained from photogrammetry; therefore it had a relatively large margin of uncertainty applicable to it.

The details for this hill were re-assessed 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 had many spot heights not on other publicly available Ordnance Survey maps and gave a 551m spot height on the bwlch area of this hill and positioned at SN 78327 70061.

The details for this hill were then 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 checked the positioning of the 551m bwlch spot height and resulted in the eventual addition of this hill.

Extract from the OS Maps website

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to detail derived from contemporary maps produced from Ordnance Survey data, resulting in a 572m summit height and a 551m bwlch height, with these values giving this hill 21m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group:  Elenydd

Name:  Trawsallt

OS 1:50,000 map:  135, 147

Summit Height:  572m (spot height)

Summit Grid Reference:  SN 77855 70410 (hand-held GPS via DoBIH)

Bwlch Height:  551m (spot height)

Bwlch Grid Reference:  SN 78327 70061 (spot height)

Drop:  21m (spot height summit and bwlch)

Myrddyn Phillips (August 2020)