Mapping Mountains – Hill Reclassifications – 100m Twmpau and 30-99m Twmpau

Fallw (SH 785 792) – 100m Sub-Twmpau reclassified to 30-99m Sub-Twmpau

There has been a reclassification from the list of 100m Twmpau to the list of 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Fallw (SH 785 792)

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

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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. 

100m Twmpau by Myrddyn Phillips

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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. 

30-99m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Fallw and this was derived from the Tithe map, and it is adjoined to the Mynydd Hiraethog group of hills, which are situated in the northern part of North Wales (Region A, Sub-Region A2), and it is positioned with the A546 road to its south-west and B5115 road to its south-east, and has the town of Deganwy towards the north-west.

When the original 100m 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 with a non-interpolated c 100m summit height.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 22m of drop, based on an estimated c 100m summit height and an estimated c 78m bwlch height, with both heights based on interpolation of 5m contouring that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

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

One of the mapping resources now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available maps and a 97m spot height is given on the summit area of this hill. 

Extract from the interactive mapping hosted on the WalkLakes website

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.

Therefore, the reclassification of this hill from 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 98.75m summit height and a 76.9m bwlch height, with these values giving this hill 21.9m of drop, and as it is below 100m in height this is sufficient for it to be classified as a 30-99m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Hiraethog 

Name:  Fallw 

OS 1:50,000 map:  115

Summit Height:  98.75m (LIDAR) 

Summit Grid Reference:  SH 78522 79260 (LIDAR) 

Bwlch Height:  76.9m (LIDAR) 

Bwlch Grid Reference:  SH 78350 79527 (LIDAR) 

Drop:  21.9m (LIDAR) 

 

Myrddyn Phillips (June 2024)