Mapping Mountains – Hill Reclassifications – 100m Twmpau

Garth Wood (ST 119 825) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been confirmation of a reclassification to a hill that is listed in the 100m Twmpau, with the summit height and its location confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Garth Wood

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

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 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.

The name of the hill is Garth Wood, 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 as its name implies the summit area and northern part of the hill comprises woodland, whilst its southern land forms a part of the Taffs Well Quarry, and it is encircled by a variety of roads with the A4054 and A470 to its east, the M4 to its south and minor roads to its north, east and south, and has the city of Caerdydd (Cardiff) towards its south-east. 

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 main P30 list with an accompanying note stating Included by contour configuration, and given a summit height of 179m which appears as a spot height on the Ordnance Survey 1:50,000 Landranger map.  Therefore, this hill was only listed as a P30 through an estimated bwlch height based on interpolation of contours, however due to the quarry workings the contours at the bwlch are not continuous on contemporary Ordnance Survey maps of the day. 

When this list was re-assessed and the sub-list standardised, with also inclusion of interpolated heights and drop values in the main P30 and sub-list this hill was listed with an estimated c 30m of drop based on the 179m summit spot height and an estimated bwlch height of c 149m based on interpolation of 10m contouring on the Ordnance Survey 1:50,000 Landranger map which showed a continuous 150m ring contour connecting to higher ground.  However, the same 150m ring contour remained broken on the 1:25,000 Explorer map.  

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

The drop value for this hill was re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website became available online; this mapping is entitled the Interactive Coverage Map.  Although this mapping has the same 5m contour intervals as the 1:25,000 Explorer map its scale is greatly enlarged giving the opportunity for better interpolation.  This resulted in Garth Wood being reclassified to 100m Sub-Twmpau status with an estimated c 28m of drop based on an estimated bwlch height of c 151m.  

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website

This area was re-examined when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals and for the majority of land comprising old mine and quarry workings it shows present day continuous contours, as opposed to the broken contours on the 1:25,000 Explorer map and the Vector Map Local hosted on the Geograph website.  Importantly this map shows the bwlch contours for this hill between 145m – 150m with the estimated height of the bwlch being c 149m.

Extract from the OS Maps 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. 

LIDAR summit image of Garth Wood

As LIDAR only covers the summit area for this hill its bwlch height remains estimated based on interpolation of the OS Maps 5m contouring.  Therefore, the confirmation of the reclassification of Garth Wood to 100m Twmpau status is due to LIDAR summit analysis and interpolation of OS Maps bwlch contouring, resulting in a 179.9m summit height and an estimated c 149m bwlch height, with these values giving this hill c 31m of drop which is sufficient for it to be reinstated as a 100m Twmpau.

The full details for the hill are:

Group:  Cymoedd Morgannwg

Name:  Garth Wood

OS 1:50,000 map:  171

Summit Height:  179.9m (LIDAR)

Summit Grid Reference:  ST 11990 82531 (LIDAR)

Bwlch Height:  c 149m

Bwlch Grid Reference:  ST 11456 82400 (I)

Drop:  c 31m

Myrddyn Phillips (June 2019)