Mapping Mountains – Summit Relocations – 100m Twmpau

Dinas Dinorwig (SH 549 652)

There has been a Summit Relocation to a hill that is listed in the 100m 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.

Dinas Dinorwig (SH 549 652)

The criteria for the list that this summit relocation 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.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Dinas Dinorwig and it is adjoined to the Glyder Fawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is encircled by an old track to its north and minor roads to its west, south and east, with the B4366 road to its north-west and the A4244 road to its north-east, and has the small communities of Bethel towards the west, Rhiwlas towards the east and Llanrug towards the south-west.

Prior to LIDAR analysis this hill was listed with 38m of drop, based on its 170m summit spot height positioned at SH 54966 65306 that appears on the Ordnance Survey 1:25,000 Explorer map (with the 1:50,000 Landranger map giving a 169m summit spot height) and a 132m bwlch height based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

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

The upper part of this hill comprises an ancient Iron Age Hillfort that has a series of steep embankments and ditches which are in the main overgrown with brambles, bracken and small trees, whilst the inner central part of the upper hill used to be grazed and is now a paddock with an extremely friendly horse when I last visited.  It is this inner central part of the summit area where the 170m spot height is positioned.

LIDAR image of Dinas Dinorwig (SH 549 652)

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 Dinas Dinorwig (SH 549 652)

The summit height produced by LIDAR analysis is 169.8m and is positioned at SH 54945 65256, this position is to the highest part of the ancient embankments, whereas the height produced by the Trimble GeoXH 6000 to the high point of the inner central part of the hill is 169.2m and is positioned at SH 54964 65301.  As the location of the listed summit of this hill is to a different feature compared to its previously listed summit position this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour either on Ordnance Survey maps or interactive mapping, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, 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.

The Trimble set-up position with the very friendly horse at the old listed summit position of Dinas Dinorwig

Therefore, the height produced by LIDAR analysis to the summit of this hill is 169.8m and this is positioned at SH 54945 65256, this position is approximately 50 metres southward from where the 170m spot height appears on the contemporary Ordnance Survey 1:25,000 Explorer map and which was surveyed as being 169.2m in height, and importantly the new summit position is to a different feature.

The summit of Dinas Dinorwig

The full details for the hill are:

Group:  Glyder Fawr

Name:  Dinas Dinorwig

OS 1:50,000 map:  114, 115

Summit Height:  169.8m (LIDAR)

Summit Grid Reference (New Position):  SH 54945 65256 (LIDAR)

Bwlch Height:  131.8m (LIDAR)

Bwlch Grid Reference:  SH 54642 64647 (LIDAR) 

Drop:  38.1m (LIDAR)

Myrddyn Phillips (March 2019)