Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Waltwood Hill (ST 386 885)

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Waltwood Hill (ST 386 885)

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 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.

Y Trechol - The Dominant Hills of Wales - Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height, with the Introduction to the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format.

The name the hill is listed by is Waltwood Hill, and it is adjoined to the Gwent Is Coed group of hills which are situated in the south-eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with the M4 motorway to its north, the A48 road to its west and the A4810 road to its south, and has the city of Casnewydd (Newport) towards the west.

When the original Welsh 30-99m P30 list was published on Geoff Crowder’s v-g.me website this hill was listed with a 68m summit height based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map and which is positioned at ST 38637 88605.

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

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 Waltwood Hill

The summit height produced by LIDAR analysis is 70.2m and this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.

Therefore, this hill’s new listed summit height is 70.2m and this was derived from LIDAR analysis, this is 2.2m higher than the previous listed summit height of 68m that appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Gwent Is Coed

Name:  Waltwood Hill

OS 1:50,000 map:  171

Summit Height (New Height):  70.2m (LIDAR)

Summit Grid Reference:  ST 38646 88595 (LIDAR)

Bwlch Height:  21.7m (LIDAR)

Bwlch Grid Reference:  ST 39426 89069 and ST 39433 89070 (LIDAR)

Drop:  48.5m (LIDAR)

Dominance:  69.07% (LIDAR)

Myrddyn Phillips (August 2020)