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

Mynydd Bach (SH 389 767)

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 confirmed by LIDAR analysis, and a subsequent survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, with the latter taking place on the 28^th October 2018.

LIDAR summit image of Mynydd Bach

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.

The name of the hill is Mynydd Bach and this was derived from local enquiry and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it has the A5 and A55 roads and the village of Gwalchmai towards its south.

When the original 30—99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was listed with a summit height of c 95m, when this list was standardised with interpolated heights and drop values also included the hill was listed with an estimated c 97m summit height based on its uppermost 95m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

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

The details for this hill were next 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 map has a 99m spot height in the vicinity of this hill’s summit, however this spot height is placed on a covered reservoir, and as this is considered a recent man-made construct it is not taken as a part of the hill’s overall height.

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

The summit height produced by LIDAR is in good accordance with that produced by the Trimble GeoXH 6000 survey, and even though the spot height that appears on the Ordnance Survey Vector Map Local hosted on the Geograph website is placed on the top of a covered reservoir, this height 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.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new listed summit height is 96.5m and this was derived from the Trimble GeoXH 6000 survey, this is 2.5m lower than the 99m spot height that appears on the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Ynys Môn 

Name:  Mynydd Bach

OS 1:50,000 map:  114

Summit Height (New height):  96.5m (converted to OSGM15)

Summit Grid Reference:  SH 38964 76773

Bwlch Height:  c 64m

Bwlch Grid Reference:  SH 40259 77918 (I)

 

Drop:  c 32m

Dominance:  33.66%

For details on the summit survey of this hill

Myrddyn Phillips (May 2019)