There has been a Significant Height Revision to a hill that is listed in the 100m 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 derived from a Trimble GeoXH 6000 summit survey and LIDAR bwlch analysis conducted by Myrddyn Phillips.
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LIDAR image of Bryn Maelgwyn (SH 795 805)
The criteria for the two listings that this height revision
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
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. The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format.
Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips
The name the hill is now listed by is Bryn Maelgwyn 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 B5115 road to its north, the A470 road to its west and minor roads to its south-east and north-east, and has the town of Llandudno 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 listed
with a non-interpolated summit height of c 100m, based on the uppermost contour that appears on the Ordnance Survey 1:25,000 Explorer map.
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| Extract from the Ordnance Survey 1:25,000 Explorer map |
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 43m of drop, based on an estimated c 106m summit height and an estimated c 63m 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.
The summit height produced by the Trimble GeoXH 6000 survey is 102.1m positioned at SH 79580 80540, 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.
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| The Trimble GeoXH 6000 gathering data from the summit of Bryn Maelgwyn |
The full details for the hill are:
Dominance: 37.76% (Trimble GeoXH 6000 summit and LIDAR bwlch)
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