Mapping Mountains – Significant Height Revisions – 100m Twmpau and Y Trechol - The Dominant Hills of Wales

Bryn Maelgwyn (SH 795 805)

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. 

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 3^rd 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.  

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.

The Trimble GeoXH 6000 gathering data from the summit of Bryn Maelgwyn

Therefore, the new listed summit height of this hill is 102.1m and this was derived from a Trimble GeoXH 6000 survey, this is 3.9m lower than the previously listed summit height of c 106m, which was based on interpolation of 5m contouring on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.

The full details for the hill are:

Group:  Mynydd Hiraethog 

Name:  Bryn Maelgwyn

OS 1:50,000 map:  115

Summit Height (New Height):  102.1m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 79580 80540 (Trimble GeoXH 6000) 

         

Bwlch Height:  63.5m (LIDAR)

Bwlch Grid Reference:  SH 79893 80558 (LIDAR)

Drop:  38.5m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

Dominance:  37.76% (Trimble GeoXH 6000 summit and LIDAR bwlch) 

Myrddyn Phillips (January 2016)

Mapping Mountains – Trimble Surveys – Mynydd Hiraethog

16.01.16  Bryn Maelgwyn (SH 795 805) and Coed Gaer (SH 799 808)

Bryn Maelgwyn (SH 795 805)

This was the third of four relatively small walks in the company of Alex, who’s role as local guide proved beneficial, especially so for the route we were about to take, as the summit of  Bryn Maelgwyn is embedded in tangled woodland with the connection between it and Coed Gear being steep and pathless.

We parked at SH 795 802 where a couple of cars can be left just off the busy A470.  The grey skies that had pervaded the first two walks of the day remained with us, spitting occasional rain drops down from their depths.  Thankfully the rain was not heavy and as the majority of the walk was in woodland we’d be sheltered if its intensity increased.

A footpath leads in to the wood from beside the busy road and I followed Alex a short distance on the path before he veered leftward in to the morass of tangled bracken and undergrowth.  No paths from here on in, seemed to exist, and I was reliant on Alex finding the high point, he’d been here a couple of times before and confidently strode out toward the first false summit and then indicated that the true summit lay further on.  From this first vantage point it was impossible to see a higher point for the hill, but as we continued through the tangle of woodland the true summit soon veered up in front of us.

Alex heading for the summit of Bryn Maelgwyn

The high point of Bryn Maelgwyn proved to be beside a moss laden and partly collapsed wall and under a wind battered tree, with other trees forming a canopy around which were the remnants of discarded bottles and clothes.  These were the after effects of either overnighting in the wood or teenage parties.  We came across three of four examples of this during our walk; all were untidy and unnecessary mess.

An unnecessary mess

Once we decided where the summit lay I set the Trimble on top of my rucksack which was placed on the partly collapsed wall and with Alex’s help measured the offset down to the high point of the hill.  It took the Trimble about 20 minutes until it was activated to gather its five minutes of allotted data, we waited, patiently, chatting a safe distance below it.

Gathering data at the summit of Bryn Maelgwyn

Once the Trimble was closed off Alex led the way down steepening slopes and eventually out of the wood toward the attractive looking Coed Gaer, this hill proved fun as a path led up its western flank which was crowned by a small rock outcrop which had a barb wired fence running its length and which was positioned over steep ground, this was our way up, and one or two moves involving hand on rock and a balancing act getting over the fence, deposited us close to the summit, which consisted of a number of small rock outcrops vying for the accolade of being the highest point.

Steep wooded slopes on Bryn Maelgwyn

The shapely profile of Coed Gaer 

Outside of the wooded summit of Bryn Maelgwyn the hills that we had so far visited had all proved to be excellent vantage points, and as the Trimble gathered another five minute data set I looked out to the south-west toward the sea and snow-capped mountains in the distance. 

The Trimble GeoXH 6000 aligned with the high point of Coed Gaer

Gathering data at the summit of Coed Gaer 

For our brief time on the summit of Coed Gaer the rain had kept at bay, but as we descended through thickening woodland to our inward path the wet stuff started to fall again and would remain with us during our last walk of the day, this was to the top of The Vardre (SH 781 794), otherwise known as Castell Deganwy.  

LIDAR image of Bryn Maelgwyn (SH 795 805)

Postscript: 

Since the summit survey of these hills full LIDAR coverage is now available.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height and positional data that is now freely available for England and Wales.  Consequently the numerical details for these hills has been analysed using this technique, resulting in the LIDAR height and position for each bwlch being used.

Survey Result:

Bryn Maelgwyn 

Summit Height:  102.1m (converted to OSGM15, Trimble GeoXH 6000) (significant height revision)

Summit Grid Reference:  SH 79580 80540 (Trimble GeoXH 6000)

Bwlch Height:  63.5m (LIDAR)

Bwlch Grid Reference:  SH 79893 80558 (LIDAR)

Drop:  38.5m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

Dominance:  37.76% (Trimble GeoXH 6000 summit and LIDAR bwlch)

Coed Gaer (significant name change)

Summit Height:  134.0m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 79924 80840 (Trimble GeoXH 6000) (summit relocation)

Bwlch Height:  60.0m (LIDAR)

Bwlch Grid Reference:  SH 81486 81889 (LIDAR)

Drop:  74.0m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

Dominance:  55.25% (Trimble GeoXH 6000 summit and LIDAR bwlch)

For further details please consult the Trimble survey spreadsheet click {here}