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

Gallt y Gog (SH 751 012) 

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, due to a survey with the Trimble GeoXH 6000 which took place on the 29^th October 2017 and conducted by Myrdyn Phillips.

The criteria for the two lists this height revision affects are:

30-99m Twmpau – Welsh hills at and above 30m and below 100m in height that have 30m minimum drop.  With the word Twmpau being an acronym standing for ‘thirty welsh metre prominences and upward’.

Y Trechol – The Dominant Hills of Wales - Welsh P30s whose prominence equal or exceed half that of their absolute height.

The name of the hill is Gallt y Gog and it is placed in the Pumlumon range of hills which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and the hill is positioned overlooking the A 489 road to its south and the Afon Dyfi to its north, with the town of Machynlleth at the base of the hill to the south. 

As the summit of the hill is not on designated open access land permission to visit should be sought, for those wishing to do so the nearest open access land is to the south-west of the hill, however there are paths in the conifer plantation that covers this hill’s summit and these indicate that people have visited this hill for many years.

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with c 66m of drop and 72.53% dominance based on an estimated summit height of c 91m which was based on a 90m uppermost contour, and an estimated bwlch height of c 25m, with the summit height produced by the Trimble being 87.5m (converted to OSGM15), with this summit height coming 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 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 a previous estimated summit height based on interpolation of contours.  Also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  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.

However, although the 5-15cm: 43.58% Estimated Accuracy recorded by the Trimble GeoXH 6000 is reasonable considering that the survey took place within a wood; the Standard Deviation of 0.5m signifies that the accuracy of the data set is questionable.  To compound this uncertainty there is question whether the uppermost 90m contour on Ordnance Survey maps is erroneous as it is placed a significant distance from where the accepted summit position of this hill is situated, according to the map the accepted summit position on the ground is above 80m but below the 90m contour.  However, this 90m uppermost contour could in fact be a depression and this ring contour signifies a 70m height, with the uppermost contour on Ordnance Survey maps being 80m, whatever the uncertainty, the result produced by the Trimble GeoXH 6000 for the summit of this hill does come within the above specified parameters. 

Therefore, this hill’s new summit height is 87.5m (converted to OSGM15) which is 3.5m lower than its previously estimated height of c 91m and 2.5m lower that what is thought to be the uppermost ring contour on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The full details for the hill are:

Group:  Pumlumon

Name:  Gallt y Gog

OS 1:50,000 map:  135

Summit Height (New Height):  87.5m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 75169 01223 (Trimble GeoXH 6000)

Bwlch Height:  24.7m (LIDAR)

Bwlch Grid Reference:  SH 75589 00921 (LIDAR)

Drop:  62.8m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Dominance:  71.74% (Trimble GeoXH 6000 summit and LIDAR bwlch)

The Trimble GeoXH 6000 gathering data at the summit of Gallt y Gog (SH 751 012) which resulted in this hill's significant height revision

For details on the survey that significantly revised the summit height of this hill

Myrddyn Phillips (January 2018)

Mapping Mountains – Trimble Surveys – Pumlumon

29.10.17  Gallt y Gog (SH 751 012)

Gallt y Gog (SH 751 012)

Gallt y Gog is a conifered summit that looks horrendously wooded from a distance but which proved easily accessible.  It rises on the northern periphery of Machynlleth and looks down on the town which nestles beside the Afon Dyfi as it continues its downward flow to the coast.

I got lost trying to find where I wanted to start the walk from, and asked directions having driven up a road beyond a housing estate.  I was directed up a near road with pre-warning that it was rough and that it led to a number of houses, its description as ‘the high road’ sounded good, as even though this hill is not great in height the higher the better for my starting point.

I found a parking place beside two cars next to one of the houses and continued up the rough road after spotting a dead Robin on the grill of my car, it was spread like a trophy, its poor little legs dangling down from its feathered breast, with its head trapped in the grill.  I levered it off and gave it an improvised burial before continuing up the rough road.

An unfortunate consequence of driving

The road led past another house before contouring around a bungalow and near barn before what seemed to be the last house on its journey.  This rough road led up and around the north-east of the hill and continued around the northern part of the summit, which bulged up before me in planted conifers.

I left the road to walk over a field toward the forest’s perimeter fence which was conveniently collapsed above steep ground, when in the wood a narrow path led beside the perimeter fence to a small bench, to my right and amongst the conifers was the summit and the ground between me and it looked horrendously brambled, I hoped that a path led from the bench to the area of the summit, and thankfully one did.

It was only a short walk to the high point from beside the bench, over one felled tree and beside a mass of unwelcoming brambles.  The summit is close to a concrete and graffitied structure that looks decidedly at odds with where it is placed. 

The concrete structure near the summit of Gallt y Gog

The Trimble was soon set up and after a short wait I activated it to gather data, as it quietly beeped away collecting its minimum of 300 allotted datum points the sun occasionally broke through high cloud casting extended shadows through the trees, otherwise all remained quiet in the wood.

Gathering data at the summit of Gallt y Gog

Once I closed the Trimble off and packed it away I retraced my route back to the bench and out of the trees and down the road to my car.  I then drove down the rough road back through the housing estate and on to the A 489 road and east heading out of the town stopping beside a track giving access toward the town’s golf course where the critical bwlch of Gallt y Gog is placed.

I’d assessed this spot using OS Maps and a Google car and had enough space to reverse my car on to the pavement and yet for the Trimble when placed on the roof of my car to still be aligned with where I deemed the critical bwlch of this hill to be positioned.

Gathering data at the critical bwlch of Gallt y Gog

I quickly measured a 1.45m offset to the road below and activated the Trimble when the 0.1m accuracy level had been attained and stood away from my car as a multitude of vehicles past it heading this way and that.  I’ve learnt my lesson with roadside surveying and if it’s safe to do so it is always best to position the Trimble on top of the car roof.

Once five minutes of data were stored I closed the equipment off, packed it away and headed off to survey the critical bwlch of Coed Pant y Glo before continuing my journey home.    

   

Survey Result:

Gallt y Gog

Summit Height:  87.5m (converted to OSGM15, Trimble GeoXH 6000) (significant height revision)

 

Summit Grid Reference:  SH 75169 01223 (Trimble GeoXH 6000)

Bwlch Height:  24.7m (LIDAR)

Bwlch Grid Reference:  SH 75589 00921 (LIDAR)

Drop:  62.8 (Trimble GeoXH 6000 summit and LIDAR bwlch)

Dominance:  71.74% (Trimble GeoXH 6000 summit and LIDAR bwlch)

For further details please consult the Trimble Survey Spreadsheet