Friday, 12 January 2018

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, and which was initiated by a survey with the Trimble GeoXH 6000 which took place on the 29th October 2017 in good, clear conditions.

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

Summit Height (New Height):  87.5m (converted to OSGM15)

Name:  Gallt y Gog

OS 1:50,000 map:  135

Summit Grid Reference:  SH 75169 01223

Drop:  62.7m (converted to OSGM15)

Dominance:  71.68%


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


Myrddyn Phillips (January 2018)


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