Thursday, 23 January 2014

Mapping Mountains – Trimble Surveys – Rhos

23.01.14  Cefn yr Ogof  (SH 916 773)

The upper slopes of Cefn yr Ogof (SH 916 773)
For a number of years Cefn yr Ogof languished as a Sub-HuMP with a listed drop of 97m, but with 5m contour intervals on the Ordnance Survey enlarged mapping on the Geograph website with bwlch contouring between c 100m – c 105m and with a map height of 204m the hill was a likely candidate for promotion to full HuMP status.  This was proposed to the hill bagging community in May 2012 and accepted as a fully-fledged HuMP in July 2012.  Cefn yr Ogof is currently listed as a HuMP with 101m of drop.

The hill is positioned in the north-west of the Mynydd Hiraethog range and overlooks the busy A55 and the north Wales coast.  It is part of limestone country with many outcrops on the hill’s upper slopes as well as on adjacent hills.

Today’s outing was the first with G&J Surveys that included the Trimble.  We met at John’s house in Mold and headed west toward the area of the bwlch which is near Abergele.

The bwlch is in a field, we sought access permission at a number of houses, this failed disastrously as the residents were either hiding from us or out, as no one answered.  However, we did speak with a farmer, who we flagged down as he steamed his way up the narrow country lane in his tractor.  It wasn’t his field but he didn’t express any objection to us wandering around with a multitude of flags and putting surveying equipment up.

An hour or so later and we had pinpointed the position of the critical bwlch.  This was done by setting out a grid of flags and taking readings to each one with a level and staff.  Once completed the readings give the relative height of the land as it ascends on the valley to valley traverse and descends on the hill to hill traverse.  It is an easy and effective way to pinpoint the position of a critical bwlch. 

The Leica GS15 set up position at the bwlch of Cefn yr Ogof.
The Leica GS15 had been set up before doing this, once it had gathered over an hour’s worth of data we packed it away and took a reading from its set up position to the actual position of the critical bwlch.  The Trimble was then positioned on the latter and gathered 16 minutes of data.

Boys and their toys.  The critical bwlch of Cefn yr Ogof.
We then drove to Rhyd- y-foel and headed up the hill.  As John and Graham are both quicker than me I normally try and get a head start and they normally catch me up and overtake me.  I opted for a barbed wired fence, brambly scrub land approach and they opted for a route through a hedge.  Once the obstacles were overcome we converged from different directions about two minutes below the summit.

The summit is a good vantage point and brought back memories of my previous visit when Des Taylor had celebrated his completion of the Welsh and English HuMps. 

By the time the three of us arrived at the summit the wind was blowing and it was bitingly cold.  We set about taking readings to the flush bracket on the trig pillar, to the trig’s base and its top.  The GS 15 was then set up over the highest embedded rock we could find amongst the summit cairn for an hour’s worth of data, whilst the Trimble was set up on the top of the trig for 30 minutes to compare its data set against the relative height we had just attained with the level and staff to the top of the trig and the highest rock.

The Leica GS15 set up over the high point of Cefn yr Ogof
By now the flashes of sunlight had disappeared and greyness had crept from the near sea and descended upon the land.   Away to the north-west the higher peaks of Eryri were swamped in deep cloud.  We all thought it was going to rain and took shelter in the near forest, more out of the wind than the impending shower.  The latter never materialised but the wind was unabating.

After the Leica GS15 was packed away the Trimble was aligned with the high point of the highest embedded rock, secured in place with small rocks and gathered ten minutes of data.  We then headed down out of the wind.  Another excellent day out on the hill and great to be out with G&J again.

The Trimble GeoXH 6000 wedged in place on the high point of Cefn yr Ogof

Survey Result:

Cefn yr Ogof

Summit Height:  204.5m (Trimble GeoXH 6000, converted to OSGM15)  204.5m (Leica GS15, converted to OSGM15)

Summit Grid Reference:  SH 91684 77301

Bwlch Height:  102.9m (Trimble GeoXH 6000, converted to OSGM15)  102.9m (Leica GS15, converted to OSGM15)

Bwlch Grid Reference:  SH 93547 75706

Drop:  101.6m (Trimble GeoXH 6000)  101.6m (Leica GS15)  (Hump status confirmed)

Dominance:  49.69% (Lesser Dominant status confirmed)

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

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