Thursday, 13 March 2014

Mapping Mountains – Trimble Surveys – Fforest Glud

13.03.14  Cefn Perfa (SO 173 579)

This is the second Trimble survey of Cefn Perfa, to read the post on the first survey please click {here}

Cefn Perfa (SO 173 579)
Two months ago I came away from Cefn Perfa wondering why weighted averages seemed to give different results when compared to manually worked out averages, and why, even after processing a data set 14 different times for both summit and bwlch comprising a variety of base station combinations the result still came out within the margin of uncertainty associated with the technical limitations of the equipment being used – OH BUGGA!!  Only one solution to this kind of problem, and that was to try and persuade John and Graham that they really wanted to line survey a prospective new Pedwar whose summit was in a lovely close cropped grassy field.  Surprisingly not much persuasion was needed as first Graham suggested that we could line survey the hill from bwlch to summit and then John also said that he wouldn’t mind doing it.  This was excellent news and meant that they were both kept on my Christmas card list!

However, Graham became very sneaky and decided that he was going on holiday, so that just left John and I to venture down to the outskirts of Llanfihangel Nant Melan for only our second line survey in the last year or so.  Prior to this we had contacted the land owner of the hill who kindly gave permission for us to visit and survey it.

By 10.20am we were knocking on the front door of Ivor and Angela Davies’ house to introduce ourselves, both were very friendly and as an introduction to what we do, we had brought one or two newspaper articles about our exploits with us, and as we left to visit the hill Angela settled down with reports of daring do on Knight’s Peak and Thack Moor as her late morning’s entertainment.

We parked about 100 metres from the bwlch on the grass verge of the dead end lane at SO173 577 quite close to house named The Lawnt.  As we investigated the area of the bwlch a vehicle pulled up and the driver looked out with a wondering expression on his face as all manner of bits and bobs of equipment was scattered across the ground straight next to the lane.  This person was Elwyn Williams the owner of the land that the bwlch is situated on.  Angela and Ivor thought that the land we wanted to visit to the immediate south of the lane was on their neighbour’s land, when asked they said they were sure he wouldn’t mind us visiting and surveying it.  Elwyn was very friendly and more than happy with what we were doing and within a few minutes his friend was telling us all about land subsidy and the classification of any land over 400m being that of moor.

Elwyn Williams - the owner of the land where the bwlch for Cefn Perfa is situated
As Elwyn and his friend drove off, John and I quickly took a number of readings in the area of the bwlch and pinpointed the critical bwlch to within an estimated 0.02m.  John then set the Leica GS15 up to record data, posed for a photo or two and did a bit to camera for the YouTube survey channel.  We then prepared for the line survey.

John beside the Leica GS15 at the critical bwlch of Cefn Perfa
As John left Mold and we then left Welshpool the whole country seemed enveloped in fog, this only cleared as we gained height on our way to Crossgates (Y Groes) at SO 088 649.  By the time we set off up the hill with John behind the optical level and noting each reading and me positioning the 5m extendable staff, the weather was blissful with warm sunshine and blue sky making the whole thing very enjoyable.

John at the optical level and me with the extendable staff
Getting the bubble in the centre is all important
The upward line survey took about 1 hr 20 minutes; we then walked back down the hill to the bwlch, leaving the Trimble on top gathering 18 minutes of data.  As the Leica GS15 was dismantled it had gathered about 90 minutes of bwlch data.  We then carried it and its quick-set tripod back up the hill, turned off the Trimble and assembled the Leica GS15 over the summit.  Once this was done we carried on with the line survey, this time down the hill.

The Trimble GeoXH 6000 on the summit of Cefn Perfa
John beside the Leica GS15 at the summit of Cefn Perfa
Once back at the bwlch the Trimble was set up over the metal surveying bolt John had positioned in the ground to indicate the survey point.  John then did the sums; they came to 30.06m of drop between bwlch and summit.  A new Pedwar confirmed by 6cm and confirmation of current Tump status.  This anomaly was due to Clem and subsequently Mark Jackson both estimating this hill's drop as 30m and a basic levelling survey I conducted on the hill in September 2003 which gave a drop value of 97ft / 29.6m.  Therefore the hill appeared as a P30 Tump but only as a Sub-Pedwar.

The Trimble GeoXH 6000 at the critical bwlch of Cefn Perfa with John doing the sums (and trying to count the mole hills) in the background
By now the Leica GS15 had been gathering data for almost 90 minutes, we packed everything up at the bwlch once the Trimble had gathered 25 minutes of data, stored it all in the car and walked back up the hill (for the third time!) to the summit to switch the GS15 off.

Before leaving we re-visited Angela Davies to thank her and Ivor again and pick up the copies of the newspapers we had left earlier in the day.  She said she had enjoyed reading the articles, we told her that we couldn’t promise that the confirmation of Cefn Perfa as a P30 would make the headlines!  Another excellent day and good to be out line surveying again.

Survey Result:

Cefn Perfa

Summit Height:  411.3m (converted to OSGM15, Trimble)  411.2m (converted to OSGM15, Leica GS15)

Summit Grid Reference:  SO 17389 57973

Bwlch Height:  381.2m (converted to OSGM15, Trimble)  381.1m (converted to OSGM15, Leica GS15)

Bwlch Grid Reference:  SO 17288 57782

Drop:  30.1m (Trimble)  30.1m (Leica GS15)  30.06m (Line Survey) (400m Sub-Pedwar reclassified to Pedwar)

Dominance:  7.31%

The line survey we conducted gave this hill a drop of 30.06m.  The Leica GS15 data and the two Trimble data sets are in good accordance with one another.  We have another reclassification to Pedwar status – YYIIIPPPPEEEEE!!!

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

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