Mapping Mountains – Trimble Surveys – Moel Hebog

23.02.16  Moel y Gadair (SH 521 391, only bwlch surveyed)   

The Trimble set-up position at the bwlch of Moel y Gadair

Having surveyed the two tops of Parc y Borth (SH 558 375) my next objective was another visit to the bwlch area of Moel y Gadair.  I’d visited this bwlch in June of 2015 and misinterpreted the contours on the map and therefore the positions where I had placed the Trimble were incorrect.  Today I planned to rectify this and gather data from a further two positions and by doing so, see if Moel y Gadair has sufficient drop to remain listed as a P30 Twmpau.

I parked beside one of the two positions I wanted to gather data from, this is the entrance to a Fishery beside the busy A 497 as it heads from Porthmadog to Cricieth.  Walking north-eastward for 100 metres brought me to a letterbox which stands close to a bus shelter, having marched all around this area in June of last year and studying the map in detail as well as driving a Google Car up and down this road, it was this spot where I judged the critical bwlch to be positioned.

As I set the Trimble up a woman walked toward me with a letter in her hand, as it was duly deposited in the letterbox I asked when the next collection was, she said 11.00am, it was now 11.25am, but collections are usually late, she told me.  When I was here last summer I’d watched a postal van quickly pull up on the grass verge, empty the letterbox of its contents and just as quickly zoom away, I didn’t want this to happen whilst gathering data as the Trimble was set up on the same grass verge.  Therefore as it beeped away gathering its allotted five minutes of data I stood beside the bus shelter looking out for any late postal vans ready to flag the vehicle down and quickly gather the Trimble up before it was potentially squashed.  Thankfully the collection must have been on time as no late arrivals appeared during my time at this bwlch.

Having gathered one data set I wanted to gather another, the second being beside the entrance to the Fishery and close to where I had parked my car.  I walked back to it, tried setting the Trimble up on top of my rucksack which was positioned on top of a wall, but the light from the low sun made it difficult to see the accuracy level on the equipment’s screen, and therefore I re-positioned my car and set the Trimble up on its roof and measured a 1.40m offset.

Gathering data at the entrance to the Fishery, with the bus stop where the previous data set had been gathered on the left of the road in the background of this photograph

As the Trimble gathered data I sat in the sun enjoying life and trying to judge how long it would take me to drive toward the A5 and my next objective, which was the bwlch of a hill whose summit I’d surveyed on Christmas Eve 2013.

Postscript:  Since the survey of this hill, the LIDAR (Light Detection & Ranging) technique is now being used to produce accurate height and position for hills.  Subsequently this hill has been analysed via LIDAR.  The details produced by the Trimble GeoXH 6000 summit survey are retained; however LIDAR bwlch contouring indicates this hill is a natural P30 with LIDAR giving a height and position for a bwlch with remaining natural ground close by and indicating the road passing over the bwlch has been artificially raised.  Therefore, as the road is considered a recent man-made construct it is excluded from the height of the bwlch, with the listed height and position of the bwlch that indicated by LIDAR analysis resulting in the confirmation of this hill being classified as a P30.

Survey Result:

Moel y Gadair

Summit Height:  66.7m (c0nverted to OSGM15) (from previous Trimble GeoXH 6000 survey)

Summit Grid Reference:  SH 52165 39135 (from previous Trimble GeoXH 6000 survey)

Bwlch Height:  35.8m (LIDAR)

Bwlch Grid Reference:  SH 52013 39319 (LIDAR)

Drop:  30.9m (Trimble GeoXH 6000 summit and LIDAR bwlch) (30-99m Twmpau reclassified to 30-99m Sub-Twmpau) (Lesser Dominant deletion) (30-99m Sub-Twmpau reclassified to 30-99m Twmpau) (Lesser Dominant addition)

Dominance:  46.30% (Trimble GeoXH 6000 summit and LIDAR bwlch)

For the post detailing the summit survey of Moel y Gadair please click {here}

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