Mapping Mountains – Significant Height Revisions – 200m Twmpau

Coed Eryr (SH 636 508)

There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, with the summit height, its location and the drop of the hill confirmed by LIDAR analysis, and a subsequent summit survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, with the latter taking place on the 6^th October 2018.

LIDAR image of Coed Eryr

The criteria for the list that this significant height revision applies to are:

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The name of the hill is Coed Eryr, and it is adjoined to the Moelwynion range of hills, which are situated in the western part of North Wales (Region A, Sub-Region A1), and it is positioned between the lakes of Llyn Dinas to ts south-west and Llyn Gwynant to its north-east, with the A498 road to its north-west, and has the village of Beddgelert towards the south-west.

When the original Welsh 200m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 241m summit height based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger map.

Extract from the Ordnance Survey 1:50,000 Landranger map

LIDAR summit image of Coed Eryr

The summit height produced by the Trimble GeoXH 6000 survey is 243.7m (converted to OSGM15), this is not a substantial height revision when compared to some revised heights, but it does come 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 interpolated height and 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 analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.  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.

Therefore, this hill’s new listed summit height is 243.7m (converted to OSGM15) and this was produced by surveying with the Trimble GeoXH 6000, this is 2.7m higher than its previously listed height of 241m which appears as a spot height on the Ordnance Survey 1:50,000 Landranger map.

 

The Trimble GeoXH 6000 gathering data at the summit of Coed Eryr

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Moelwynion

Name:  Coed Eryr

OS 1:50,000 map:  115

Summit Height (New height):  243.7m (converted to OSGM15)

Summit Grid Reference:  SH 63612 50890

  

Bwlch Height:  181.6m (LIDAR)

Bwlch Grid Reference:  SH 64230 51021 (LIDAR)

Drop:  62.1m (Trimble summit and LIDAR bwlch)

For details on the summit survey of this hill

Myrddyn Phillips (March 2019)

Mapping Mountains – Trimble Surveys – Moelwyn Mawr

06.10.18  Coed Eryr (SH 636 508)  

I’ve visited Eryri many times over the years, mainly concentrating on its higher peaks, but also taking in a number of its lower hills, but more remain to visit and having the Trimble as an effective and accurate lightweight accessory on any hill walk only encourages me to continue visiting these hills.

A hill that until today I had not visited is Coed Eryr, which is situated between the waters of Llyn Dinas and Llyn Gwynant and rises in a conifered bulk above Nant Gwynant.  Reports on the Hill Bagging website advised an approach on an unmarked track leaving the confines of the valley from the convenience of a large lay-by adjacent to a row of houses.  However, reports also stated that the last 50 metres or so to the summit were problematic.

As I left my car and started up the track the first colours of autumn were on show as dulled browns of bracken intermingled with moss strewn trees.  The track made good steady progress uphill as all good tracks should! 

The track was mainly enclosed by conifers but the view did open to show one whitened house against a backdrop of Llyn Gwynant and the rising and steep grassed slopes of Gallt y Wenallt.  As the track crested the hill’s northerly ridge it veered south toward the land containing the summit.

Gallt y Wenallt and Llyn Gwynant

In places the track was muddy with an occasional fallen tree hanging over it, thankfully any quagmire could easily be bi-passed and the fallen trees could be walked under.  I left the track just beyond its high point and started the difficult bit, which at its beginning proved relatively easy, but fun soon arrived as the debris of rhododendron clearing had left a jungle of wooded branches beyond which, somewhere through the mass of entanglement was the summit.

Somewhere up there is the summit

Somewhere through there is the summit

Thankfully the difficulties were short lived and after a good branch bash I arrived beside a tree that had an aerial fixed to it, within a metre or so this position matched that produced by LIDAR analysis and once I had visually determined the high point I placed the Trimble at the summit, took a photograph and then positioned it on top of my rucksack for data collection.

The aerial on the tree marks the summit area

The Trimble marks the summit

I waited an inordinate amount of time for the equipment to ebb down to its 0.1m accuracy level before data should be logged, and then proceeded to gather ten minutes of data, during this wait I spent a few minutes breaking small branches off clearing a path toward the summit.

Gathering data at the summit of Coed Eryr

Once data were stored I retraced my inward route back to the track, which on the descent seemed easier when compared to the uphill bash, stopping to admire Llyn Gwynant on the way and talk with a couple who were heading up the track toward the Bryn Gwynant Youth Hostel.

It had been a good small circuit with another hill visited, next stop were two hills that I’d often thought about visiting but again, never had; Foel Gron (SH 583 621) and Pen y Bigil (SH 576 621).

Survey Result:

Coed Eryr

Summit Height:  243.7m (converted to OSGM15, Trimble GeoXH 6000) (significant height revision)

Summit Grid Reference:  SH 63612 50890 (Trimble GeoXH 6000)

Bwlch Height:  181.6m (LIDAR)

Bwlch Grid Reference:  SH 64230 51021 (LIDAR)

Drop:  62.1m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Dominance:  25.49% (Trimble GeoXH 6000 summit and LIDAR bwlch)

For further details please consult the Trimble Survey Spreadsheet