Saturday, 17 June 2017

Mapping Mountains – Hill Reclassifications – 700m Twmpau


700m Twmpau – Hill Reclassifications

The 700m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 700m and below 800m in height that have a minimum drop of 30m.  Accompanying the main P30 list is a sub list entitled the 700m Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 700m and below 80om in height with 20m or more and below 30m of drop.  The Introduction to this list is linked in the above title.

The list is authored by Myrddyn Phillips and the posts that have appeared on Mapping Mountains detailing the additions, reclassifications and deletions to the main P30 list and the sub list appear below presented chronologically in receding order.







Mapping Mountains - Hill Reclassifications - 700m Twmpau

Fan y Big (SO 036 206) - 700m Twmpau reclassified to 700m Sub-Twmpau (2nd reclassification)


There has been a reclassification to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams.  The hill was listed in the 700m height band of P30 Twmpau hills and it has now been reclassified to a 700m Sub-Twmpau, with the criteria for the sub hills being 700m or more and below 800m in height with 20m or more and below 30m of drop.  Details to this height band of the Twmpau were published on the Mapping Mountains site on 15th April 2015.

The hill had been listed with 30m of drop, based on the 719m summit spot height that appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and the 689m bwlch spot height that appears on the Ordnance Survey enlarged mapping hosted on the Geograph website. 

The hill is situated in the central part of the main ridge that forms the Bannau Brycheiniog (Brecon Beacons) group of hills with its Cardinal Hill being Pen y Fan (SO 012 215) and is placed in the Region of South Wales (Region C, Sub-Region C2).  It is situated between the towns of Aberhonddu (Brecon) to its north and Merthyr Tudful to its south.

The hill can be accessed from public footpaths originating from its north and from its south; or it can be combined as a longer expedition taking in the main Bannau Brycheiniog ridge either from its west or from its east.

The name of the hill is Fan y Big and its reclassification to a 700m Sub-Twmpau is due to the analysis of LIDAR data by Aled Williams.  LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Aled’s analysis of LIDAR data gives the hill the following details:


Fan y Big

Summit Height:  716.3m

Summit Grid Reference:  SO 03651 20662

Bwlch Height:  688.1m

Bwlch Grid Reference:  SO 03773 19876

Drop:  28.2m


Therefore, the 716.3m LIDAR data produced for the summit position at SO 03651 20662 and the 688.1m LIDAR data produced for the bwlch position at SO 03773 19876 is sufficient for this hill to be reclassified from a 700m Twmpau to a 700m Sub-Twmpau with 28.2m of drop.

This now revises the total in the 700m Twmpau which will be updated accordingly.


The full details for the hill are:


Cardinal Hill:  Pen y Fan

Summit Height:  716.3m (LIDAR data)

Name:  Fan y Big

OS 1:50,000 map:  160

Summit Grid Reference:  SO 03651 20662 
  
Drop:  28.2m (LIDAR data)


Fan y Big (SO 036 206) now reclassified to a 700m Sub-Twmpau

My thanks to Aled Williams for analysing the details of this hill.

Myrddyn Phillips (March 2017)







Mapping Mountains - Hill Reclassifications - 700m Twmpau

Bera Mawr (SH 674 682) - 700m Twmpau reclassified to 700m Sub-Twmpau (1st reclassification)

Survey post for Bera Mawr


There has been a reclassification to the 700m Twmpau (thirty welsh metre prominences and upward) list due to a survey with the Trimble GeoXH 6000.   The hill is situated in the north-western part of the Carneddau in Eryri, and is positioned south of Llanfairfechan and eastward of Bethesda.

These details are retrospective as the Trimble GeoXH 6000 survey that resulted in the reclassification of this hill from 700m Twmpau to 700m Sub-Twmpau status was conducted on 10th March 2014 on a bright early spring day.

The hill is relatively remote for a Welsh hill and it can be accessed from above Abergwyngregyn to the north, or from the upper parts of Bethesda to the west or from the main Carneddau ridge to its south-east.  Most approaches enable the walker to visit other hills on the way.

The hill is named Bera Mawr and its summit is positioned at SH 674 682.  The hill was listed with c 31m of drop based on a 794m summit spot height and an estimated bwlch height of c 763m.  The area of the bwlch is comprised of relatively short grass and the set-up position of the Trimble for the bwlch survey was given a 0.3m margin of uncertainty for height.

The survey with the Trimble GeoXH 6000 resulted in a 793.6m (converted to OSGM15) summit height with Estimated Accuracies of 15-30cm: 55.22%, this figure is below the average and therefore it would be favourable for this hill to be revisited and another data set gathered for comparison.  The bwlch survey resulted in a 763.9m (converted to OSGM15) height and the normal Estimated Accuracies of 5-15cm: 100.00%.

This now brings the overall total for the hills listed in the 700m Twmpau to 33 and the total for the 700m Sub-Twmpau to 5,  please click {here} to access the list and {here} for its detailed Introduction.


The full details for the hill are:


Cardinal Hill:  Carnedd Llywelyn

Summit Height:  793.6m (converted to OSGM15)

Name:  Bera Mawr

OS 1:50,000 map:  115

Summit Grid Reference:  SH 67482 68270

Drop:  29.7m


Gathering data at the summit of Bera Mawr, which resulted in this hill being reclassified from a 700m Twmpau to a 700m Sub-Twmpau

For details on the survey that reclassified this hill from 700m Twmpau to 700m Sub-Twmpau status please click {here}

Myrddyn Phillips (March 2014)


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