Friday, 26 June 2020

The 500-Metre Tops of England and Wales – the Deweys



Twr Pen-cyrn (SO 202 144) – Dewey deletion

The listing to The 500-Metre Tops of England and Wales appeared in Michael’s Mountain tables book which was published by Constable in 1995.  This list mixed metric and imperial height in its criteria to bookend up to the 2000ft height band and listed hills between 500m – 2000ft (609.6m) in height with 30m minimum drop.

Mountain tables by Michael Dewey

Since publication of this list there have been a number of status changes to it. In recent years this has involved independent surveyors and the use of LIDAR.  Both have enabled an accurate re-assessment of many of the hills included in Michael’s list.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR summit image of Twr Pen-cyrn (SO 202 144)

The details for this hill were forwarded to Michael Dewey in February 2017.  These details were produced by LIDAR analysis conducted by Aled Williams:


Dewey deletion:

Name:  Twr Pen-cyrn

OS 1:50,000:  161

Summit Height:  528.4m

Summit Grid Reference:  SO 20283 14511

Bwlch Height:  499.6m

Bwlch Grid Reference:  SO 18452 15398

Drop:  28.8m


Subsequent LIDAR analysis conducted by Myrddyn Phillips produced the following result:


Name:  Twr Pen-cyrn

OS 1:50,000:  161

Summit Height:  528.9m

Summit Grid Reference:  SO 20281 14491

Bwlch Height:  499.7m

Bwlch Grid Reference:  SO 18531 15394

Drop:  29.3m


As this hill has a large ancient cairn positioned a few metres in distance from a triangulation pillar, the discrepancy in the summit height and position given above in the two evaluations is dependent upon where natural ground is judged to be positioned, with the caveat that higher natural ground may exist under the ancient cairn.  However, this was judged not to influence this hill’s demotion from Dewey status.  The difference in bwlch position is given as 2mm by LIDAR with the slightly lower position on the hill to hill traverse presented in the latter evaluation.

LIDAR bwlch image for Twr Pen-cyrn

Michael has now been consulted regarding this hill, and this has resulted in him confirming the deletion of Twr Pen-cyrn from Dewey status.


Aled Williams and Myrddyn Phillips (June 2020)
















 



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