Thursday, 25 May 2017

Mapping Mountains – Hill Reclassifications – Y Trichant


Allt y Genlli (SN 985 950) – Sub-Trichant reclassified to Trichant

There has been a reclassification to the listing of the Y Trichant due to analysis of LIDAR data by Aled Williams.  Y Trichant is the title for the hills in the 300m height band of the Twmpau (thirty welsh metre prominences and upward) and takes in all Welsh hills at or above 300m and below 400m in height that have a minimum 30m of drop, with the introduction to the re-naming of this list appearing on Mapping Mountains on the 13th May 2017.

The hill was listed in the sub category that accompanied the original Welsh P30 lists when published on Geoff Crowder’s v-g.me website, this sub category has now been standardised and named the Sub-Trichant and comprises all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.

Prior to analysis of LIDAR data the hill was listed as a Sub-Trichant and its reclassification is dependent upon its adjacent southerly hill of Mynydd Garth Pwt, because of this the previously listed details of both hills based on map data are given below: 



Allt y Genlli (previously listed as a Sub-Trichant)

Summit Height:  394m (OS 1:25,000 Explorer map)

Summit Grid Reference:  SN 985 950

Bwlch Height:  367m (OS enlarged mapping hosted on the Geograph website)

Bwlch Grid Reference:  SN 983 944

Drop:  27m




Mynydd Garth Pwt (listed as a Trichant)

Summit Height:  397m (OS 1:50,000 Landranger and 1:25,000 Explorer maps)

Summit Grid Reference:  SN 981 941

Bwlch Height:  c 325m

Bwlch Grid Reference:  SN 973 941

Drop:  c 72m



The hill is a part of the Pumlumon range of hills with its Cardinal Hill being Bryn Amlwg (SN 921 973) and is placed in the Region of Mid and West Wales (Region B, Sub-Region B2).  The hill is situated to the south of the A 470 and is positioned between the small communities of Carno to its west north-west and Clatter to its east.

The upper part of the hill is a part of designated open access land, however it is also immersed in conifer plantation with its summit encircled by a forest track, and for those who wish to visit its high point an approach from the north using this track may be the best option.

The name of the hill is Allt y Genlli and its reclassification from Sub-Trichant to Trichant status is dependent upon its adjacent southerly hill of Mynydd Garth Pwt and 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 these two hills the following details:


Allt y Genlli

Summit Height:  398.9m

Summit Grid Reference:  SN 98508 95042

Bwlch Height:  325.7m

Bwlch Grid Reference:  SN 97387 94159

Drop:  73.2m




Mynydd Garth Pwt

Summit Height:  397.8m

Summit Grid Reference:  SN 98154 94173

Bwlch Height:  367.3m

Bwlch Grid Reference:  SN 98385 94438

Drop:  30.5m


Therefore, as the 398.9m LIDAR data produced for the summit position of Allt y Genlli at SN 98508 95042 is higher than the 397.8m LIDAR data produced for the summit position of Mynydd Garth Pwt at SN 98154 94173, the listed bwlch position of each hill is swapped as the lower bwlch is the critical bwlch for the higher of the two hills.


The full details for the two hills are:


Cardinal Hill:  Bryn Amlwg

Summit Height:  398.9m (LIDAR data)

Name:  Allt y Genlli

OS 1:50,000 map:  136

Summit Grid Reference:  SN 98508 95042  

Drop:  73.2m (LIDAR data)




Cardinal Hill:  Bryn Amlwg

Summit Height:  397.8m (LIDAR data)

Name:  Mynydd Garth Pwt

OS 1:50,000 map:  136

Summit Grid Reference:  SN 98154 94173  

Drop:  30.5m (LIDAR data)




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


Myrddyn Phillips (May 2017)






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