Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Allt Fawr (SH 346 685) – 30-99m Sub-Twmpau addition 

There has been confirmation of an addition to the list of 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps produced from Ordnance Survey data and LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR bwlch image of Allt Fawr (SH 346 685)

The criteria for the list that this addition applies to are: 

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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 30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Allt Fawr and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the coast to its west, a minor road to its immediate east and the A4080 road to its north-east, and has the village of Aberffraw towards the east north-east. 

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category. 

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 24m of drop, based on the 42m summit spot height adjoined to a triangulation pillar and an estimated c 18m bwlch height, based on interpolation of 5m contouring between 15m – 20m that appears on the Ordnance Survey 1:25,000 Explorer map. 

Extract from the Ordnance Survey 1:25,000 Explorer map

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

Therefore, the confirmation of the addition of this hill to 30-99m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data and LIDAR analysis, resulting in a 42m summit height and a 17.0m bwlch height, with these values giving this hill 25m of drop, which is sufficient for it to be classified as a 30-99m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Allt Fawr 

OS 1:50,000 map:  114

Summit Height:  42m (triangulation pillar) 

Summit Grid Reference:  SH 34639 68534 (triangulation pillar)               

Bwlch Height:  17.0m (LIDAR) 

Bwlch Grid Reference:  SH 34974 69389 (LIDAR) 

Drop:  25m (triangulation pillar summit and LIDAR bwlch) 

 

Myrddyn Phillips (September 2021)

 

 

 

 

  

Mapping Mountains – Summit Relocations – 30-99m Twmpau

Allt Fawr (SH 383 356)

There has been a Summit Relocation to a hill that is now listed in the 30-99m Twmpau, with the summit height and its position, drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

The criteria for this list are:

30-99m Twmpau – Welsh hills at and above 30m and below 100m in height that have a minimum 30m of drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at and above 30m and below 100m in height with 20m and 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 Allt Fawr and it is adjoined to the Pen Llŷn range of hills, which is situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A 499 road to its west and the A 497 road to its east, and has the town of Pwllheli towards its south-west.

As the hill is not a part of designated open access land permission to visit should be sought, for those wishing to do so the proximity of the roads will give relatively easy access.

Prior to LIDAR analysis this hill was listed with c 18m of drop based on an estimated c 46m summit height with the hill given three uppermost 45m ring contours on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website, and an estimated bwlch height of c 28m based on bwlch contouring at 5m intervals and between 25m – 30m on this same map.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

Extract from OS Maps

The position of the three uppermost 45m ring contours were analysed via LIDAR with a fourth point also examined resulting in the below:

48.889m at SH 38347 35695 (new summit position)

46.569m at SH 38145 35521 (c 46m old listed summit)

44.962m at SH 38406 35898

44.944m at SH 38383 35833

The summit height produced by LIDAR analysis is 48.9m and is positioned at SH 38347 35695, this position is not given a spot height or an uppermost 45m ring contour on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website, but is given a 45m ring contour on OS Maps (the recent replacement for OS Get-a-map).  The new summit position for this hill is approximately 250 metres north-eastward from the previously listed summit position.

 

LIDAR image of Allt Fawr

The full details for the hill are:

Group:  Pen Llŷn

Summit Height:  48.9m (LIDAR)

Name:  Allt Fawr

OS 1:50,000 map:  123

Summit Grid Reference (New Position):  SH 38347 35695 (LIDAR)  

Drop:  23.1m (LIDAR)

Myrddyn Phillips (April 2018)

Mapping Mountains – Significant Height Revisions – 30-99m Twmpau

Allt Fawr (SH 383 356)

There has been a Significant Height Revision to a hill that now appears in the 30-99m Twmpau, with the summit height, drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

The criteria for this list are:

30-99m Twmpau – Welsh hills at and above 30m and below 100m in height that have a minimum 30m of drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at and above 30m and below 100m in height with 20m and 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 Allt Fawr and it is adjoined to the Pen Llŷn range of hills, which is situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A 499 road to its west and the A 497 road to its east, and has the town of Pwllheli towards its south-west.

As the hill is not a part of designated open access land permission to visit should be sought, for those wishing to do so the proximity of the roads will give relatively easy access.

Prior to LIDAR analysis this hill was listed with c 18m of drop based on an estimated c 46m summit height with the hill given three uppermost 45m ring contours on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website and an estimated bwlch height of c 28m based on bwlch contouring at 5m intervals and between 25m – 30m on this same map.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

The summit height produced by LIDAR analysis is 48.9m, this is not a dramatic 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:

LIDAR image of Allt Fawr

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 summit height is 48.9m and this was produced by LIDAR analysis, this is 2.9m higher than its previously listed height of c 46m which was estimated from the Ordnance Survey Interactive Coverage map hosted on the Geograph website.

The full details for the hill are:

Group:  Pen Llŷn

Summit Height (New Height):  48.9m (LIDAR)

Name:  Allt Fawr

OS 1:50,000 map:  123

Summit Grid Reference:  SH 38347 35695 (LIDAR)  

 

Drop:  23.1m (LIDAR)

Myrddyn Phillips (April 2018)

Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Allt Fawr (SH 383 356) – 30-99m Sub-Twmpau addition

There has been an addition to the 30-99m Twmpau due to LIDAR analysis conducted by Myrddyn Phillips, with the criteria for this list being:

30-99m Twmpau - Welsh hills at and above 30m and below 100m in height with 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at and above 30m and below 100m in height with 20m and more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

This hill did not appear in the accompanying Hills to be surveyed sub list when the original Welsh P30 hills were published on Geoff Crowder’s v-g.me website as it did not meet the criteria for this sub category; however this sub list has now been standardised and interpolated heights also included.

Prior to LIDAR analysis this hill was listed with an estimated c 18m of drop based on an estimated summit height of c 46m with the hill being given three uppermost 45m ring contours on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website and an estimated bwlch height of c 28m based on bwlch contouring at 5m intervals and between 25m – 30m on this same map.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

The name of the hill is Allt Fawr and it is adjoined to the Pen Llŷn group of hills, which is situated in the north-west part of North Wales (Region A, Sub-Region A1), and is positioned with the A 499 and the A 497 roads to its west and east respectively and has the town of Pwllheli towards the south-west.

If wanting to visit the hill permission to do so should be sought as it is not a part of designated open access land and does not have any rights of way on it.

The addition of Allt Fawr to 30-99m Sub-Twmpau status is due to LIDAR analysis conducted by Myrddyn Phillips.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR analysis gives the hill the following details:

Allt Fawr

Summit Height:  48.9m

Summit Grid Reference:  SH 38347 35695

Bwlch Height:  25.8m

Bwlch Grid Reference:  SH 38411 35961

Drop:  23.1m

LIDAR image of Allt Fawr

Therefore, the 48.889 LIDAR analysis for the summit position at SH 38347 35695 and the 25.753m LIDAR analysis for the bwlch position at SH 38411 35961 gives this hill 23.136m of drop, which is sufficient for 30-99m Sub-Twmpau status.

Three other tops were also analysed by LIDAR on the hill of Allt Fawr, resulting in the following; 46.569m at SH 38145 35521, 44.962m at SH 38406 35898 and 44.944m at SH 38383 35833.

The full details for the hill are:

Group:  Pen Llŷn

Name:  Allt Fawr

Summit Height:  48.9m (LIDAR)

Summit Grid Reference:  SH 38347 35695 (LIDAR)

OS 1:50,000 map:  123

Drop:  23.1m (LIDAR)

Myrddyn Phillips (April 2018)