Mapping Mountains – Significant Height Revisions – The Welsh P15s

 

Twyni Mawr (SN 606 938)

There has been a Significant Height Revision to a hill that was listed in The Welsh P15s, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

Twyni Mawr (SN 606 938)

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

The Welsh P15s – Welsh hills with 15m minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s, with the criteria for this sub category being all Welsh hills with 14m or more and below 15m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list appearing on Mapping Mountains on the 10^th May 2019. 

The Welsh P15s by Myrddyn Phillips

The name the hill is listed by is Twyni Mawr, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the north-western part of South Wales (Region B, Sub-Region B1), and it is positioned with the coast to its west, a minor road to its east and the B4353 road to its south-east, and has the village of Y Borth towards the south.

When the listing that became known as The Welsh P15s was being compiled, this hill was not included in either the main list or the accompanying sub list, as with no significant contours of note on the contemporary Ordnance Survey 1:25,000 map, it was hard to know whether any hill with qualifying prominence existed. 

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

Since the compilation of The Welsh P15s first started there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites. 

Extract from the interactive mapping hosted on the WalkLakes website

One of the mapping resources now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available maps and for this hill a 20m uppermost contour is shown. 

Extract from the DataMapWales

Another resource now available online is the interactive mapping hosted on the Welsh Government website and entitled the DataMapWales.  This mapping has 5m contours and its detail matches that produced from the OS Terrain 5 product, which compliments much of that produced from LIDAR, and for this hill there are two 23m spot heights shown. 

LIDAR image of Twyni Mawr (SN 606 938)

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. 

LIDAR summit image of Twyni Mawr (SN 606 938)

The summit height produced by LIDAR analysis is 22.8m and when compared to the detail on the Ordnance Survey 1:25,000 Explprer map, this comes within the parameters of the Significant Height Revisions used within this page heading, and these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 22.8m and this was derived from LIDAR analysis.  This is 22.8m higher than the detail on the Ordnance Survey 1:25,000 Explorer map, but is complimentary to the detail on the interactive mapping hosted on the WalkLakes website and also that on the DataMapWales.

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Twyni Mawr 

OS 1:50,000 map:  135

Summit Height (New Height):  22.8m (LIDAR)                                                           

Summit Grid Reference:  SN 60614 93828 (LIDAR)                                                  

Bwlch Height:  4.4m (LIDAR) 

Bwlch Grid Reference:  SN 60576 92484 (LIDAR) 

Drop:  18.4m (LIDAR) 

                                                           

Myrddyn Phillips (April 2026)

 

Mapping Mountains – Hill Reclassifications – The Welsh P15s

Twyni Mawr (SN 606 938) – Welsh P15 addition 

There has been an addition to the list of The Welsh P15s, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

Twyni Mawr (SN 606 938)

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

The Welsh P15s – Welsh hills with 15m minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s, with the criteria for this sub category being all Welsh hills with 14m or more and below 15m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list appearing on Mapping Mountains on the 10^th May 2019. 

The Welsh P15s by Myrddyn Phillips

The name the hill is listed by is Twyni Mawr, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the north-western part of South Wales (Region B, Sub-Region B1), and it is positioned with the coast to its west, a minor road to its east and the B4353 road to its south-east, and has the village of Y Borth towards the south.

When the listing that became known as The Welsh P15s was being compiled, this hill was not included in either the main list or the accompanying sub list, as with no significant contours of note on the contemporary Ordnance Survey 1:25,000 map, it was hard to know whether any hill with qualifying prominence existed. 

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

Since the compilation of The Welsh P15s first started there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

One of the mapping resources now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available maps and for this hill a 20m uppermost contour is shown. 

Extract from the interactive mapping hosted on the WalkLakes website

Another resource now available online is the interactive mapping hosted on the Welsh Government website and entitled the DataMapWales.  This mapping has 5m contours and its detail matches that produced from the OS Terrain 5 product, which compliments much of that produced from LIDAR, and for this hill there are two 23m spot heights shown. 

Extract from the DataMapWales

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. 

LIDAR image of Twyni Mawr (SN 606 938)

Therefore, the addition of this hill to Welsh P15 status is due to LIDAR analysis, resulting in a 22.8m summit height and a 4.4m bwlch height, with these values giving this hill 18.4m 0f drop, which is sufficient for it to be classified as a Welsh P15. 

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Twyni Mawr 

OS 1:50,000 map:  135

Summit Height:  22.8m (LIDAR)                                                           

Summit Grid Reference:  SN 60614 93828 (LIDAR)                                                  

Bwlch Height:  4.4m (LIDAR) 

Bwlch Grid Reference:  SN 60576 92484 (LIDAR) 

Drop:  18.4m (LIDAR) 

 

Myrddyn Phillips (March 2026) 

Mapping Mountains – Trimble Surveys – Banc Llechwedd Mawr

 

03.09.25  Twyni Mawr (SN 606 938) 

Twyni Mawr (SN 606 938)

Blanks on the map can be interesting to investigate.  These are the places that Ordnance Survey designate in flux, as in the ground they inhabit is unstable.  These include quarries, landfills and other places terra-formed by human interference.  However, they also include places that have not been altered by man’s need to make big holes or bury a lot of stuff in the ground. 

The blank on the map

Nowadays other online mapping show contours for such places, this gives perspective to the lay of land.  Today I was visiting friends and if the weather acted kindly we planned on visiting one of these blanks on the map.  I mention the weather as leaving Welshpool and driving west the cloud base soon closed in with drizzle becoming ever more present.  Approaching the coast this turned into heavy showers with the narrow roads leading to Ynys-las becoming water-logged.  I suppose after an unseasonably warm summer, a bit of rain was a change! 

Extract from the interactive mapping hosted on the WalkLakes website

I was heading to the Searivers Caravan Park, where Eryl and Rita were enjoying a few days relaxation, including coastal walks between what was proving to be a particularly wet spell of weather. 

The Searivers caravan site at Ynys-las

By the time I pulled up outside their home from home the rain was bucketing down, so much so that I ran from the car to the front door and hoped it was opened without delay.  The next hour or so I was very happily settled on a chair, with a mug of tea and assorted biscuits at hand as we caught up with all manner of things, whilst the rain continued to fall.  As time progressed a glimmer of semi-brightness appeared out to sea, this quickly pushed inland and heralded a tempting flash of sunlight as the rain calmed its deluge.  Rita said; ‘right if we’re going to go we need to get ready and make the most of whatever dry spell we have’.  This we did, and soon were walking through the site toward the narrow dead end road and the gate on its opposite side which gives access to the complex of dunes.

Prior to my visit I’d examined this blank on the map via LIDAR, building up the coloured contours until the high point and the connecting bwlch of these dunes was found.  LIDAR gives this point as having 18.4m of drop, so it easily qualifies as a Welsh P15, whilst maps record the name of Twyni Mawr for this area, which is apt as a translation of this can be big dunes. 

LIDAR summit image of Twyni Mawr

Whatever the occasion it’s always good to see Eryl and Rita, whom I’ve shared so much with over the years, especially so with Eryl; including travelling and treks around the Alps, Nepal, Tibet, Vietnam and Cuba, nowadays our ambitions are reduced, but the friendship is still strong. 

Information board at the access to the dunes

Our access to the high point of Twyni Mawr was via a wooden walk way which continues to the coastal sands and the sea beyond; where the walk way crests the dunes a convenient bench had been installed.  Not surprisingly this point gives extensive views out to sea and also inland to what today were cloud enshrouded hills.  Although the view was to be admired the cloud out to sea looked like more showers were on their way, so not lingering I followed Eryl as he started down the walk way whilst suggesting the best way to the high point of the dunes was to cut back using one of many valleys that interweave amongst the dunes.  Eryl had investigated some of these and cautioned that when in them it’s hard to know where exactly you are. 

The wooden walk way heading into the dunes

The path beyond the high point of the wooden walk way

Leaving the walk way we headed into the dunes and accessed the valley directly below the western flank of the high point.  It was here that Eryl stayed, watching me and Rita as we headed the short distance across the valley to the base of the steepening dune.  

The valley leading to the high point of Twyni Mawr.  Photo: Eryl Selly

Heading toward the summit of Twyni Mawr.  Photo: Eryl Selly

I’d brought an umbrella, which is a norm for me even on some higher hills, as if confronted by rain it can shield my camera equipment much better that a Goretex coat, and although it was never used today as whatever rain did come across was never heavy enough to seek protection, it proved an ideal replacement for a walking pole for Rita, who used it to steady herself as we slid ourselves uphill on the steepening sand dune.

Once at the top of the steep bit it was only a short walk on a narrow path across the ridge of the highest dune to its summit.  Here we posed for the customary photographs before continuing on the narrow ridge path back toward where Eryl was patiently waiting for us on the wooden walk way. 

At the summit of Twyni Mawr.  Photo: Rita Selly

Heading back to the caravan the whole adventure had only taken 55 minutes, and we were ever so lucky with the weather as only a skidding light shower had blown across the land whilst on the dunes, which was fortunate considering how heavy the showers had been on my way toward Ynys-las and on my arrival.

We ended the day with a trip into Aberystwyth for a lunchtime meal at the University Art’s Centre before settling down to watch The Roses in the cinema.  It even brightened up for our drive back to Ynys-las and my continued journey to Welshpool.  It proved an excellent day out. 

 

Survey Result: 

 

Twyni Mawr 

Summit Height:  22.8m (LIDAR) (significant height revision)  

Summit Grid Reference:  SN 60614 93828 (LIDAR)

Bwlch Height:  4.4m (LIDAR)

Bwlch Grid Reference:  SN 60576 92484 (LIDAR) 

Drop:  18.4m (LIDAR) (Welsh P15 addition)

Dominance:  80.55% (LIDAR)

 

 

For further details please consult the Trimble Survey Spreadsheet