Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Ynys Hir (SN 679 957) 

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill derived by Joe Nuttall who produced a summit analysis programme using LIDAR, and then by LIDAR analysis initially conducted by Chris Crocker and subsequently by Myrddyn Phillips. 

LIDAR image of Ynys Hir (SN 679 957)

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

30-99m Twmpau - Welsh hills at or 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 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

Y Trechol - The Dominant Hills of Wales - Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height, with the Introduction to the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Ynys Hir, and it is adjoined to the Pumlumon group of hills which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with the Afon Dyfi (River Dovey) and the coast to its west and has the A487 road to its east and the village of Y Borth towards the south-west. 

When the original 30-99m height band of Welsh P30 hills was published on Geoff Crowder’s v-g.me website this hill was included in the main P30 list with a summit height of c 40m.  

When 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 41m of drop, based on an estimated c 48m summit height and an estimated c 7m bwlch height, with both values based on interpolation of 10m contouring that appears on the Ordnance Survey 1:25,000 Explorer map, with 40m being the uppermost contour given the hill on this 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. 

The summit height produced by LIDAR analysis is 61.1m and this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are: 

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. 

Therefore, this hill’s new listed summit height is 61.1m and this was derived from LIDAR analysis, this is 13.1m higher than the previous listed summit height of c 48m which was derived from interpolation of the uppermost contour given the hill on contemporary Ordnance Survey 1:25,000 Explorer maps.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Pumlumon 

Name:  Ynys Fach 

OS 1:50,000 map:  135

Summit Height (New Height):  61.1m (LIDAR) 

Summit Grid Reference:  SN 67972 95798 (LIDAR) 

Bwlch Height:  6.8m (LIDAR) 

Bwlch Grid Reference:  SN 68354 95558 & SN 68368 95541 (LIDAR) 

Drop:  54.35m (LIDAR) 

Dominance:  88.91% (LIDAR)

 

Myrddyn Phillips (August 2021)

 

 

  

Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Ynys Hir (SH 566 396) – 30-99m Twmpau addition

There has been an addition to the Twmpau (thirty welsh metre prominences and upward) list with this hill being included in the 30-99m height band.  Accompanying all the 100m height band of hills within the Twmpau are sub lists with the one attached to this list being entitled the 30-99m Sub-Twmpau.

The criteria for 30-99m Twmpau status is all Welsh hills at or above 30m and below 100m in height with 30m minimum drop, whilst the criteria for 30-99m Sub-Twmpau status is all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop.

The details relating to this hill’s addition are retrospective as it was proposed as a possible new P30 by Aled Williams, and subsequently surveyed with the Trimble GeoXH 6000 on the 1^st February 2014.

Prior to the survey with the Trimble this hill was unclassified as the Ordnance Survey 1:50,000 Landranger map, 1:25,000 Explorer map and the enlarged mapping hosted on the Geograph website all give the hill an uppermost 20m ring contour, implying that its height is not sufficient for P30 status.

The hill is in the Moel Hebog group of hills and is placed in the Region of North Wales (Region A, Sub-Region A1) with its Cardinal Hill being Pt. 734m (SH 525 502) and is situated between the communities of Porthmadog to its south and Tremadog to its north north-west, with the A 487 road to its immediate south.  The hill can be visited from this road where a lay-by gives access through a gate into woodland that covers this and other adjacent small hills.

The name of the hill is Ynys Hir and as its name implies it forms an island, which is now landlocked, it is included as a 30-99m Twmpau as the survey with the Trimble GeoXH 6000 produced a summit height of 37.3m (converted to OSGM15), and with an estimated bwlch height of c 3m it gives this hill c 34m of drop.

The full details for the hill are:

Cardinal Hill:  Pt. 734m (SH 525 502)

Summit Height:  37.3m (converted to OSGM15)

Name:  Ynys Hir

OS 1:50,000 map:  124

Summit Grid Reference:  SH 56695 39693

Drop:  c 34m

Beyond the cliffs and vegetation is the summit of Ynys Hir (SH 566 396) which has been confirmed as a 30-99m Twmpau

For details on the survey that confirmed this hill’s 30-99m Twmpau status

Myrddyn Phillips (July 2017)

Mapping Mountains – Hill Reclassifications – Y Trechol – The Dominant Hills of Wales

Ynys Hir (SH 566 396) - Dominant addition

There has been an addition to the listing of Y Trechol – The Dominant Hills of Wales due to a survey with the Trimble GeoXH 6000.  This has resulted in the hill being added to the Dominant list.  With the criteria for inclusion to this list being those Welsh P30 hills whose prominence equal or exceed half that of their absolute height.

The details relating to this hill’s inclusion as a Dominant hill are retrospective as it was surveyed on the 1^st February 2014 and later appeared in the Moel Hebog group of Dominant hills when published on the Mappingn Mountains site on the 28^th January 2016.

Prior to the survey with the Trimble this hill was unclassified as the Ordnance Survey 1:50,000 Landranger map, 1:25,000 Explorer map and the enlarged mapping hosted on the Geograph website all give the hill an uppermost 20m ring contour, implying that its height is not sufficient for P30 status and consideration as a Dominant hill.  However, it was Aled Williams who first proposed that this hill may attain P30 status and it was subsequently surveyed in his company.

The hill is in the Moel Hebog group of hills and is placed in the Region of North Wales (Region A, Sub-Region A1) and is situated between the communities of Porthmadog to its south and Tremadog to its north north-west, with the A 487 road to its immediate south.  The hill can be visited from this road where a lay-by gives access through a gate into the woodland that covers this and the other adjacent small hills.

The name of the hill is Ynys Hir and as its name implies it forms an island, which is now landlocked, it is included as a Dominant hill as the survey with the Trimble GeoXH 6000 produced a summit height of 37.3m, and with an estimated bwlch height of c 3m it gives this hill 91.21% Dominance.

The full details for the hill are:

Group:  Moel Hebog

Name:  Ynys Hir

Dominance:  91.21%

OS 1:50,000 map:  124

Summit Grid Reference:  SH 56695 39693

Summit Height:  37.3m (converted to OSGM15)

Drop Summit to Bwlch:  c 34m

Drop Bwlch to ODN:  c 3m 

Beyond the cliffs and immersed in woodland is the summit of Ynys Hir (SH 566 396) which is now classified as a Dominant hill

For details on the survey that confirmed this hill’s Dominant status

Myrddyn Phillips (February 2017)

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

Ynys Hir (SH 566 398)

There has been a Significant Height Revision in the 30-99m Sub-Twmpau (thirty welsh metre prominences and upward) list initiated by a survey with the Trimble GeoXH 6000, with the following details being retrospective as the  survey that resulted in this height revision was conducted on 1^st February 2014.

The criteria for the listing that this height revision affects are:


30-99m Twmpau - these are the Welsh hills at or above 30m and below 100m in height that have a minimum drop of 30m. 

30-99m Sub-Twmpau - these are the Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop

The listed name of the hill is Ynys Hir with its adjacent and higher hill taking on the same name, and it is situated in the lowlands outside of Porthmadog in north-west Wales, and the survey was conducted in the company of Aled Williams who had suggested the walk, as he considered at least one of the hills he proposed for us to visit would become a new P30 – he was right!  With the new P30 being Ynys Hir (SH 56695 39393) which is adjacent to the hill this blog post relates to; Ynys Hir (SH 56649 39840).

The hill is situated to the north of Porthmadog in the area known as Traeth Mawr, and it can be accessed from a lay-by on the A487 to the south of the hill, however its ascent can be problematic due to the undergrowth.

The summit height of this hill has been dramatically increased from its current Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map height, as prior to the survey with the Trimble GeoXH 6000 the hill was not listed as it only had an uppermost ring contour of 20m on both of these maps, with an estimated summit height of c 24m.  Its new summit height is 36.9m (converted to OSGM15), which is 12.9m higher than its previously estimated height and 16.9m higher than the 20m uppermost ring contour on current Ordnance Survey maps.

Although the new summit height of 36.9m (converted to OSGM15) is dramatically different to current Ordnance Survey map detail, the Ordnance Survey Historical 1:25,000 map has an uppermost ring contour of 100ft for this hill which is correct when compared to the surveyed height produced by the Trimble GeoXH 6000. 

The full details for the hill are:

Cardinal Hill:  Hir Ynys

Summit Height (New Height):  36.9m (converted to OSGM15)

Name:  Ynys Hir

OS 1:50,000 map:  124

Summit Grid Reference:  SH 56649 39840

  

Drop:  25.8m (converted to OSGM15)

Setting up the Trimble geoXH 6000 at the summit of Ynys Hir.  Photo: Aled Williams

For details on the survey that significantly revised the summit height of this hill

For details on the bwlch survey that confirmed the drop value of this hill

Myrddyn Phillips (August 2015)

Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol - The Dominant Hills of Wales

Ynys Hir (SH 566 396)

There has been a Significant Height Revision to the 30-99m Twmpau and Y Trechol - The Dominant Hills of Wales listings initiated by a survey with the Trimble GeoXH 6000, with the following details being retrospective as the survey that resulted in this height revision was conducted on 1^st February 2014.

The two listings that this height revision affects are:


30-99m Twmpau - these are the Welsh hills at or above 30m and below 100m in height that have a minimum drop of 30m. 

Y Trechol - The Dominant Hills of Wales - these are the Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.

The name of the hill is Ynys Hir and it is situated in the lowlands outside of Porthmadog in north-west Wales, and the survey was conducted in the company of Aled Williams who had suggested the walk as he considered at least one of the hills he proposed for us to visit would become a new P30 – he was right!

The hill is situated to the north of Porthmadog in the area known as Traeth Mawr, and it can be easily accessed from a lay-by on the A487 to the south-west.  The term easily accessed is only applicable to where the road is situated in relation to the hill as it can be a bit of a battle to reach its top.

The summit height of Ynys Hir has been dramatically increased from its current Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map height, as prior to the survey with the Trimble GeoXH 6000 the hill was not listed as it only had an uppermost ring contour of 20m on both of these maps, with an estimated summit height of c 23m.  Its new summit height is 37.3m (converted to OSGM15), which is 14.3m higher than its previously estimated height and 17.3m higher than the 20m uppermost ring contour on the current Ordnance Survey maps.

Although the new summit height of 37.3m (converted to OSGM15) is dramatically different to current Ordnance Survey map detail, the Ordnance Survey Historical 1:25,000 map has an uppermost ring contour of 100ft for this hill which is correct when compared to the surveyed height produced by the Trimble GeoXH 6000.  

The full details for the hill are:

Cardinal Hill:  Hir Ynys

Summit Height (New Height):  37.3m (converted to OSGM15)

Name:  Ynys Hir

OS 1:50,000 map:  124

Summit Grid Reference:  SH 56695 39693 

 

Drop:  c 34m

Dominance:  91.21%

Beyond the trees, vertical cliffs and undergrowth is the summit of Ynys Hir, a new P30 which has a significant height revision

For details on the survey that significantly revised the summit height of this hill

Myrddyn Phillips (August 2015)

Mapping Mountains – Trimble Surveys – Moel Hebog

28.02.15  Ynys Hir (SH 566 398, bwlch only)

Thirteen months ago Aled took me to a magical land of small wooded islands, all situated around Porthmadog, with most having been tidal islands with their land area dictated by the inward wash of the sea.  The first of these islands we had visited was Ynys Hir, in affect this name is given to two hills, both of which are next to the A487.  I’d passed these hills on many occasions travelling between mid and north Wales, but I had never thought to investigate their inner realms.  On that first island adventure the Trimble had surveyed the respective summits but not the connecting bwlch.  The data produced on that day complimented that from the Ordnance Survey Historical 1:25,000 map and not that from current maps, which do not include an uppermost 30m ring contour as the two Ynys Hir summits were surveyed as 37.3m (SH 566 396) and 36.9m (SH 566 398) respectively.

Having visited Ynys Galch (SH 567 393) and Ynys Pen Syflog (SH 561 395) earlier in the day we parked in a large lay-bi beside the wooden gate that gives access into the island realm.  As soon as one enters this land the atmosphere changes from that of busy road and overhead sky to a labyrinthine land of trees and undergrowth, all overshadowed by rising wooded slopes and impressive small crags.

A path leads from the gate upto the bwlch between the two Ynys Hir hills, bisecting the bwlch is a solid stone wall, on the other side of which is a manicured green lawn adjoined to buildings named Bodawen.  The contrast between one side of this wall and the other is immediate as within the island territory the land consists of overgrown brambles, fallen trees and a snaking growth of ivy, whilst above are the islands and a thick canopy of mature trees.

The critical bwlch was judged to be a couple of metres from the wall and the remnants of a rolled up wire fence proved ideal to lay the Trimble on so it was at least partly elevated above the undergrowth.  Soon we had also found the remains of a broken glass cabinet which when positioned on top of the rolled up wire fence elevated the Trimble 1.09m above the bwlch.

Gathering data at the critical bwlch

Measuring the offset between the ground at the bwlch and the internal antenna

As the equipment rhythmically beeped each of its gathered data points we stood under a tree and chatted, 14 minutes later the Trimble was switched off and we delicately made our way over a fallen tree and back onto the path.

A few minutes later and I was driving toward Nantlle with the first spots of rain falling, and once at my brother’s house the wind howled across the land and the rest of the afternoon the rain fell.

Survey Result:

Ynys Hir

Bwlch Height:  11.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 56713 39759

Drop:  25.8m (converted to OSGM15) (30-99m Sub-Twmpau status confirmed)

Dominance:  69.90%

To read the blog post on the survey of this hill’s summit please click {here}

For further details please consult the Trimble survey spreadsheet click {here}

Mapping Mountains – Trimble Surveys – Moel Hebog

01.02.14  Ynys Hir (SH 566 396), Pt. 36.9m, Ynys Hir (SH 566 398) and Ynys Fadog (SH 564 398)

The wooded summit of Ynys Fadog (SH 564 398)

The ‘islands’ of Ynys Hir and Ynys Fadog are just some of the many small outcrops of land that are now landlocked having once been islands in the tidal estuary of Afon Glaslyn.  They are on the flatland's known as the Traeth Mawr, the translation of which can be ‘large beach’ or ‘big sands’.  Each is situated on the outskirts of, and between Porthmadog and Tremadog with the flatland's having been reclaimed from the sea in the late 1700’s. 

The Traeth Mawr extends north-eastward from Ynys Hir and Ynys Fadog toward Nantmor, whist following this reclaimed land on the map one is struck by these landlocked islands, many are wooded and some are P30’s.  It was our intention to survey the two ‘islands’ known as Ynys Hir and the one known as Ynys Fadog, the latter seems lower in height when compared to the other two, but all three have an uppermost c 20m contour on current Ordnance Survey maps.  All three are densely wooded, could one or two of them attain P30 status?  

Our intention had been to re-survey Foel (SH 450 506) for potential HuMP status, but the forecast was so dreadful that Aled suggested a wonder in a wood, as this would give us shelter from the predicted rain and high wind and would give Twm (Aled’s dog) a good bit of exercise, as well as giving each ‘island’ an absolute height, something that they lacked on any Ordnance Survey map we had examined.

We met at 9.30am and spent the next 90 minutes chatting in Aled’s van as the rain swept in.  Once there was a break in the wet stuff we gathered Twm up and set out in to the wood.  Once beneath the trees it was like entering another world, one consisting of canopy, steep slopes and cliffs.  These islands may be low in height but they are little marvels, packed full of interest and undergrowth.  Paths exist in the wood, mainly at lower elevation around the base of each ‘island’, and stone steps partly up them (if found), otherwise they are overgrown in fern, bramble, ivy and copious amounts of trees.  Interspersed amongst the pleasantries of foliage is rock, much of this is cliff shaped and some overhanging.  Ways to each summit do exist but much of it is through branches on a zig zag route ever upward.

The whole wood is known as Y Nursery, with the island at SH 566 396 and SH 566 398 both being known as Ynys Hir.  We headed for the one at SH 566 396 first and encountered steep rising ground full to the brim with lots of undergrowth and woody stuff.  Aled led the way closely followed by Twm and soon found a series of stone steps that gained height around the middle section of the hill.  Further progress involved bashing through branches until we reached the high point.  Beside this was a telegraph pole, looking rather out of place amongst the ivy, bramble and detritus.  We set the Trimble up on its pole and stuck it in the earth taking a measurement from tip of screw (just under the antenna) to ground level and input this in to the equipment as a measurement offset.  The antenna was fixed in place and we gathered 18 minutes of data.  The downward route was as much fun as the upward, full of interest and swacks in the face by branches.

Aled and Twm beside the high point of Ynys Hir (SH 566 396)

We considered surveying the bwlch between the two summits known as Ynys Hir but pressed on to the second summit.  More stone steps led up the middle part of the hill past an impressive overhanging cliff and then it was bush whacking again as the summit was eventually reached through a thick passage of intervening shrubbery.

Underneath the overhanging cliff on our way up to the second summit.

The high point consisted of a rock on top of a larger rock with the customary amounts of ivy, fern and all sorts of Tolkienesque forms of undergrowth pervading everything in sight.  Near to the high point a gap in the trees gave us a view toward our first wooded island and our yet to be visited wooded island; Ynys Fadog.  Close by was a cliff.  Twm was settled beside a tree as the Trimble was again set up on its pole; we gathered ten minutes of data and found an easier slithery way down.

The high point of the second summit (SH 566 398)

The third island was by far the easiest to get to the top of, once there the pole was pushed in to the earth beside the high point, which was another small rock.  We measured from the tip of the screw thread to the tip of the rock, put this measurement offset in to the equipment and set it gathering data.

At the summit of Ynys Fadog (SH 564 398)

The day had proved quite an unexpected adventure on small islands of overgrown wonder that are well sheltered and rather magical.

Survey Result:

Ynys Hir

Summit Height:  37.3m (converted to OSGM15) (significant height revision)

Summit Grid Reference:  SH 56695 39693

Drop:  c 34m (30-99m Twmpau addition confirmed)

Dominance:  91.21% (Dominant addition confirmed)

Pt. 36.9m, Ynys Hir

Summit Height:  36.9m (converted to OSGM15) (significant height revision)

Summit Grid Reference:  SH 56649 39840

Drop:  25.8m (converted to OSGM15) (30-99m Sub-Twmpau addition confirmed)

Dominance:  69.90% 

To read the blog post on the survey of this hill’s bwlch please click {here}

Ynys Fadog

Summit Height:  27.8m (converted to OSGM15)

Summit Grid Reference:  SH 56464 39823

Drop:  c 25m (Double Sub-Twmpau addition confirmed) 

Dominance:  89.77%

For further details please consult the Trimble survey spreadsheet click {here}