Determining the position of the Welsh – English border relative to the summit of Twyn Llech (SO 255 353)

Before detailing our on-site adventures, it may be prudent to briefly concentrate on the name of this hill.

The hill is known by many as Black Mountain; however research in to the use of this name suggests that it was supplanted by early hill list compilers from the old ridge name given to the main easterly ridge of this hill range and does not apply to the hill itself. 

Extensive local enquiries with the farming community unearthed the feature name of the thief’s stone; this name was given by David Gains, who farms from the Olchon Valley, situated on the English side of the hill.  This feature name still appears on contemporary Ordnance Survey maps in its Welsh form of Llech y Lladron.  It is given to a rock outcrop on the upper and northerly part of this hill.  This and the name of Twyn Llech are the two main named features on this hill, and it is the latter that many know as this hill’s referred to Welsh name.  The name of Twyn Llech, which can be translated as hill of the stone (or crag), still appears on Ordnance Survey maps and is positioned to the west of its summit.  Therefore, for the remainder of this article this hill is referred to as Twyn Llech.

Mark on Llech y Lladron; the thief's stone 

Twyn Llech is positioned in the Black Mountains range of hills which are situated in the south-eastern part of Wales.  This hill range has four main extended ridges oriented in a north-west to south-east direction.  Twyn Llech is the highest point of the most easterly of these main ridges.

Twyn Llech (SO 255 353)

The hill comprises grass and heather with a base of peat; it is not dramatic in nature but is a great bulk of a hill.  Its high point is situated toward the northerly section of the main easterly ridge, which extends for approximately 15km (over nine miles).  It has two main recognised and individual summits on it, one of which is Twyn Llech and the second the southerly outlier of Hatterrall Hill (SO 308 256).

Part of the main easterly ridge of the Black Mountains

Our intention was to determine the summit position of Twyn Llech, survey this point for absolute height and discover where, in relation to this point, the border between Wales and England is positioned.  If clarity could be given to the above, we hoped this would reveal whether the summit of Twyn Llech is positioned in Wales or in England, or whether it is a Dual National Hill. 

The summit of Twyn Llech was surveyed over two days, each nine months apart.  Before detailing the first survey of this hill, we need to detail the map height and classifications of this hill.

Twyn Llech is given a 703m summit spot height on contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.  This is positioned in the southerly part of a 700m continuous contour ring at SO 25586 35010.  However, Harvey maps give a 706m summit spot height on their 1:40,000 Offa’s Dyke South map.  This is positioned in the northerly part of a 705m continuous contour ring. 

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

Extract from Harvey maps

Many peat based moorland hills have relatively flat summit areas, and Twyn Llech is no exception.  The spot heights on Ordnance Survey and Harvey maps indicate a southerly high point and a northerly high point, each separated by approximately 350 metres.  This is so on the ground, with the northerly high point more distinguishable than its southerly counterpart which is undulating in nature.

This hill is classified in many categories as befits its prominence which is over 150m, these are; Marilyn, Hump, Tump and Twmpau, and as its height exceeds 700m it is also classified as a Simm, Hewitt, Nuttall and Uchaf.  It is also included in many past hill lists which are now viewed as being historic.  Twyn Llech is also classified as a Historic County Top, current County/UA Top and an Administrative County Top.

I’d visited Twyn Llech on 18 occasions prior to the first survey of its summit, with the majority of these approaching the high point of the hill from the south.  As I walked up the broad southerly ridge of this hill I always thought that land to the north of where the Ordnance Survey 703m spot height appears to be slightly higher, but I had no instrumentation to prove this.

The first survey of Twyn Llech took place on the 28^th September 2017 whilst on an extended walk, taking in a number of hills.  The conditions for the survey were excellent with good visibility and little breeze.

I approached from the direction of Hay Bluff (SO 244 366) and found the northerly high point relatively easy to pinpoint.  The hill was surveyed using a Trimble GeoXH 6000 GNSS receiver.  The first survey was conducted between 12.34pm – 12.39pm, with five minutes of data gathered. The area surrounding the summit consists of moor grass and heather.

The position of the southerly high point is harder to determine, as its land undulates.  It is of similar nature to its northerly counterpart and consists of moor grass and heather.  I chose a narrow path leading from a small cairn placed beside the flagged ridge path for the Trimble set-up position and again gathered five minutes of data between 12.56pm – 1.01pm.

The results of these two surveys appear below:

Northerly high point:  703.639m (converted to OSGM15) at SO 25520 35383

Southerly high point:  702.721m (converted to OSGM15) at SO 25572 35032

First survey at the northerly high point - the summit of Twyn Llech

First survey at the southerly point

With a 0.9m difference in height I was confident that the northerly point is where the summit of Twyn Llech is positioned, but as the land where the southerly point undulates I wanted to revisit and take further data sets to provide confirmation of this first survey.  The opportunity to do so came when Mark suggested two walks; the first to the summit of Hatterrall Hill, and the second to the summit of Twyn Llech.  As Mark knew of the first survey result, he wanted to visit the northerly high point and confirm his Marilyn ascent, but he was also enthusiastic to pinpoint where the Welsh - English border is positioned in relation to the summit of this hill.

Prior to the visit I had conducted extensive research with the aim being to consult any written text that gives details of where the border between Wales and England is positioned and specifically for the land border passing over the Black Mountains hill range.  I had contacted a number of individuals, public bodies and organisations including the Powys Archives, Powys County Council, National Library of Wales, Longtown Historical Society, Ewyas Study Group, Hereford Record Office, Brecon History Society, Clwyd Powys Archaeological Trust, Hereford Border Authorities, Electoral Services, Boundary Commission, Ordnance Survey, HMSO, Electoral Commission, local historians and the Senedd.  None of these individuals and organisations could either supply or give information on any form of written document to where the land border between Wales and England is positioned.  In fact many said that such a document did not exist.  The only organisation that gave any indication of where this border is positioned was the Ordnance Survey, and they instructed me to consult their maps, as these would show the position of the border.

We visited Twyn Llech in the early evening of 12^th July 2018, approaching from the Gospel Pass car park via the summit of Hay Bluff.  As we reached steepening ground heading toward the hill’s summit plateau, we visited the rock outcrop named Llech y Lladron.  Once back on the main ridge path Mark activated his Garmin and started to plot on the ground where the 1:50,000 map positions the border.  This was consistently to the east of the flagged ridge path which forms a part of the Offa’s Dyke long distance footpath. 

Once at the northerly high point the Trimble was set up to gather data.  We checked each high point for best position and the measurement offset between the internal antenna in the Trimble and the ground at the base of the rucksack which is used as an improvised tripod for many of the Trimble surveys.  Meanwhile Mark also undertook the task of determining where the land border, according to the Ordnance Survey 1:50,000 map, exists on the ground as it crosses the summit ridge of Twyn Llech.  He used a Garmin GPSMap 64s hand-held device to do this, as it is uploaded with the Ordnance Survey 1:50,000 Landranger mapping.  This can be examined in fine detail on the screen with the dotted line of the border followed on the hill.

Second survey at the northerly high point - the summit of Twyn Llech

We paced out the distance between what proved to be the summit of the hill and the flagged path.  This came to approximately 12 metres, and from here we continued eastward to where the 1:50,000 map places the border.  It was a further 38 metres to this point, totalling 50 metres in all from the summit to the border, according to the 1:50,000 Landranger map.

We repeated this process for the southerly high point and found the Trimble position for the higher of the two surveys conducted to be 15 metres from the flagged path and a further 30m to the land border, according to the 1:50,000 map.  This totalled 45 metres in all from the Trimble set-up position to the border.

Second survey at the southerly point

The second survey of Twyn Llech was again conducted in excellent conditions with good visibility and not a breath of breeze.  A ten minute data set was gathered from each of three points, the first on the northerly high point and two from the southerly point.  The survey of the northerly high point was conducted between 6.13pm – 6.23pm and the grid coordinates produced from post processing matched those from the first survey of this same point in September 2017; SO 25520 35383.

As the ground at the southerly highpoint slightly undulates, I wanted to take a further two data sets from here, but in different positions to where the Trimble was placed during the September 2017 survey.  I easily found the point where the Trimble had previously been set up on the narrow path that leads west from the small cairn beside the flagged ridge path, and with Mark’s assistance, we studied the lay of land and chose two further points.  Each of these was of similar height to the narrow path and each we considered as potential highpoints for this southerly land.  The Trimble GeoXH 6000 gathered two further data sets from these points between 6.43pm – 6.53pm and 6.58pm – 7.08pm respectively. 

Third survey at the southerly point

Over two days, almost nine months apart, two data sets were gathered from the northerly high point and three from land comprising the southerly high point, with all Estimated Accuracies given as 5-15cm: 100.00% and the Standard Deviation as 0.0m, meaning all data sets are good.

The results of these three further surveys appear below:

Northerly high point:  703.656m (converted to OSGM15) at SO 25520 35383

Southerly high point:  702.787m (converted to OSGM15) at SO 25558 35018

Southerly high point:  702.699m (converted to OSGM15) at SO 25566 35054

These results show excellent correlation with the first set of survey results produced in September 2017 with the exact same position for the Trimble on the northerly high point with the two surveys; 703.639m and 703.656m only being 0.017m apart.

Three positional surveys were conducted on the southerly high point, all within a grid approximately 14 metres by 36 metres.  Their results; 702.721m, 702.787m and 702.699m are within 0.088m of one another, confirming that the southerly high point is lower than the northerly high point.  Therefore the summit of Twyn Llech is positioned at SO 25520 35383 and its height given by the Trimble GeoXH 6000 is 703.6m (average of two summit surveys).

However, what of the border between Wales and England and does this result confirm that the summit of Twyn Llech is positioned in Wales as the Ordnance Survey 1:50,000 map implies?

Our on-site visit had confirmed where the summit of Twyn Llech is positioned and had given the hill an accurate height.  We now compared our findings to the series of Ordnance Survey Six-Inch maps.

For many years the Ordnance Survey Six-Inch map was considered their base map.  This was the map for information to be fed onto.  The scale was superseded in the 1950s by the 1:10,000 series of maps and was available as sheets until the 1980s when these maps were digitised.  The Six-Inch map is still one of the best for giving detail, either numerical or positional, and it was this map that we turned to.

We consulted the series of Six-Inch maps and looked at detail surveyed in 1887 and published in the same year.  The land border between Wales and England appears on this map as a split black line, as it does on the publicly available scales of 1:50,000 and 1:25,000 maps.  However, the line of this border on the Six-Inch map has one extremely important word placed against it – ‘Watershed’.

Extract from the Ordnance Survey Six-Inch map published in 1887 with the all-important word 'Watershed' following the course of the border

The word ‘Watershed’ also appears beside this border on the Ordnance Survey Vector Map Local that is hosted on the Geograph website and entitled the Interactive Coverage Map.  Therefore this word and its implication have remained in the system of Ordnance Survey mapping for over 130 years.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map with the all important word 'Watershed' still appearing

As there was discrepancy between our on-site findings using the 1:50,000 mapping and the Six-Inch and Vector Map Local maps the Ordnance Survey were consulted and they kindly forwarded the following information:

‘Ordnance Survey are not directed by the Ordnance Survey Act 1841 or any other Act to show National boundaries, although they are shown on certain scales of mapping (1:50k / Landranger, does depict National Boundaries between England-Wales, England-Scotland).  In Os’s most detailed mapping product OS MasterMap the Welsh / English border is located accurately and correctly it is displayed as a County/UA Boundary.

‘OS’s 1:25k and 1:50k leisure maps are purely contextual to provide a navigational guide to the landscape.  Given their scale, these maps should never be used for measuring and surveying purposes.  Our leisure maps are derived from the accurate large-scale data but are generalised for clarity, therefore not every feature is shown in the same position as the source data, and we are satisfied with how they currently depict the location of the Welsh English border.’

The OS MasterMap is the Ordnance Survey digitised mapping which receives upwards of 10,000 individual updates per day and as its name implies, this is the master map for Ordnance Survey data.

I was advised by Ordnance Survey that the placement of the border on MasterMap is 12 metres to the east of the grid coordinates I supplied them for the summit position of Twyn Llech, and this means that MasterMap places the summit of this hill in Wales.

After consultation with Ordnance Survey I was advised to contact the Boundary Commission for Wales, it is this organisation that if a change in border placement is required would instigate it.  This organisation kindly forwarded the below:

‘There is no legal mechanism for reviewing the boundary between England and Wales.  The boundary is fixed and would require primary legislation to develop a mechanism to make a change to it.  The England-Scotland boundary does have such a mechanism and the review is performed jointly by the English and Scottish Boundary Commissions.

As far as the Commission is concerned the boundary provided by Ordnance Survey, particularly that in its MasterMap product, is the definitive boundary.

With regard to the specific area to which you referred, I confirm that it is the convention of this Commission that if we were to draw a boundary that it would follow either the top of a ridge-line to its peak or the bottom of a valley depending on the circumstance; where we are not following another easily identifiable boundary (road, railway line, centre of a river, etc.).  It may be that when the boundary was originally drawn at the larger scale that that was the intention.  We have no records which would ascertain that intention’.

My conclusion from the research and survey I have conducted is that the border should be moved and placed on MasterMap going through the summit of Twyn Llech and that primary legislation should be initiated to instigate the mechanism required for such a change.  This is based on the convention the Boundary Commission for Wales would adopt and the evidence that the border followed the watershed on this hill’s summit plateau on selective Ordnance Survey maps for over 130 years.  It is also based on the fact that it is the natural course of any boundary on an open mountain such as Twyn Llech to follow the ridge crest which is also the watershed.  The simple fact of the matter is that no one had ever surveyed this hill for where its summit is positioned prior to our visit, therefore the border placement on MasterMap although very close to the summit is still 12 metres from it and in all probability the cartographer who placed it was doing so to where they thought the watershed lay.

Myrddyn Phillips and Mark Trengove (March 2019)

Grough article detailing the research and survey relating to Twyn Llech and the border   

UKHillwalking article detailing the research and survey relating to Twyn Llech and the border