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Map heights in the UK

When you read a map you will see that various points have their height marked.

It is commonly believed that these Spot Heights are taken from Sea Level. However the phrase Sea Level is a misnomer as the tides rise and fall, plus many other things such as natural coastal erosion or the seasons can also affect the average height of the Sea Level.

For this reason The Board of Ordnance (which since the 1810 Survey of the Isle of Wight has been known as The Ordnance Survey) decided in the 1840’s to gauge all heights in the UK from the average tide level at the Albert Dock in Liverpool.

However, due to multiple inaccuracies, at the start of the 20th century The Ordnance Survey decided that a new Datum was required – in this case a Datum is simply a fixed reference point.

After assessing the results from 3 different sites, Newlyn in Cornwall was selected to be the single reference Datum for all heights in the UK.  A gauge was placed on a stone pier at the harbour’s entrance, so it would be exposed to the full range of movement of the Atlantic Ocean and, for more than six years, it took hourly readings of the rise and fall of the tides.  The average height of the sea was calculated and this became the new Ordinance Datum Newlyn (ODN).  

So all map heights in the UK are taken from a line on a wall in Newlyn, Cornwall.

Height:  The vertical difference between two points, such as the top and bottom of a building. 
Altitude:  The vertical difference between a point and a Datum, which is a fixed reference.
In this piece I have used the term “height” as this is common practice, when map reading, when referring to “altitude”.

 Benchmarks and Trig Points

In 1935 the Retriangulation of Great Britain was started. This involved dividing the entire country in to an inter-linked series of triangles and other geometric shapes, so the angles between them could be calculated.

Following on from the Ordinance Datum Newlyn almost 200 Fundamental Benchmarks were placed around the UK.  Each Fundamental Benchmark’s height difference (from the Newlyn Datum) was measured to a very high degree of accuracy.  As well the main Fundamental Benchmarks there were hundreds of thousands of Lower Order Benchmarks sited around the country on buildings, monuments and other permanent features although their height was not as accurately measured as the Fundamentals.

 Ordinance Survey Fundamental Benchmarks

 Have you ever seen a mark like this engraved on to a building ?
There are still hundreds of thousands Lower Order Benchmarks to be found.

Note the height mark is the top rectangle or line

Also a large number (over 6,500) of Triangulation Pillars (Trig Points) were constructed from concrete or rock.  Each had a metal brace built in to the top, to hold a theodolite which is a  sighting device used to measure angles to other visible Trig Points. Either a metal bracket or an inscription was placed on the side of each pillar which gave the exact point at which the height of the trig point was taken.

 The Trig Point on the summit of Ben Nevis

This is one of the very few which is build on to a raised platform – so it can be seen above the snow in winter

Contrary to popular belief (and almost all books / websites on the topic) Trig Point are NOT always at the top of mountains. They are placed so they can be seen from other Trig Points, so many are placed on the summits, but some are not, for example Nurton Hill (England), Green Brae (Northern Ireland), Neuk (Scotland), Coppet Hill (Wales).

As those who participated in one of our courses in the Chiltern Hills will tell you “the Trig Point on Deacon Hill is not at the top”.

Spot heights

With the advance of the GNSS (Global Navigation Satellite System) surveyors no longer need to take manual readings from various points to calculate their height above the harbour in Newlyn.

There are a number of methods which the Ordinance Survey uses to obtain the height of a point in the UK. Mainly they use only two, either photogrammetry or GPS.

  • Photogrammetry (Air)
    This is a process where an aircraft equipped with sensors and high resolution cameras will fly over an area collecting data. This information can be used to create a 3D image from which heights can be obtained. This will normally be accurate to within 3 or 4 meters.
  • GPS (Ground)
    In this case someone will take a geodetic survey grade GPS or other, similar, device to a location and take a reading of their height. The more satellites are read the more accurate the reading. Of course satellites only orbit the Earth slowly so this process can take quite a few hours. Later the data will be processed and combined with information from other GPS receivers in static (known) locations. In this manner height readings to within a fraction of a centimetre can be obtained.

 Note. Heights taken from the ground are Black on a map and those from the air are Orange.
The height shown is for the Spot Height marked with a Dot on the map.

Maps contain a great deal of information and can become cluttered so sometimes the Spot Height Dots can be quite difficult to distinguish.
In this case – look between the F and E in Fell and on the bridleway above the 2 in 492

Heights are rounded up and down
The height information shown on standard OS maps is only shown in meters. Of course this will hardly ever be exact as heights are rounded to the nearest meter. For example the most recent survey of Ben Nevis in 2016 gave the height (of the highest natural point) of 1344.527m. After rounding all standard maps show this as 1,345m.

 A geodetic survey grade GPS

Oh and just out of interest (and as I had some time to spare) I looked through the Ordinance Survey’s list of 517,011 Lower Order Benchmarks.  Not one single height was recorded as being exactly to the meter, but 30 were to within 1mm and 59 were to within 2mm. 

These were all manually calculated in the 1940’s, 50’s and 60’s from spirit level readings, whether this level of accuracy was actually possible at that time would take another blog to discuss !!