Arniston House and the origins of Ordnance Survey maps

It could be considered that the origins of Ordnance Survey maps that we love and rely on, can be traced to Arniston House and the Dundas family.

It could be considered that the origins of Ordnance Survey maps that we love and rely on, can be traced to Arniston House and the Dundas family.

The age of the Scottish Enlightenment was the late 1600’s and early 1700’s.  During this time the rich and educated took a keen interest in everything scientific.  They craved accuracy in everything they did and spent a lot of money investing in new scientific instruments.  Maps were destined to be based on real measurements rather than panoramic sketches.

In 1712 Robert Dundas of Arniston, age 27, a lawyer and Solicitor General for Scotland, married Elizabeth Watson.  At the time of their marriage, both Elizabeth’s parents were dead.  Her youngest brother, David Watson, was only 8, and came to stay with them at Arniston.  Robert Dundas took a keen interest in geography and amassed a large collection of maps, globes and surveying instruments. Young David was also fascinated by this collection and learned how to use them to make his own maps.

A few years later in his late teens, David Watson joined the British army, spending most of the next twenty years on the continent much of the time as an engineer for the board of Ordnance.  After leaving the army, Robert Dundas helped David, now in his early thirties, move to the Board of Ordnance in London.  This had a small section devoted to map making.

Shortly before the battle of Culloden, Hanoverian commanders complained that their maps of the northwest highlands were useless – features, roads and names were often wrong or non-existent.  This was one of the reasons they took two months after Culloden trying to catch up with Bonnie Prince Charlie and Lord Lovett.  In 1747, a year after the battle of Culloden, David Watson persuaded the Duke of Cumberland that their maps of the Highlands were not fit for purpose.

This was the trigger for a national military map, initially of Scotland and later of England’s south coast. This is how it came about and another famous name creeps into the story.

William Roy, age 21, and his father worked as factors on an estate near Carluke.  William had no training in maths. The estate they worked on had originally been owned by Robert Dundas’s second wife’s father. The new landowners at Carluke decided they needed a detailed map of their estate.  Possibly at a family lunch when the Dundases visited Hallcraig, David Watson was introduced to the enthusiastic young William Roy then aged just 21.  David, working as he was for the Board of Ordnance, (and, remember, stepson of Robert Dundas of Arniston), employed William Roy.  He was given the job of mapping the Highlands of Scotland and later the Scottish Lowlands.  On completion four years later, he moved to London and moved onto mapping the coastal regions of the south of England.  Within seven years of his death, his successor, Charles Lennox, persuaded King George the 3rd to fund mapping of the nation using the new triangulation system.  This is the beginnings of Ordnance mapping as we now know it.

So in summary, Robert Dundas of Arniston’s interest in maps and surveying instruments rubbed off on his young stepbrother who later employed William Roy.  He created the most accurate and detailed maps ever seen of much of Britain.  It was the vision of David Watson to produce a map of the country that eventually came to be at the hands of William Lennox starting in 1791.

A footnote on Board of Ordnance mapping

When William Roy created his maps of the Scottish Highlands they used a chain to measure straight lines.  At a corner in a track he recorded the angle to the next straight line.  This meant that measurement errors gradually increased.  By the time he reached the lowlands the error had reached something in the order of 20 miles.  The triangulation method that was introduced 30 years later, meant the for the most part, that errors tended to cancel.

In 1784 William Roy laid down the trigonometrical baseline (5.2 miles) on Hownslow Heath   (now the edge of Heathrow Airport) initially using wooden rods and later glass rods which were less affected by temperature and moisture.  This was used to create a triangulated map between Greenwich and the Paris Observatory.  This would allow the map makers to provide sailors with significantly more accurate charts.  The baseline was re-surveyed by Charles Lennox in preparation for creating the first Board of Ordnance maps.


When the full triangulation of Britain was started, the original baseline was used as the starting point.  In 1794, a second baseline of 7 miles was accurately measured on Salisbury Plain by Mudge & Dalby to check the accuracy of the first stage of triangulation.  Measurement of the new baseline confirmed their triangulation to have been “very accurate”.  In general, any triangulation errors tend to cancel themselves out whereas with the original end to end measurements plus measurements of angles (Roy’s original technique) the errors tend to be cumulative.  While Roy and later Mudge & Dalby measured very large triangles, a separate team of surveyors, created smaller triangles and noted the small detail topography producing useful maps as we now know them.  On the triangulation survey, the surveyors marked their theodolite points with a small pile of stones (cairns) which they asked the locals not to disturb.  This original triangulation survey remained the basis for all OS mapping until the 1935 when the small cairns began to be replaced by the modern concrete obelisks.

Ian Brown

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