IHAG ver
null
First Ordnance Survey Map of Devon - 1809

Early Ordnance Survey Mapping

Land Surveying

 

The profession of land surveying is referred to in the earliest texts with references to land ownership (The Iliad), the marking of property boundaries (modern Israel), re-establishment of boundaries (Egypt, after Nile floods), setting out of towns (Greece) and determination of the size of the Earth (Greeks in Egypt). Romans used Agrimensors to set out their forts and their roads.

 

The modern equivalents of these still exist and the work may be undertaken by land surveyors or other specialists (geodesists, civil engineers) who concentrate on one small part of the topic.

 

When Francis Drake needed to determine a gentle waterflow from the River Meavy (the weir & take-off are now under Burrator Reservoir) to Plymouth, he employed Robert Lampan (Hemery, 1986), who found a route with a gradient steep enough for the water to flow downhill but not so steep as to run out of height before reaching Plymouth. Along its route, Drake had sufficient power to run 6 mills. In doing so it crossed out of one valley (the Meavy) at a height that would bring it down to Millbay. This crossing of a watershed is a finely balanced achievement.

 

Many leats were dug around the country, to supply water for power, for washing (as in the leat on the left bank of the Erme for the Mill at Ivybridge) or for drinking water (possibly the upper leat on the right bank of the Erme). One that clearly displays a slow descent is Kings Gutter, which nearly follows various contour lines but gradually drops down as it makes its way towards Dinnaton

Kings Gutter2
King’s Gutter, Longtimber Woods, Ivybridge

Land surveying used new technologies as they became available:

  • Directions: telescopes; graduated horizontal and vertical circles to determine directions and deduce angles, first in theodolites and now in modern Total Stations;
  • Distances: tapes, chains (e.g. Gunters Chain), glass rods, the use of microwaves, infra-red light for land-based, portable instruments, measuring from satellite transmissions or the random noise from Pulsars;
  • 3D Position: Transit satellites, GPS (and other systems from other nations including the EU’s Galileo)
  • Object Detection: Laser Scanners and satellite-based remote sensing

 

Height measurement also uses a telescope, coupled with a vertical, telescopic staff to measure differences in height. GPS can also be used for this too.

 

Land surveying was important for the construction of the canals, road improvements, the building of the railways, and the developments of new towns. With the Enlightenment and the early Industrial Revolution, there was significant research into the size and shape of the Earth, the nature of sea level, and what may be usefully chosen as points of reference when processing field data into a series of maps.

 

The profession continues to change and develop with new technologies and improved mathematics.

William Mudge & Thomas Colby

Mark Cardale, a descendent of the Mudge family, wrote,

 

“William Mudge was born in Plymouth where his grandfather the Reverend Zachariah Mudge had established the family when he became the vicar of St Andrew’s in Plymouth in 1732. This Zachariah became well known as a theologian and preacher, and three of his sons also achieved renown, as
  • a physician (doctor)/instrument maker (John),
  • a watch and clock maker (Thomas), and
  • a musical composer (Richard).
Thomas made clocks for the English and Spanish royal families, and competed with John Harrison for the Longitude Prize for marine chronometers. The Mudges made connections with many prominent people in Plymouth and beyond, including the family of Joshua Reynolds in particular, and William Cookworthy. Through Joshua Reynolds they came to know Samuel Johnson, creator of the first English dictionary, who became William’s godfather. Another of the Reverend Zachariah’s grandsons, also called Zachariah, became an Admiral and settled in Plympton in the early 1800s, and the family continued to live in Plympton for the next 150 years or so.”
(Cardale, 2017)

 

William Mudge was the effective leader of the Ordnance Survey (OS) from 1791, its principal surveyor 1791-1820, and its official leader from 1798. On his retirement, he was promoted to Major General.

 

Thomas Colby succeeded Mudge as leader of the Ordnance Survey. He was very experienced, having worked under Mudge from 1802. He led and surveyed in Ireland from 1824 to 1838 (it was completed in 1847).

 

Mudge had encouraged Colby to work with the French scientist, Jean-Baptiste Biot (though that had an acrimonious ending and Mudge’s son, Richard, was important in continuing the international relationship). Colby completed the triangulation of northern England and then Scotland. He established the vertical reference datum in Liverpool, though this is now succeeded by a reference in Newlyn Cornwall. Colby’s campaign to provide height information for the country, to supplement Mudge’s measurements in the horizontal plane led to teams levelling all across England and Wales, including Ivybridge and Butterton (stet) Hill.

Triangulation, with reference to Kit Hill, Eddystone Rocks and Butterdon Hill

 

To gather the data for a map (one aspect of ‘land surveying’), it is best to start with a control network, and visible reminders of the later Re-triangulation of Great Britain (1935-1962) remain today on many hilltops. The triangles fix a horizontal framework and much time was spent in determining directions from one apex to all others in view and from these to derive angles. All three angles of a triangle had to be determined because, at variance from a triangle on the plane, the three angles on a sphere add up to more than 180 degrees. This can be imagined with a simple thought experiment, taking two lines from the North Pole, one drawn South along the Greenwich Meridian to the Equator, and the other separated by 90 degrees travelling south from the North Pole via Lake Superior and eastern Mexico to the Galapagos Islands on the Equator. Closing the triangle along the Equator gives 270 degrees within the triangle. Mudge’s triangles had a much smaller “spherical excess” yet each value had to be determined. As a result, any error due to the observations themselves could be minimised.

 

Determining the three angles of a triangle establishes its shape but not its size. Scaling of each and every triangle is achieved by measuring one side of one triangle but as adjacent triangles have sides in common, the computed scale carries through the network.

 

The rigorous mapping of the United Kingdom (which included Ireland from 1801) was built on Mudge’s (and later, Colby’s) triangles and then infill mapping. It was the triangulation of southern England and Wales which was principally Mudge’s work, with the whole country triangulated between 1791 and 1841. Major-General Thomas Colby (who was attached to the OS from 1802, and led the OS from 1820-1846) took over the triangulation from 1820 and instituted the first precise levelling of the country.

 

Precisely measuring distances greater than 100m was particularly difficult prior to the electronics developed in the 1950s. Measuring baselines was extremely careful work involving Ramsden’s Chains and later 18’ glass rods, trestle legs, and shade against the elements. William Roy measured the Hounslow Base in 1784, Mudge in 1791, and by 1858 it had been measured eleven times. Mudge measured a check base on Salisbury Plain in 1794, at Sedgemoor, and others.

 

From a reconnaissance in 1792, the triangulation was extended from the Hounslow Base through to Land’s End in 1797 . In October 1795, Mudge observed directions at Furland, determining angles between Butterton (very close to the pillar on the hill now named Butterdon) and Bolt Head and Rippin (now Rippon) Tor. At Butterton he derived six angles. In 1796, he took further observations to Butterton from Kit Hill, Maker Heights, Bolt Head, Rippin Tor and Carraton (now Caradon) Hill.

Butterdon Hill
Butterdon Hill is located close to Western Beacon on the southern edge of Dartmoor near Ivybridge and is marked by a cairn of stones gathered from the moor. 

 

A single stone row runs for over 2km along a ridge between Butterdon Hill and Piles Hill and is the second longest stone alignment on Dartmoor.
Benchmark at Butterdon Hill

The current concrete Butterdon pillar stems from the Re-triangulation of Great Britain but the 1795 point is a nearby mark on the ground. Butterton was also used for a local meridian (Wheeler, 2013) (this being a North/South line, which in theory extends to the poles), and a perpendicular to this. The latitude of the point was determined (Mudge, 1801) as 50o 24’ 46.3” North of the Equator, and the longitude as 3o 52’ 47.5” West of Greenwich. Mudge chose to observe six meridians spread across southern England at intervals of about sixty miles with Black Down, Somerset to the east, and St Agnes, Cornwall to the west (Adams, 1994). Being both a geographical latitude and longitude and an orientation, they were used in the drawing of detail on the individual map sheets.

New Bridge
Land Surveying tab

Benchmarks

 

Thomas Colby took over the leadership of the Ordnance Survey in 1820, and the First Geodetic Levelling of England and Wales was undertaken between 1840 and 1860. Of the 184 levelling lines, one passed through Ivybridge, possibly in 1848.

 

Benchmarks used the broad arrow design, sometimes with a circular mark, often with a horizontal line for the referenced height. At New Bridge, Ivybridge, this incorporates a bolt (now well-worn) in the horizontal line. At the Methodist Church, the simple cut mark is clear to see. This design is the most commonplace (about 500,000 across the UK) and there are perhaps a dozen examples in Ivybridge. The Flush Bracket is less common (there were about 13,000 across the UK) with two in Ivybridge (both post 1921), one at Fore Street (No. 11025) and another on the northern side of the Anglican Church (No. S4301).

 

Not all those from the 1840s have survived, with the railway crossing at Stowford Bridge (established by 1848 (MacDermot, 1927)) being the most obvious. On the 1885/7 mapping the Benchmark is clearly shown on the centre line of the railway and this matches with the description. However, the railway is Brunel’s original 7’1/4” gauge (1838-1892; change of gauge in the Plymouth area, 1892). The railway line was originally single track being doubled in 1893 (but only connected with the replacement of the viaduct in 1894 (Wikipedia_Disused Railway Stations, 2017)). By the time of the 1905/6 mapping, the line is shown doubled and is standard gauge. The bridge must have changed to the present cast iron structure. The Benchmark has gone.

Benchmark tab

The First Ordnance Survey Map of Devon, 1809

 

Maps are printed in a variety of scales, from maps of the world to plans of houses. A map of Great Britain at 1:1,000,000 is termed, “small scale”, arising from the fractional display of the value: 1/1,000,000. A “large scale” map may be 1/2,500 and show local streets. The first Ordnance Survey maps were printed at 1:63,360 (!), because, for 1” to represent 1 mile there are 1,760 yards in a mile, 5,280 feet in a mile, and 63,360 inches.

 

The military connection with these maps is in the name, Ordnance Survey, where ordnance refers to artillery and military stores. William Roy’s pre-OS maps were undertaken to support patrols in Scotland but by 1803, Britain was at war with France. Understanding our own country was seen as a first line of defence and maps of the south coast were undertaken first. Nevertheless, scientific cooperation continued and joint operations with the French were made in 1813, two years before Waterloo.

 

The first map was of Kent but privately published in 1801, the first by the OS of Essex in 1805, and the map of Devon in 1809. To cover such a large county at the medium scale of 1: 63,360, takes seven sheets (21-27). Sheet 24 covers Ivybridge. It is accessible from http://nla.gov.au/nla.obj-231919894/view and you must pan to the NE and zoom in. You can move north, west and east using the green arrows.

A section of the first Ordnance Survey Map of Devon, Sheet XXIV, 1809.
Reproduced with the permission of the National Library of Australia  https://nla.gov.au

You might think the map shows little, but the mill is there; the railway isn’t as it didn’t arrive until 1848; the Ivy Bridge is there (since at least 1250) but New Bridge is not (1834). Factory Bridge, removed by the building of the current A38, still exists being clearly met by Park Street and Keaton Road. The road at Bittaford does not include the current viaduct and diverts up the valley to cross the stream; although more difficult to discern, and requiring sheet 25 too, the current road from Bittaford to South Brent has not been constructed and the route climbs up the hill – the old road to Wrangaton (Wrangerton).

Benchmarks in the vicinity of the Stowford railway bridge on the 1885 mapping
Reproduced with the permission of the National Library of Scotland  https://maps.nls.uk/index.html
Benchmarks in the vicinity of the Stowford railway bridge on the 1905/6 mapping
Reproduced with the permission of the National Library of Scotland  https://maps.nls.uk/index.html
References
Adams B, 1994, Parallel to the meridian of Butterton Hill – do I laugh or cry, Sheetlines, 38, pp15-18 https://www.charlesclosesociety.org/files/Issue38page15.pdf
Cardale M, 2017, Personal Communication
Hemery E, 1986, Walking the Dartmoor Waterways: a guide to retracing the leats and canals of Dartmoor, Maps by Ethan Danielson, David & Charles, Newton Abbot, ISBN 0-7153-8627-1
James H, 1861, Abstracts of the Principal Levelling Lines of Spirit Levelling in England and Wales, Ordnance Survey,
https://www.deformedweb.co.uk/trigs/data/1GL/1GLA_074.png
and
https://www.deformedweb.co.uk/trigs/data/1GL/1GLA_084.png
MacDermot ET, 1927, History of the great Western Railway, Volume 1 1833-1863, London, Great Western Railway
Mercator P, 2010, Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=37442201
Mercator P, 2014, Licensed under the Creative Commons Attribution-Share Alike 4.0 International licence, being a scan by Peter Mercator from a report on the principal lines of spirit levelling, 1860
Mudge W, 1801, A Trigonometrical Survey of England and Wales, 1797-1799, https://ia800401.us.archive.org/17/items/bub_gb_wWcOAAAAYAAJ_2/bub_gb_wWcOAAAAYAAJ.pdf
Williams, Mudge and Dalby, 1797, An Account of the Trigonometrical Survey, Carried on in the Years 1795, and 1796, by Order of the Marquis Cornwallis, Master General of the Ordnance. By Colonel Edward Williams, Captain William Mudge, and Mr. Isaac Dalby, 1795, Philosophical Transactions of the Royal Society of London (1776-1886). 1797-01-01. 87:432–541, pp436, 437 http://rstl.royalsocietypublishing.org/content/87/432.full.pdf+html

EARLY ORDNANCE SURVEY MAPPING

Land Surveying

 

The profession of land surveying is referred to in the earliest texts with references to land ownership (The Iliad), the marking of property boundaries (modern Israel), re-establishment of boundaries (Egypt, after Nile floods), setting out of towns (Greece) and determination of the size of the Earth (Greeks in Egypt). Romans used Agrimensors to set out their forts and their roads.
The modern equivalents of these still exist and the work may be undertaken by land surveyors or other specialists (geodesists, civil engineers) who concentrate on one small part of the topic.
When Francis Drake needed to determine a gentle waterflow from the River Meavy (the weir & take-off are now under Burrator Reservoir) to Plymouth, he employed Robert Lampan (Hemery, 1986), who found a route with a gradient steep enough for the water to flow downhill but not so steep as to run out of height before reaching Plymouth. Along its route, Drake had sufficient power to run 6 mills. In doing so it crossed out of one valley (the Meavy) at a height that would bring it down to Millbay. This crossing of a watershed is a finely balanced achievement.
Many leats were dug around the country, to supply water for power, for washing (as in the leat on the left bank of the Erme for the Mill at Ivybridge) or for drinking water (possibly the upper leat on the right bank of the Erme). One that clearly displays a slow descent is Kings Gutter, which nearly follows various contour lines but gradually drops down as it makes its way towards Dinnaton.
Land surveying used new technologies as they became available:
  • Directions: telescopes; graduated horizontal and vertical circles to determine directions and deduce angles, first in theodolites and now in modern Total Stations;
  • Distances: tapes, chains (e.g. Gunters Chain), glass rods, the use of microwaves, infra-red light for land-based, portable instruments, measuring from satellite transmissions or the random noise from Pulsars;
  • 3D Position: Transit satellites, GPS (and other systems from other nations including the EU’s Galileo)
  • Object Detection: Laser Scanners and satellite-based remote sensing
Height measurement also uses a telescope, coupled with a vertical, telescopic staff to measure differences in height. GPS can also be used for this too.
Land surveying was important for the construction of the canals, road improvements, the building of the railways, and the developments of new towns. With the Enlightenment and the early Industrial Revolution, there was significant research into the size and shape of the Earth, the nature of sea level, and what may be usefully chosen as points of reference when processing field data into a series of maps.
The profession continues to change and develop with new technologies and improved mathematics.

William Mudge & Thomas Colby

 

Mark Cardale, a descendent of the Mudge family, wrote,
“William Mudge was born in Plymouth where his grandfather the Reverend Zachariah Mudge had established the family when he became the vicar of St Andrew’s in Plymouth in 1732. This Zachariah became well known as a theologian and preacher, and three of his sons also achieved renown, as
  • a physician (doctor)/instrument maker (John),
  • a watch and clock maker (Thomas), and
  • a musical composer (Richard).
Thomas made clocks for the English and Spanish royal families, and competed with John Harrison for the Longitude Prize for marine chronometers. The Mudges made connections with many prominent people in Plymouth and beyond, including the family of Joshua Reynolds in particular, and William Cookworthy. Through Joshua Reynolds they came to know Samuel Johnson, creator of the first English dictionary, who became William’s godfather. Another of the Reverend Zachariah’s grandsons, also called Zachariah, became an Admiral and settled in Plympton in the early 1800s, and the family continued to live in Plympton for the next 150 years or so.” (Cardale, 2017)
William Mudge was the effective leader of the Ordnance Survey (OS) from 1791, its principal surveyor 1791-1820, and its official leader from 1798. On his retirement, he was promoted to Major General.
Thomas Colby succeeded Mudge as leader of the Ordnance Survey. He was very experienced, having worked under Mudge from 1802. He led and surveyed in Ireland from 1824 to 1838 (it was completed in 1847).
Mudge had encouraged Colby to work with the French scientist, Jean-Baptiste Biot (though that had an acrimonious ending and Mudge’s son, Richard, was important in continuing the international relationship). Colby completed the triangulation of northern England and then Scotland. He established the vertical reference datum in Liverpool, though this is now succeeded by a reference in Newlyn Cornwall. Colby’s campaign to provide height information for the country, to supplement Mudge’s measurements in the horizontal plane led to teams levelling all across England and Wales, including Ivybridge and Butterton (stet) Hill.

Triangulation, with reference to Kit Hill, Eddystone Rocks and Butterdon Hill

 

To gather the data for a map (one aspect of ‘land surveying’), it is best to start with a control network, and visible reminders of the later Re-triangulation of Great Britain (1935-1962) remain today on many hilltops. The triangles fix a horizontal framework and much time was spent in determining directions from one apex to all others in view and from these to derive angles. All three angles of a triangle had to be determined because, at variance from a triangle on the plane, the three angles on a sphere add up to more than 180 degrees. This can be imagined with a simple thought experiment, taking two lines from the North Pole, one drawn South along the Greenwich Meridian to the Equator, and the other separated by 90 degrees travelling south from the North Pole via Lake Superior and eastern Mexico to the Galapagos Islands on the Equator. Closing the triangle along the Equator gives 270 degrees within the triangle. Mudge’s triangles had a much smaller “spherical excess” yet each value had to be determined. As a result, any error due to the observations themselves could be minimised.
Determining the three angles of a triangle establishes its shape but not its size. Scaling of each and every triangle is achieved by measuring one side of one triangle but as adjacent triangles have sides in common, the computed scale carries through the network.
The rigorous mapping of the United Kingdom (which included Ireland from 1801) was built on Mudge’s (and later, Colby’s) triangles and then infill mapping. It was the triangulation of southern England and Wales which was principally Mudge’s work, with the whole country triangulated between 1791 and 1841. Major-General Thomas Colby (who was attached to the OS from 1802, and led the OS from 1820-1846) took over the triangulation from 1820 and instituted the first precise levelling of the country.
Precisely measuring distances greater than 100m was particularly difficult prior to the electronics developed in the 1950s. Measuring baselines was extremely careful work involving Ramsden’s Chains and later 18’ glass rods, trestle legs, and shade against the elements. William Roy measured the Hounslow Base in 1784, Mudge in 1791, and by 1858 it had been measured eleven times. Mudge measured a check base on Salisbury Plain in 1794, at Sedgemoor, and others.
From a reconnaissance in 1792, the triangulation was extended from the Hounslow Base through to Land’s End in 1797. In October 1795, Mudge observed directions at Furland, determining angles between Butterton (very close to the pillar on the hill now named Butterdon) and Bolt Head and Rippin (now Rippon) Tor. At Butterton he derived six angles. In 1796, he took further observations to Butterton from Kit Hill, Maker Heights, Bolt Head, Rippin Tor and Carraton (now Caradon) Hill.
The current concrete Butterdon pillar stems from the Re-triangulation of Great Britain but the 1795 point is a nearby mark on the ground. Butterton was also used for a local meridian (Wheeler, 2013) (this being a North/South line, which in theory extends to the poles), and a perpendicular to this. The latitude of the point was determined (Mudge, 1801) as 50o 24’ 46.3” North of the Equator, and the longitude as 3o 52’ 47.5” West of Greenwich. Mudge chose to observe six meridians spread across southern England at intervals of about sixty miles with Black Down, Somerset to the east, and St Agnes, Cornwall to the west (Adams, 1994). Being both a geographical latitude and longitude and an orientation, they were used in the drawing of detail on the individual map sheets.

Benchmarks

 

Thomas Colby took over the leadership of the Ordnance Survey in 1820, and the First Geodetic Levelling of England and Wales was undertaken between 1840 and 1860. Of the 184 levelling lines, one passed through Ivybridge, possibly in 1848.
Benchmarks used the broad arrow design, sometimes with a circular mark, often with a horizontal line for the referenced height. At New Bridge, Ivybridge, this incorporates a bolt (now well-worn) in the horizontal line. At the Methodist Church, the simple cut mark is clear to see. This design is the most commonplace (about 500,000 across the UK) and there are perhaps a dozen examples in Ivybridge. The Flush Bracket is less common (there were about 13,000 across the UK) with two in Ivybridge (both post 1921), one at Fore Street (No. 11025) and another on the northern side of the Anglican Church (No. S4301).
Not all those from the 1840s have survived, with the railway crossing at Stowford Bridge (established by 1848 (MacDermot, 1927)) being the most obvious. On the 1885/7 mapping the Benchmark is clearly shown on the centreline of the railway and this matches with the description. However, the railway is Brunel’s original 7’1/4” gauge (1838-1892; change of gauge in the Plymouth area, 1892). The railway line was originally single track being doubled in 1893 (but only connected with the replacement of the viaduct in 1894 (Wikipedia_Disused Railway Stations, 2017)). By the time of the 1905/6 mapping, the line is shown doubled and is standard gauge. The bridge must have changed to the present cast iron structure. The Benchmark has gone.

The first Ordnance Survey Map of Devon, 1809

 

Maps are printed in a variety of scales, from maps of the world to plans of houses. A map of Great Britain at 1:1,000,000 is termed, “small scale”, arising from the fractional display of the value: 1/1,000,000. A “large scale” map may be 1/2,500 and show local streets. The first Ordnance Survey maps were printed at 1:63,360 (!), because, for 1” to represent 1 mile there are 1,760 yards in a mile, 5,280 feet in a mile, and 63,360 inches.
The military connection with these maps is in the name, Ordnance Survey, where ordnance refers to artillery and military stores. William Roy’s pre-OS maps were undertaken to support patrols in Scotland but by 1803, Britain was at war with France. Understanding our own country was seen as a first line of defence and maps of the south coast were undertaken first. Nevertheless, scientific cooperation continued and joint operations with the French were made in 1813, two years before Waterloo.
The first map was of Kent but privately published in 1801, the first by the OS of Essex in 1805, and the map of Devon in 1809. To cover such a large county at the medium scale of 1: 63,360, takes seven sheets (21-27). Sheet 24 covers Ivybridge. It is accessible from http://nla.gov.au/nla.obj-231919894/view and you must pan to the NE and zoom in. You can move north, west and east using the green arrows.
You might think the map shows little, but the mill is there; the railway isn’t as it didn’t arrive until 1848; the Ivy Bridge is there (since at least 1250) but New Bridge is not (1834). Factory Bridge, removed by the building of the current A38, still exists being clearly met by Park Street and Keaton Road. The road at Bittaford does not include the current viaduct and diverts up the valley to cross the stream; although more difficult to discern, and requiring sheet 25 too, the current road from Bittaford to South Brent has not been constructed and the route climbs up the hill – the old road to Wrangaton (Wrangerton).

References

 

Adams B, 1994, Parallel to the meridian of Butterton Hill – do I laugh or cry, Sheetlines, 38, pp15-18 https://www.charlesclosesociety.org/files/Issue38page15.pdf
Cardale M, 2017, Personal Communication
Hemery E, 1986, Walking the Dartmoor Waterways: a guide to retracing the leats and canals of Dartmoor, Maps by Ethan Danielson, David & Charles, Newton Abbot, ISBN 0-7153-8627-1
James H, 1861, Abstracts of the Principal Levelling Lines of Spirit Levelling in England and Wales, Ordnance Survey,
https://www.deformedweb.co.uk/trigs/data/1GL/1GLA_074.png
and
https://www.deformedweb.co.uk/trigs/data/1GL/1GLA_084.png
MacDermot ET, 1927, History of the great Western Railway, Volume 1 1833-1863, London, Great Western Railway
Mercator P, 2010, Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=37442201
Mercator P, 2014, Licensed under the Creative Commons Attribution-Share Alike 4.0 International licence, being a scan by Peter Mercator from a report on the principal lines of spirit levelling, 1860
Mudge W, 1801, A Trigonometrical Survey of England and Wales, 1797-1799, https://ia800401.us.archive.org/17/items/bub_gb_wWcOAAAAYAAJ_2/bub_gb_wWcOAAAAYAAJ.pdf
Williams, Mudge and Dalby, 1797, An Account of the Trigonometrical Survey, Carried on in the Years 1795, and 1796, by Order of the Marquis Cornwallis, Master General of the Ordnance. By Colonel Edward Williams, Captain William Mudge, and Mr. Isaac Dalby, 1795, Philosophical Transactions of the Royal Society of London (1776-1886). 1797-01-01. 87:432–541, pp436, 437 http://rstl.royalsocietypublishing.org/content/87/432.full.pdf+html