The Ordnance Survey
The first Ordnance Survey map
This map (right) was the result of the first series of surveys made by the newly-founded Ordnance Survey in the 1790s. The Royal Observatory was used by the Survey as a triangulation point and a tower was constructed over what is now the Meridian Building to support their instruments. The scale is 1 inch to 1 mile.
To commemorate 200 years since the map was published, it was displayed during 2001 and 2002 alongside much earlier maps from our collections, and the instruments used to make them. The Observatory provided a key site in the survey as the position of longitude zero.
A period of solutions
The second-half of the 18th century saw the solution of several long-standing problems. Harrison completed 'H4', the first marine timekeeper in 1759. The sextant was invented in the 1750s for measuring Moon and star positions on board ships and the Nautical Almanac was first published in 1766. Each helped solve the problem of finding longitude at sea. Cook's voyages between 1768 and 1780 contributed greatly to improvements in the accuracy of sea charts. Land maps, however, were still very misleading.
Land surveying was originally carried out by landowners, using small tools of limited accuracy to map their estates. The Ordnance Survey, which began in 1791, was the first to produce detailed maps of Britain. A surveying method called triangulation was used. In this method, the land is divided into adjoining triangles. A baseline and two angles are carefully measured and used to calculate distance. A key point for measuring these angles was the Bradley Meridian here at the Royal Observatory in Greenwich. A tower was built over the Bradley transit instrument and a theodolite set up on top.
Making maps
Once the survey had been completed the draughtsmen transferred the information to the copper plates ready for printing. The design was engraved onto sheets of polished copper coated with wax, using a sharp, needle-like tool called a burin.
Plate-making was a hierarchical process and an individual's contribution to the plate was dependent on experience. First the coastal areas, roads and buildings were outlined. Next vegetation and depth elements were added to give the plate its third dimension. Finally the lettering was added. This was the most skilled job since the plate was a mirror image and all text had to be written in reverse.
The plate was then coated with acid, which permanently etched all the exposed areas of copper: those still coated with wax remained unmarked. When ink was wiped over the cleaned plate it adhered only to the etched lines. Damp paper was then applied under pressure, producing the first positive image of the map.
On display
The Great Ramsden Theodolite, 1791
This is the theodolite used in the original ordnance survey. It was built by the London instrument maker Jesse Ramsden and the base measures 3-ft (910 mm) in diameter. To carry out observations at Greenwich, a 38-ft (12m) tower was built over the Bradley transit instrument and this theodolite was placed on it.
Waywiser, by Benjamin Martin, about 1750
Sometimes called the surveyor's wheel, this type of instrument dates back to Roman times and is used for measuring roads. Each revolution of the wheel is recorded on a large dial, allowing distances to be measured.
Czech Astrolabe by Erasmus Habermel, about 1585
An astrolabe is primarily an astronomical tool but, with the addition of a compass, it could be used flat to measure angles on land. The first tool to measure the angles needed for surveying by triangulation was a specially adapted astrolabe.
French recipiangle, 1607
A recipiangle is a tool for measuring angles. It consists of a protractor and two pivoted arms, each with sights. The sights are lined up with particular landmarks and the angle between them can then be read-off from the protractor.
Britannia, by Joan Blaeu, 1672
Joan Blaeu was the official cartographer to the Dutch East India Company from 1638 to 1673, having succeeded his father, Willem Janszoon, in the post. As well as atlases, the family business also produced and sold globes and mathematical instruments.
D5255/5
Parallel rule, 1750
A parallel rule is used to draw parallel lines. This silver engraved example was probably designed more for show than practical technical drawing.
NAV0598
John Troughton's Greenwich Transit Instrument, a print published in 1828
When the Ordnance Survey began in 1791, this transit instrument, situated on what we now call the Bradley Meridian, marked longitude 0o for all of Britain. A 12m (38 ft) tower was built above this instrument to carry the Ramsden theodolite, with which fundamental measurements of the Survey were made.
B655
The Airy Transit Circle; from The Graphic, 11 December 1880
The Airy Transit Circle was installed in 1851, moving the position of the Greenwich Meridian for the last time. To take account of this in the Ordnance Survey maps would mean re-surveying the whole of Britain. For this reason, Bradley's meridian, not Airy's – which is 5.9 m (19 ft) east – remains the 'longitude zero' on these maps.
A5005
Theodolite by Humphrey Cole, 1574
This is the earliest known example of a theodolite – an instrument which enables both the vertical and horizontal angles between two points to be read at the same time. Used mainly in surveying, the theodolite allows the size of a given area of land to be easily calculated. It includes a compass and plumb line, enabling it to be correctly positioned for use.
NAV1448 - A0082
Theodolite by Jonathon Sisson, 1737
By the mid-18th century the theodolite was rapidly being recognized as the most important surveying instrument. Sisson improved it by adding a telescope instead of the traditional open sights, reducing the margin for error during observation.
NAV1451 - D9382_1
German trigometre, 1595
Survey sightings were taken from either end of a chosen baseline to a common point, a method called triangulation. The central sliding scale of this instrument was used to measure the angles between the triangulation points and then apply those measurements directly to a drawing.
NAV1475 - 8730
Graphometer by Choizy, Paris, 1667
This instrument was an early form of theodolite with which surveyors could measure angles between two specific points. This was usually done horizontally, although this example has a ring allowing it to be suspended in order to measure vertical angles. The compass enabled the instrument to be accurately positioned.
NAV1409 - 8732
Surveyor's quadrant by J. Sisson, London, about 1730
The quadrant was originally an instrument to measure the angular height of a celestial body. Since altitude was also an essential factor in land measurement it became a popular tool for the surveyor, although less so as use of the theodolite increased. This example has a compass for determining direction and a geared movement to allow fine adjustment.
NAV1443 - C4839/4
Circumferentor by Butterfield, Paris, about 1690
This instrument was often referred to as a 'surveyor's compass'. It comprises a compass fitted with sights so that bearings to be taken with reference to magnetic north. This type of instrument was later widely used in mine surveying.
NAV1373
Set of drawing instrument by George Adams, London, about 1780
Following in father's trade, Adams was a leading instrument maker in 18th century London. These are steel and ebony and fit into the silver shagreen (or rayskin) . Sets like this were used by mathematicians, architects and navigators as well as surveyors.
NAV0651 - D1865-2