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
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.
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.
