A map has been described as a graphic representation of features on a plane surface that appear on the curved surface of the Earth. However, before the cartographer (mapmaker) can design and produce a map, a surveyor has to collect the basic data.
The Earth is described as a spheroid, being slightly flattened at both ends near the Poles, and surveyors' measurements need to be reduced to the map projection plane.
Natural and man-made features, such as rivers, roads, buildings, boundaries and hills have to be observed, measured and recorded. Over the centuries, surveyors have used chains, theodolites, tapes, telescopes, spirit levels and lasers to achieve this. In the twenty-first century, new technologies like satellite positioning systems and computerisation are altering the way that surveyors work.
Before any survey can take place, some form of control must be laid down.
In the past, a network of triangles covered the area to be surveyed. Points of reference with known positions were situated at the corner of each triangle, allowing the surveyor to take detailed measurements. In Great Britain, these fixed points are marked by the four-feet-high concrete pillars often seen on hilltops or in the corner of fields. A theodolite - which measures angles between points - was fitted on top of the pillar to determine their relative positions.
The advent of satellite technology has greatly improved the accuracy and speed of establishing basic mapping control networks, enabling a position to be fixed within centimetres. This network ensures the particular projection being used to present the map data is of the right size, orientation and position to the Earth. The projection employed for Great Britain is the Transverse Mercator. The science of studying the shape of the Earth is called Geodesy; hence the Primary Network of a large area is called a geodetic network.
Having established control, the surveyor fits the details on the ground into the network provided by the control points. The calculation of surveys takes account of the different accuracy and methods of survey and control.
Filling the detail
A topographical survey records the physical features of the Earth showing their relative position both horizontally and vertically. Several surveying techniques are employed to do this:
Photogrammetry and Air Photography
In some surveys up to 80% of ground detail is collected using aerial photography. Specially modified aircraft mounted with sophisticated cameras fly over the designated area. Each photograph (approximately 23 cm by 23 cm) overlaps the next by 60%. When developed, two adjacent prints are viewed through the binocular eyepiece of a stereo-plotting machine. Because of the reaction of our eyes and brain, the image appears three-dimensional. A floating mark in the eye-piece, represented by a black dot, can be manipulated to trace detail onto the survey document which is then returned to the surveyor for the addition of detail not recorded by the camera. Because of the stereo effect it is also possible for accurate measurements to be taken from individual features. Corrections have to be made for the roll and tilt of the aircraft. This is achieved by having the extract orientation (the relative position) of the aircraft recorded on each frame as it is exposed. It is also possible to calculate the relative heights of ground detail, above or below a Mean Sea Level, thus recording the information needed for contours that appear on some maps.
From control stations (perhaps 20 km apart) the system must be broken down further so the surveyor has a control point to use to fix map detail. These points must be situated approximately 100 metres apart. They used to be fixed by traversing with theodolites, ie measuring angles and distances in a zigzag line from one known point to another. All intervening points were then fixed using simple trigonometry. Modern methods use Global Positioning Systems (GPS) to position these points without theodolite traversing.
Levelling is a technique for measuring differences in height between established points, relative to a datum or base-point. Over short distances levelling telescopes are used to view a staff or pole and, with the aid of a bar code, the height is recorded, in relation to the previous station.
It is now possible to use the GPS network to collect height information. By using satellite receivers on the ground and a model of the Earth's gravity field, it is possible to obtain the three-dimensional data necessary to compute height. GPS is not yet as accurate as spirit levelling over short distances.
Detailed ground survey
It is not possible to collect all ground detail using aerial photography. Shadows, clouds, tree cover and overhanging eaves on buildings mean that the surveyor still has to take instruments into the field. In addition, details such as street names, house numbers, and the position of public buildings can only be discovered on the ground. This data is collected by the surveyor.
Until recently, new information was measured and fixed into position by the surveyor relating it to existing detail and drawing directly onto a Master Survey Document. The surveyor used an optical prism or 'popeye' to establish sight lines and right angles between detail already shown on the map. These sight lines were then lightly drawn on the survey document.
With the advent of GPS, Electronic Distance Measuring instruments (EDMs) and digital theodolites, the co-ordinates of new detail can be keyed directly into the map on a PRISM handheld computer. The new data can then be downloaded to a central map data archive.
Surveyors with a GPS handset will no longer need to clamber up church towers or hillsides. Satellite readings will allow new map detail to be accurately positioned simply by placing the receiver on the feature being recorded, and saving the co-ordinates directly into PRISM.