A topographical survey gathers information about the natural and man-made features of a piece of land. This information consists of the position and level of each feature along with its description. The data collected is used to produce a highly detailed map of the site being surveyed. Surveys have traditionally been issued as drawings on paper or stable polyester film although they are now more commonly issued as a digital drawing, usually in AutoCAD DWG format. Other digital formats such as PDF or 3D files that can be used to model the ground are also commonly issued.
A land survey can be used for many different purposes. Some typical examples would be an architect using one so he can design a new building to fit on a site, a highways engineer would use one to design a new road layout or the Environment Agency would use a topographical survey of a river to calculate its flow or the likely flood risk for surrounding properties.
Most topographical surveys are carried out using a combination of equipment such as laser scanners, total stations, precise GPS receivers and levels.
A total station is an advanced version of a theodolite. Whereas a theodolite only has the ability to measure horizontal and vertical angles, a total station can also measure distances to a glass prism or reflective target with its built in EDM system (electromagnetic distance measurement - using light beams to measure). Modern total stations have many additional features; some can be used robotically allowing surveys to be carried out by just one person, some have a reflector-less measurement mode that can measure to objects up to several hundred metres away without a target, others have built in cameras to record extra information about each measured point.
The GPS (global positioning system) receivers used for topographical surveying use the same technology as a car SatNav, except they can fix a position to within a centimetre or two. Signal from network of satellites circling the Earth are used to fix the receiver's position using either a system called differential GPS that uses two receivers, or network GPS that uses one receiver linked to a correction service over the Internet. Network GPS gives the user accurate Ordnance Survey grid coordinates and levels instantly which the survey can then be related to. Many modern receivers can also use signals from the Russian satellite system, GLONASS, which gives much greater signal coverage.
A level is used with a levelling staff to transfer heights accurately from one point to another. This may be used to relate to the survey to a datum such as an Ordnance Survey bench mark or to more accurately obtain levels for survey stations or detail points.
The survey is carried out by setting up the total station over a point on the ground known as a station which is usually marked by a nail in a hard surface or a wooden peg hammered into the ground. Angles and distances are measured between each station to establish the site control network. This process is known as traversing. Each station is placed in a suitable position to be able to observe the detail that needs surveying. The angles and distances to each point of required topographical detail is observed along with a description - usually an abbreviation of the description known as a field code. If the kerb lines along a road were being surveyed the surveyor might use a field code of KB. Other information can also be included as part of the field code such as the shape of the feature being surveyed. If the kerb line was curving for instance then the surveyor could use KBC as a code to describe that.
GPS can also be used to measure the positions of survey stations and to survey detail points. The great advantage of GPS is that no 'lines of sight' is needed unlike when surveying with a total station. The surveyor is free to roam wherever he can get a GPS signal which is particularly useful when surveying large open areas.
Once the site work is complete the surveyor returns to the office and processes the data collected on site. The traverse is computed and coordinates are calculated for each detail point. If the survey is to be based on Ordnance Survey coordinates and datum then the coordinates obtain by the GPS can be applied to the stations. The field codes are used to define how each feature is drawn and how it is stored in the data base. The processing software has a feature code library that allows it to recognise the field codes and use them to automatically draw every feature correctly. Our KB code will draw a line linking all the other kerb line points surveyed. That line will be drawn in the correct colour, with the correct levels written alongside it - this will all appear on the correct layer in a CAD program. Different line styles are applied to different linear features to define them and different symbols along with text descriptors are applied to individual point features such as trees and lamp posts. Any mistakes, such as incorrect coding can be corrected in the processing software and addition descriptive text, title blocks, north points etc. can be added either from within the processing software or in a separate CAD program. The survey can then be printed or emailed directly to the client.