SURVEYING EQUIPMENT

The auto level is an optical instrument used in surveying to transfer, measure or set horizontal levels, as well as make distance measurements (distance is usually only accurate to around 100mm).

The level instrument is set up on a tripod and, depending on the type, either roughly or accurately set to a levelled condition using foot screws (levelling screws). This levelling component is known as the ‘tribrach stage’. The operator looks through the eyepiece of the telescope while an assistant holds a graduated staff (E staff) vertical at the point being measured. The instrument and staff are used to gather and/or transfer elevations (levels). Measurement generally starts from a benchmark with known height determined by a previous survey, or an arbitrary point with an assumed height.

'E' Staff and Staff Bubble

The staff is broken up into 1cm sections and has heights marked every 0.1m.  Quite often the sections are grouped into a 5cm block in the shape of the letter 'E'.

The Staff bubble is held against the staff. The ‘bubble’ should be centred in the circle as shown in the picture. A centred staff bubble indicates that the staff is being held vertical, and accurate readings are being made.

The three-leg tripod is used to position, support, or stabilize instruments such as a total station, auto level, theodolite, survey prism or GPS antenna.  These tripods are normally hinged at the top, sturdily constructed of nonferrous materials (typically timber and/or aluminium), and are strong enough to support heavy measuring instruments. Most are fitted with metal ground spikes for use in soil or turf to stop the tripod legs spreading under the weight of the equipment attached to the top of the tripod.  Equipment is attached via a 5/8 Whitworth thread.

Theodolite

A theodolite is an instrument for measuring both horizontal and vertical angles, as used in triangulation networks. It is a key tool in surveying and engineering work, particularly on inaccessible ground. A modern theodolite consists of a movable telescope mounted within two perpendicular axes—the horizontal or trunnion axis, and the vertical axis. When the telescope is pointed at a desired object, the angle of each of these axes can be measured with great precision, typically on the scale of arcseconds.

In digital theodolites, the reading out of the horizontal and vertical angles is done electronically and displayed via a liquid crystal display.

Total Station

A total station is a combination of an digital theodolite, an electronic distance meter (EDM) and software running on an internal (or external) computer known as a data collector.

With a total station the operator can determine angles and distances from the instrument to points to be surveyed. With the aid of trigonometry and triangulation, the angles and distances may be used to calculate the coordinates of actual positions (X, Y, and Z or northing, easting and elevation) of surveyed points, or the position of the instrument from known points, in absolute terms.

The data may be downloaded from the total station to a personal computer and with the aid of software, will generate a map/plan of the surveyed area.

Some total stations also have a GPS interface which combines these two technologies to make use of the advantages of both (GPS - line of sight not required between measured points; Traditional Total Station - high precision measurement especially in the vertical axis compared with GPS) and reduce the consequences of each technology's disadvantages (GPS - poor accuracy in the vertical axis and lower accuracy without long observation periods; Total Station - requires line of sight observations and must be set up over a known point or within line of sight of 2 or more known points).

The best quality total stations are capable of measuring angles down to 0.5 arc-second. Inexpensive "construction grade" total stations can generally measure angles to 5 or 10 arc-seconds.

Some modern total stations are robotic (hence called 'robotic total stations') allowing the operator to control the instrument from a distance via remote control. This eliminates the need for an assistant staff member to hold the reflector prism over the point to be measured. The operator holds the reflector and controls the total station instrument from the observed point.

Most total stations use a purpose-built glass prism as the reflector for the EDM signal, and can measure distances out to a few kilometres, but some instruments are "reflectorless", and can measure distances to any object that is reasonably light in colour, out to a few hundred meters.

360° Prism

The 360° prism negates the need to align a particular face of the prism directly back at the EDM / total station. Its special construction means that as long as the pole it is attached to is held vertical and the prism can be seen (not obscured by anything), the distance measuring beam from the total station will get a return reflection.

 Laser Level & Receiver

The laser level emits a rotating horizontal beam up to an approximate radius of 500 metres.  The operator slides the receiver up and down the surveyor's staff until the receiver is at the same height as the laser beam.  The operator then reads off the height indicated on the survey staff via a 'notch' that is moulded into the side of the receiver.

Laser Scanner

A laser scanner is a device that analyses a real-world object or environment to collect data on its shape and possibly its appearance (i.e. colour). The collected data can then be used to construct digital, three dimensional models useful for a wide variety of applications. These devices are used extensively in industrial design, reverse engineering and prototyping, quality control/inspection and documentation of cultural artefacts.

Many different technologies can be used to build these 3D scanning devices; each technology comes with its own limitations, advantages and costs. It should be remembered that many limitations in the kind of objects that can be digitized are still present: for example optical technologies encounter many difficulties with shiny, mirroring or transparent objects.

Measurement of distance is accomplished with a modulated microwave or infrared carrier signal, generated by a small solid-state emitter within the instrument.  The signal is bounced off of the object to be measured. The modulation pattern in the returning signal is read and interpreted by the onboard computer. The distance is determined by emitting and receiving multiple frequencies, and determining the integer number of wavelengths to the target for each frequency. Some EDM’s use a reflector and can measure distances out to a few kilometers, but some instruments are "reflectorless", and can measure distances to any object that is reasonably light in color, out to a few hundred meters.  The picture shown is a ‘stand-alone’ EDM, but they can also be integrated into total stations.