Construction

Electronic theodolites are almost identical to optical instruments in every way except for the method used to determine horizontal and vertical angles.

Circle Encoding
In order for the horizontal and vertical circles of theodolites to be 'read' electronically they need to be encoded so that angles can be determined. This is normally achieved by engraving the circles in a binary coded pattern, similar to that shown below (schematic only).

Different instrument companies use different methods, but the most common is to have an array of light emitting diodes (LEDs, shown right above) and light sensitive diodes either side of a glass circle with patches of light and dark Light is either transmitted or not, depending on the position of the circle over the diode array. This can either denote a unique position (as does engravings on an optical theodolite) or can start a counter which converts 'counts' to degrees minutes seconds through the use of the on-board micro-processor. The instruments can 'read' angles either clockwise or anti-clockwise, angles can be summed and the mean calculated automatically, and zero can be set at the push of a button.

Tilt Sensing
Most electronic theodolites have automatic tilt sensors which can measure the orientation of the alidade and correct for this in the angle observations. Unlike the horizontal circle, which generally does not have a ‘zero’ point, the vertical circle must orient the zero to the zenith. This is done on optical theodolites by adjusting the vertical circle bubble, or by an automatic compensator like that found in an auto level. Electronic instruments ‘sense’ the verticality of the instrument using a device that acts under gravity, and then orients the ‘zero’ on the vertical circle to the zenith. Some sensors use a reflecting surface of mercury, others use prisms suspended by fine wires, but all refer somehow to the direction of gravity.

Data output

As the count signals in the instrument are processed by computer, this digital signal is often available via a data port on the outside of the instrument. This means that the angle measurements can be taken from the instrument electronically and either stored for later processing or manipulated on-line to give position or deformation results in 'real-time'. Shown to the left is an electronic theodolite with a electromagnetic distance meter (EDM) attached. If an EDM is included either as an add-on or as an integrated part of the instrument’s telescope then it is possible to determine all elements of a three dimensional vector, that is direction, elevation and length. The micro-processor also processes the EDM phase measurements and is capable of transmitting this distance as well as the angular measurements through a communications port.