The digital level is an instrument that uses electronic image processing to evaluate the staff reading. The observer is in effect replaced by a detector which derives a signal pattern from a bar-code type levelling staff. A correlation procedure within the instrument translates the pattern into the vertical staff reading
and the horizontal distance of the instrument from the staff. Staff-reading errors by the observer are thus eliminated. The basic field data are automatically stored by the instrument thus further eliminating booking errors.
The design of both the staff and instrument are such that it can be used in the conventional way as well as digitally.
(1) The levelling staff:
The staff is usually made from a synthetic material, which has a small coefficient of expansion. The staff may be in one or more sections. There are precise invar staves for precise levelling. On one side of the staff is a binary bar code for electronic measurement, and on the other side there are often conventional graduations in metres.
The black and white binary code comprises many elements over the staff length. The scale is absolute in that it does not repeat along the staff. As the correlation method is used to evaluate the image, the elements are arranged in a pseudo-random code. The code pattern is such that the correlation procedure can be used over the whole working range of the staff and instrument. Each manufacturer uses a different code on their staffs therefore an instrument will only work with a staff from the same manufacturer.
2) The digital level:
The digital level has the same optical and mechanical components as a normal automatic level. However, for the purpose of electronic staff reading a beam splitter is incorporated which transfers the bar code image to a detector. Light reflected from the white elements of the bar code is divided and sent to the observer and to the detector.
The detector is a form of charge couple device (CCD) which turns the black and white staff pattern into a binary code. The angular aperture of the instrument is quite small of the order of 1◦–2◦, resulting in a short section of the staff being imaged at the minimum range and up to the whole staff at the maximum range. The bar code image is compared with a stored reference code to find the height collimation on the staff. The instrument may not need to see the part of the staff where the cross-hairs lie. The distance from instrument to staff is dependent on the image scale of the code.
The data processing is carried out within the instrument and the data are displayed in a simple format. The measurement process is initiated by a very light touch on a measure button. A keypad on the eyepiece face of the instrument permits the entry of further numerical data and pre-programmed commands.
The data can be stored and transferred to a computer when required. The instrument may have an interface, which permits external control, data transfer and power supply.
There are two external stages to the measuring procedure; pointing and focusing on the staff and triggering the digital measurement. The whole process takes a few seconds. Triggering the measurement determines the focus position, from which the distance to the staff is measured, and initiates monitoring of the compensator.
Acoarse correlation approximately determines the target height and the image scale and a fine correlation using calibration constants produces the final staff reading and instrument to staff distance. For best results a number of observations are taken automatically and the result averaged. This reduces biases due to oscillations of the compensator and air turbulence within the instrument.
The results may be further processed within the instrument, displayed and recorded. The programs incorporated will vary from instrument to instrument but typically may include those for:
(1) A single measurement of staff reading and horizontal distance.
(2) The start of a line of levelling and its continuation including intermediate sights. Automatic reduction of data. Setting out of levels.
(3) Calibration and adjustment of the instrument (two-peg test).
(4) Data management.
(5) Recognition of an inverted staff.
(6) Set the parameters of the instrument; a process similar to the initializing procedures used when setting up electronic theodolites.