digital levelling

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.

Instrumentation :

The design of both the staff and instrument are such that it can be used in the conventional way as well as digitally.

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 lengh . 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. 

East manufacture

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 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 1ُ-2ُ 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 out within the instrument and the data are displayed in a simple format , The measurment 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.

Measuring procedure :

There are two external stages to the measuring procedure : pointing and focusing on the staff and triggering the digital measurment . 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.

A coarse correlation approximately determines the target height and the image scale and 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 trubulence 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 for :

(1) A single measurment of staff reading and horizontal distance.

(2) The start of 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) Recongnition of an inverted staff .

(6) Set the parameters of the instrument : a process iimilar to the initializing procedures used when setting up electronic theodolites .