The following discussion on digital levels has been primarily taken from Schoffield (2002). Main components of digital level consist of two parts: Hardware (Digital level and levelling staff) and Software.
Both digital level and associated staff are manufactured so that they can be used for both conventional and digital operations.
As mentioned earlier, digital levelling staves have dual marking. One side is binary bar-coded for digital recording. For example, Sokkia SDL30 uses a RAB (RAndom Bi-directional Code) staff. The other side is marked as the conventional staff for conventional staff reading. The staff is made from a glass-fiber-strengthened synthetic materia with low coefficient of thermal expansion for high accuracy. For highest precision work, Invar bar coded staves are also available.
Typically 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 diode array. Figure 1.1 shows components of a typical digital level (Schoffield, 2002). The light, reflected from the white elements only of the bar code, is divided into infrared and visible light components by the beam splitter. The visible light passes on to the observer, the infrared to diode array. The acquired bar code image is converted into an analogous video signal, which is then compared with a stored reference code within the instrument. The image correlation procedure then obtains the height relationship by displacement of codes, while the distance from instrument to staff is dependent on the scale of code.
The data processing is carried out on a microprocessor and the results are displayed on matrix display. The measurement process is initiated by an interactive keypad and data can be stored onboard.
Data from digital levels is stored onboard (e.g. REC module GRM10, GPC1 with NA2002/2003 or on PCMCIA cards with Topcon DL 101C/102C and DL 103/103AF) and can be transferred to computer for further processing. For example, NA 2002 and 2003 from Leica Geosystems use DELTA/LEVNET software which can carry out adjustment, profiling, instrument tests, etc.
Various capabilities of digital levels are as follows:
measuring height difference
measuring height difference with multiple instrument positions
setting out with horizontal distance
levelling of ceilings
Figure 1.1 Components of a typical digital level (Schoffield, 2002)