Lesson Note On Digital Levelling Operating features

Digital Levelling Operating features
The resolution for most instruments is 0.1mm for height and 10mm for distance or better with instrumental ranges up to 100 m. At such distances the effects of refraction and curvature become significant. The effect of curvature can be precisely calculated, the effect of refraction cannot. Most digital levels can also be used as conventional optical automatic levels but in that case the standard error of 1 km of double-run levelling becomes less. Although the digital level can also measure distance, the precision of the distance measurement is only of the order of a few centimetres.

Advantages of digital levelling
One advantage claimed for digital levelling is that there is less fatigue for the observer. While it is true that the observer does not have to make observations the instrument still needs to be set up, pointed at the target and focused. The digital display needs no interpretation such as reading the centimetre from the E on a conventional staff and estimating the millimetre. Measurements are of consistent quality, subject to the observer taking the same care with the instrument to ensure consistency of target distances and illumination of the staff. Also the staff holder must not move the staff between the forward reading in one bay and the back reading in the next, and that the staff must be kept vertical.

There is an acceptable range of illumination, but too much or too little light may make observations
impossible. Some, but not all, digital levels will recognize when staffs are inverted, others will indicate an error if not told that the staff is inverted. Like any automatic level, the digital level will need to be at least coarsely levelled for the compensator to be in range.
Although exact focusing may not be required, the instrument will not work if the focusing is too far out but if the instrument has automatic focusing this would not be a problem. Automatic data storage eliminates the need for manual booking and its associated errors, and automatic reduction of data to produce ground levels eliminates arithmetical errors. However, checks for levelling circuit misclosure need to be made or at least checked and an adjustment to the intermediate points for misclosure needs to be made.
As with all surveying instruments the digital level should be allowed to adapt to the ambient air
temperature.
The scale of the height measurements is primarily fixed by the scale of the staff. An invar staff will
vary less with change of temperature. The scale will also be dependent on the quality of the CCD. How the dimensional stability of CCDs may vary with time is not well known.
There are a number of menus and functions that can be called on to make the levelling process easier,
in particular the two-peg test for collimation error and calibration.

Overall, digital levelling is generally a faster process than levelling with an automatic level. Data can
be directly downloaded to a suitable software package to enable computation and plotting of longitudinal sections and cross-sections. The digital level can be used in just about every situation where a conventional level can be used, and should the batteries fail it can be used as a conventional level if necessary.