Every operation in a measurment procedure is a possible error source and as such requires careful consideration in order to assess the effect on the final result .
pointing and focusing
Obviously the instrument will not work if it is not pointed at the staff . The amount of staff needs to be read depends on the range of the instrument to the staff , However , there will be a minimum amount necessary at short ranges . it may not be critical to have staff pointing directly at instrument ,
The precision of the height measurment may be independent of sharpness of image : however , a clear sharply focused image reduces the time required for the measurment . if the image is too far of focus then the instrument may not read at all . some instrument have an auto-focus function to avoid potential focusing problems .
(2) Vibrations and heat shimmer
Vibration of the compensator caused by wind , traffic , etc ,, has a similar effect on the bar code image as that of heat shimmer . However as digital levelling does not require a single reading , but instead is dependent Similarly scale errors on the staff are averaged .
(3) lllumination
As the method relies on reflected light from the white intervals of the bar code , illumination of the staff is important . During the day , this illumination will be affected by cloud sun , twilight and the effects of there may be an increase in the measuring time .
(4) Staff coverage
In some conditions part of the bar code section being interrogated by the instrument may be obscured Consult the manufacturer's handbook to ensure that sufficient of the staff is showing to the instrument .
(5) Collimation
The collimation value is set in the instrument but can be checked and changed as required . The method of determining the collimation is based upon one the two peg methods described earlier . once the collimation value has been determined it is applied to subsequent readings thereby minimizing its effect .
Note , however , that it can never be completely removed and appropriate procedures according to the precision required must still be applied .
(6) Physical damage
It is likely that the instrument will be seriously damaged if it is pointed at the sun .
Operating features
The resolution for most instruments is 0.1 mm for height and 10 mm for distance or better with instinstrumental 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 standard error of 1 km of double - run measurment is only of the order of a few centimeters
Advantages of digital levelling
once advantage claimed for digital lvelling is that there is less fatigue for the observer . while it is true that the observer does not have tomake 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 millimeter .
Measurments are of consistent quality . subject to the observer taking the same care with the instrument to ensure consistency of distance 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 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 byt 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 to the intermediate point 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 on to make the levelling process easier in particular the two - pegs 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 as a conventional if necessary .
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