Factors affecting the measuring procedure
Every operation in a measurement procedure is a possible error source and as such requires careful consideration in order to assess the effect on the final result.
(1) Pointing and focusing
It can be shown that at a range of 2 m only 0.3 mm of the staff width need be imaged and at 100 m only 14 mm. As the bar code is 50 mm wide, positioning the staff to face the instrument is not critical and results can be obtained even when the staff is at 45° to the line of sight.
The precision of the height measurement is independent of sharpness of image; however, a clear, sharply focused image reduces the time required for the measurement.
Read More Methods of Levelling
(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 on a section of the code, the effects of shimmer and vibration are not critical. Similarly, scale errors on the staff are averaged.
(3) Illumination
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 shadows. All these variations are catered for by the instrument and are indicated by an increase in the measuring time as illumination decreases.If used in artificial light, its spectral distribution must be comparable with daylight.
(4) Staff coverage
In some conditions part of the bar code section being interrogated by the instrument may be obscured. A minimum of 30 code elements are necessary to determine height and distance, requiring at least 70 mm of the staff section to be available. This means that for ranges greater than 5 m up to 30% of the staff section may be obscured. Below 5 m, all the section is required.
