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Description
This study compares the vertical measurement accuracy of two Leica laser trackers: the AT500, which uses Orientation to Gravity (OTG), and the AT403, which provides continuous dual-axis compensation. Laboratory tests using a network of ten stations with a maximum baseline of 18 m showed that OTG correction produces vertical angle uncertainties of up to 0.4 arcsec, making single-instrument vertical positioning 25–50% less precise than continuous compensation. Long-term monitoring over 10.5 hours revealed high-frequency fluctuations of 0.8 arcsec peak-to-peak for the AT500. Network simulations based on a 560 m particle accelerator alignment showed that combining OTG measurements with traditional leveling produces vertical positioning errors about 30% larger than continuously compensated systems with leveling, while omitting gravity correction entirely increases errors by 135%. OTG procedures therefore provide acceptable vertical accuracy for large-scale applications when combined with leveling, but measurement networks must account for the resulting uneven precision distribution, especially for points far from leveling lines. The dual-OTG protocol, which brackets measurements with initial and final gravity determinations, reduces the vertical angle uncertainty from 0.4 arcsec to 0.2 arcsec (a 50% reduction) compared to single-OTG measurement.
