Instrument errors 



The Zeiss has not been subject to systematic instrument errors, except for requiring 

 occasional servicing. However, the Geodimeter has a number of idiosyncracies that were not 

 fully understood during the early Geodimeter surveys. 



One of the instrument errors of the Geodimeter results from the separation of the tracking 

 unit and the angle measurement unit. For accurate vertical measurements, the two units must 

 be parallel. Instead of attempting to fme adjust the parallelism, the angular error was 

 computed and applied to the measured vertical angle in the collection software. 

 Unfortunately, the angle can change during a survey, apparently resulting from temperature 

 changes. Although checks of the vertical angle correction were made, they were not always 

 made frequently enough to fully remove the error. The associated vertical error increases 

 with distance from the instrument. The vertical angle change was usually less than 30 sec. 

 Over a distance of 1,000 m, a 30-sec angle error can result in a maximum elevation error of 

 16 cm. 



Another error associated with the vertical angle correction resulted from the instrument 

 shitting out of level caused by the slight shifting of the pier and the FRF building, probably 

 by differential heating. Some of this movement is automatically compensated for by the 

 Geodimeter and it is designed to stop acquiring data if the instrument tilts outside the range of 

 the compensator. However, this applies only for the horizontal compensation, not for the 

 vertical. Consequently, even if tilted, the instrument continued to collect erroneous vertical 

 data with no indication, except for the bubble level on the instrument being off-center. 

 Because the operator watched a PC screen, and not the instrument, a tilted instrument could 

 go undetected. From the beginning, this error was minimized by sheltering the Geodimeter 

 with an umbrella or by setting it up in a "dome shelter" located on the roof of the FRF 

 building. Frequently sighting and re-sighting a prism of known location was also used as a 

 setup check and for computing corrections. Unfortunately, during the first year of use, some 

 of the out-of-level errors were wrongly corrected for by recomputing the vertical angle 

 correction. This unfortunately added to the error. Once the tilt error was fully understood, a 

 program of frequent level checks was instituted. 



The other instrument error resulted from oscillations of the telescope caused by an 

 improperly adjusted tracker amplitude as it searches to locate the center of the prism. This 

 resulted in jagged data with an oscillatory amplitude of a few centimeters. Although the data 

 appeared to follow the true profile shape somewhat, it was difficult to remove the oscillations 

 since they were not centered about the true profile. A final problem was interference from 

 sunlight, which could overwhelm the tracker power. This problem affected early morning 

 surveys of lines 188 and 190. The problem was eliminated by surveying the northern profile 

 lines, 58 and 62, first. 



External errors 



Electronic survey instruments are sensitive to atmospheric and climatic variation since they 

 use the speed of light to determine distance and optical aiming to measure the angles. The 

 instruments allow for rough adjustment for these variables. During the summer months heat 



10 Chapter 2 Profile Data 



