Flow chart of the main survey task 

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... using an angular precision of ±2" and the specified distance precision from the manufacturer, the lateral and vertical coordinate precisions of the rail locations were ±0.5 and ±0.3 millimetres respectively. The longitudinal coordinate precisions of the rail locations were as large as ±1.5 millimetres, but this coordinate was not critical. As could be expected, altering the distance precision to ±3 millimetres improved the overall reliability and, in particular, the non-critical longitudinal coordinate precision improved to ±1.1 millimetres. More importantly, reducing some of the rail locations to only two radiations substantially degraded the overall reliability and reduced the coordinate precisions to ±(1.8 to 2.3), ±(0.7 to 1.2), ±(0.3 to 0.5) millimetres in the longitudinal, lateral and vertical directions respectively. The variations are due to the different geometries generated by the relative positions of the rail locations and the instrument stations (see Figure 3). Although the results for the critical coordinates are still acceptable with respect to the nominal ±1 millimetre specification, it is clear that three radiations to each rail location would be very desirable to guarantee the required precision. In order to complete measurement to the rail centre line it was necessary to design and build two different types of targets. The most critical of these were the two identical rail targets, whilst the central resection targets (see Figure 2) were of somewhat lesser importance. Both of these types of targets required a purpose built EDM reflector and telescope alignment cross-hair combination so that a single, rapid, precise pointing would be possible from close range. Standard prism reflector targets were used for the resection targets near the ends of the warehouse (see Figure 2) as they would be sighted over longer ranges and from the same general direction. There were two primary constraints which pointing and a circle of retro-reflective tape governed the design of the rail targets, to return the EDM signal. namely the height of the rails above the instrument stations and the need to accurately define the rail centre-line. In order to be seen from each of the instrument stations it was necessary to design a target with an offset so that it could be seen clearly above the rail and the rail support structure. As measurements were to be taken to the rail centre-line it was very important to ensure that this line was accurately defined. Symmetrical targets were designed (Figure 4) to precisely straddle the rails at a constant vertical offset above the rail centre-line. Metal shims were inserted at the base of the rail to force the target centring at each measurement location. The target design incorporated an engraved, colour contrasting cross for theodolite Retro-reflective tape is an effective substitute for the return of EDM signals over short ranges, however there can be a degradation in distance precision and potential for variation in the reflector constant with distance (Rüeger, 1990). Due to the possible degradation it was decided to adopt the specified distance precision from the manufacturer of the total stations. Empirical testing showed that the variation in the constant was negligible over the expected range of distances. There were a number of preliminary tasks that needed to be completed prior to the commencement of the main survey measurement task : • establish datum points at each end of the warehouse and an approximate centre-line axis • establish the instrument station locations as determined from the simulation but with regard to line of sight obstructions • locate the resection targets around the warehouse • determine instrument station heights through precise levelling The precise levelling observations were reduced on site and the 600 metre run closed to 1.2 millimetres. The main survey task was then begun with the initial three instrument stations. The routine shown in Figure 6 was executed to complete the measurement process. Approximately six positions on each rail were observed from each set up of three instruments. Inevitably some rail locations could only be observed from two of the three stations due to foreground obstructions. A one metre precise scale bar was measured from the initial three stations and the last three stations, in order to enhance the overall scale determination for the network. Two serious problems emerged once the survey was commenced. The most threatening of these was the unexpected difficulty of accurately sighting to resection targets over distances greater than approximately 200 metres. The problems were primarily caused by poor lighting conditions, a heat haze due to high ambient temperatures, temperature variations caused by air conditioning units and various line of sight obstructions. This resulted in a progressive change in resection targets used as the survey station occupations moved along the centre-line, with the most distant resection targets often being unusable. The loss of data certainly weakened the overall network, but in all cases at least five resection targets were available to independently fix each instrument, as well as the influence of the common measurements to rail positions. The second problem was that the time taken to move targets from point to point far exceeded the initial estimates. It was envisaged that two 8 metre ladders would used to reach the rails and locate the targets. However, the point to point measurement time of this routine was in excess of ten minutes because of the positioning and manipulation of the ladders. It became apparent that the time constraints for the measurement could not be met without speeding up the target relocation process. To this end two members of the team elected to sit on the rails and slide the targets between points. This effectively reduced the point to point measurement time to a function of the operator pointing speed, but was literally a pain in the posterior for the rail sitters. Overall, the time taken to complete the collection of all 120 rail points was approximately twelve hours, of which one hour could be assigned to the resolution of on-site problems. The time taken was not significantly greater than that anticipated. This was not by chance but through good initial planning and on-site improvisation. The first phase of the measurement processing was the downloading of the field data from the SDR33 recorders. The field data was then converted to TDVC compatible format using a utility program known as SDR2TDVC (Shortis, 1994). This program computes means of different face pointings for all observed horizontal directions for an instrument station and then reduces the mean directions based on the mean direction to the reference object adopted for each instrument station. The program also computes means of different face pointings for vertical angles and mean distances where multiple observations have been taken. The second phase was a merge of all the instrument station files and the estimation of approximate locations for all stations, resection marks and rail positions. The levels, or vertical coordinates, of the instrument stations were already known from the precise levelling. Using the site plan and the known geometry of the survey, all other coordinates were visually estimated to sufficient accuracy for TDVC to compute the adjustment of the network. the dominant factor, or with the longest lines of sight where poor atmospheric conditions were an influence. Further processing of the coordinate data was necessary to convert the results into an easily interpreted format. Essentially, displacements from the standard gauge and differences in level between the two rails had to be presented in both numerical and graphical form. The first step in this process was to reformat the output coordinate file from TDVC into a plain text file. This file was then imported into the PC version of the Microsoft Excel spreadsheet program. Spreadsheet functions were then used to perform the following preliminary calculations: • compute the mean horizontal alignment of each rail • realign the coordinate system to the mean horizontal alignment of the two rails • compute the mean height of all rail positions The following values were then computed at each pair of rail location positions : • the lateral deviation of each rail with respect to the mean position of the rail • the deviation from the standard gauge • the vertical deviation of each rail from the mean rail height • the difference in height between the two rails • plot sheet coordinates of rail deviations The largest deviations from the standard gauge were +21 and -14 millimetres whilst the largest height differences were +12 and -10 millimetres. The average gauge was some 6 millimetres greater than standard. Deviation values greater than 5 millimetres were deemed to be worthy of remedial action, which would have affected ...