Spatial positioning of sidewall stations in a narrow tunnel environment: a safe alternative to traditional mine survey practice

Phase 3. Observation protocol

The observation of newly installed survey stations needs to follow specific protocols in order to ensure the accuracy of the survey network. The procedure for surveying a sidewall survey station is similar to the forward observation of a new hangingwall survey point. Check measurements to reference objects to ensure correct identification of the points should be made. Instrument software should be able to identify any such error. In some instances it has been found that the illumination of targets become difficult under certain lighting conditions, by lighting the side of telescope with a cap lamp the target prism can be illuminated sufficiently to perform an observation. [146]. It is effective practice to use the instrument red-laser to identify the point that needs to be illuminated by the survey crew. The sighting of a target through the telescope while the laser is switched on can place excessive strain on the observer’s eye. It is possible to utilize Automatic Target Recognition capabilities if a robotic instrument is used. This capability will allow the instrument to observe a target after a rough alignment using the red-laser has been made. The motorized or robotic instrument will then be able to take a face left and face right reading without the assistance of the surveyor. This process has the potential to simplify the identification and orientation of sidewall stations.

Geometry of observations
It is important that the surveyor follows the fundamentals of good observation protocol during the orientation of the setup as well as the installation of new control points. In the dynamic underground environment it may not be possible to obtain the conditions that will allow the optimal placement of survey stations in the best possible geometric alignment. In order to ensure the best possible geometry under real conditions, the following points are to be taken into consideration:

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  Ensure that the instrument is setup to a position where the reference points are as far as possible evenly distributed around the setup point to ensure the best geometry possible.
  
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  Observing an additional point away from the cluster of reference points used, preferably in a direction that is not the same as the rest of the points.
  
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  Performing a distance measurement to at least one reference point if only angular observations are taken.
  
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  If the sum of the internal angles of the observations to three of the unknown points and the angle subtended by the three known points approximates 180 degrees or a multiple thereof, the polygon of the three known points and the unknown point can be regarded as a cyclic quadrilateral and therefore would be a configuration of points that would fall in the “danger circle” [181]
  
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  Ensure that the setup point does not exceed the maximum distance of 42 metres from the last cluster of reference points.
  
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  The minimum observations from the setup should be at least 3 points in the case of a resection in order to ensure redundancy for observations.
  
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  If a setup must be made with fewer than 3 reference stations or poor geometry it is adviseable that the classification of the resection and subsequent survey stations installed from that setup be “downgraded” to a Class “C” localized type survey with a lower confidence rating. The confidence in such points can then be upgraded at a later stage after additional observations and/or a check survey has been performed.
  
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  The setup points should ideally be situated between four survey stations, if such a setup is not possible, the following criteria should be considered:
  

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  It is best to avoid angles between reference points smaller than 2 degrees
  
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  If possible take check observations to additional control points that are not from the same cluster of points.
  

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  All observations should at least consist of one set of angles made up of one “face left” and one “face right” observation.
  

Field notes
It is recognized that the instruments and software used on mines may differ significantly between operations. The output and storage format of each of these systems must be able to be edited in such a manner that the observations and calculations made on-board of the Instrument can be stored and printed in a format that can be easily understood by an individual not familiar with the on-site equipment. It is critical that sufficient back-ups be made of all observations and calculations to reproduce any part of a survey network when required to do so. Special care must be taken to ensure that the data storage and calculation format remains accessible as software and operating systems improve and change.
It is recommended that the output of the resection information, follow a similar output format than that used for normal observations. It is suggested that a small printout of the points used in the setup is plotted at the bottom of the field observations in order to form part of the permanent record of the observations at that specific point. An example layout for a hardcopy of the field observations is shown below:

Phase 4. Calculation methodology

All observations must provide sufficient redundancy in order to calculate the final position of the resection point. In order for the instrument calculation software to perform a resection, observations to at least three known survey points must be made. It is advisedable that a minimum of four points should be observed in order to ensure sufficient redundancy in the observations to ensure an accurate fix of the setup position. The surveyor must ensure that the tolerances pre-set on the instrument for positional fix on angles and distance are within realistic tolerances for the class of survey required for the survey network. Most survey instrument software has the ability to identify reference objects that are outside these determined tolerances. In the case where the position fix of the setup is outside the pre-determined tolerances of accuracy, some software programs have the ability to display the individual reference points with the error estimated at each point. The user is then offered the opportunity to remove any points that may fall outside the tolerances and request a re-calculation. In the case where the instrument software does not provide the information it is possible by a process of elimination to use different combinations of the visible reference points to re-calculate the position fix until an accurate position fix can be obtained. The surveyor must take special care that the points used in the resection are identified correctly and that the co-ordinates of these points are in fact correct. Prior to installing new survey control it is recommended that the surveyor verified the following:

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  Have the reference stations been identified correctly?
  
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  Does the position fix fall within the specified minimum standards of accuracy?
  
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  Has the correct prism size and prism constant been used?
  
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  are the survey station co-ordinates correct?
  
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  Have the correct instrument settings been used?