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

. Three case studies involving sidewall station networks in the mining environment

6.1. Introduction

The test phase of the sidewall survey station method was concluded in the underground tunnel of the University of Johannesburg. The results proved that the sidewall station method of surveying could meet the accuracy requirements of a Class A type survey network. This change in traditional mine survey practice has enormous potential for underground surveying because of its contribution to “zero-harm” by removing the mine survey crew from a high risk area namely the hangingwall of deep level tunnels. Apart from the risk associated with vertical rock stresses it will also eliminate the risk of working at heights as well as moving the survey setup away from the high risk centre of the tunnel where machinery is bound to travel.

In order to evaluate the accuracy and efficiency of the survey method, it was decided to test a similar network in the workings of an active mine. The assumption was made that the survey method would meet the prescribed accuracy requirements and at the same time prove to be a safer and faster method of extending an underground development survey network. The sidewall station survey method would have to be properly evaluated to ascertain whether or not it can realistically be implemented on a working mine under production conditions. In order to evaluate the accuracy parameters obtained during this process it would be required to investigate the different classes of accuracy and the approved definition of each of these with reference to the types of network to which it applies.
In total three case studies are discussed, investigating the methodology of network installation and then focussing on specific aspects of the network unique to each mine. A sidewall network was established in a deep level platinum mine in Rustenburg, an existing method used on a deep level goldmine on the Witwatersrand used for secondary networks and a network used in main development of a copper mine in Palabora was investigated as part of a check survey.

6.2. Rustenburg Platinum mines, Siphumelele 2 shaft

Rustenburg Platinum Mine, Siphumelele 2 Shaft is situated in the Western limb of the Bushveld complex, approximately halfway between Rustenburg and Marikana in the Rustenburg magisterial district. The mine produces platinum from two reef horizons, namely the Merensky reef and the Upper Group 2 Chromitite layer (UG2). The reef workings are accessed through a vertical shaft system. The 11 Level Main Cross-cut (X/Cut) is a near-horizontal tunnel in the footwall of the reef horizons designed to serve as an access and transportation tunnel for mining personnel, material and rock to- and from the working panels on that level. The tunnel is used to introduce ventilation airflow to the workings and act as water drainage from the workings. It contains electricity cables, water- and compressed air pipelines that are suspended in the roof of the excavation. The cross-cut³⁵ is designed for conventional rail-bound tramming of material.

Level 11 Station is at an elevation of 839m AMSL which is approximately 342m below the surface. The tunnel has been extensively supported with roofbolts and in some areas of geological disturbance supported with a heavy-gauge wire meshing. The Cross-cut is orientated in a North-South direction. The dimensions of the 11 Level Main Cross-cut South is 3.0m wide by 3.0m high. The total length of the cross-cut is approximately 3km. The gradient of the tunnel is 1:200 and has been surveyed by conventional hangingwall stations spaced 60metres apart on average. Please refer to the original development sheet³⁶ in Figure .

Figure . 11 Level Main Cross Cut

Source: Anglo Platinum Mines Survey Office

1. The establishment of the Sidewall survey station network

The underground survey network was initiated by establishing a long baseline on 11 Level in the cross-cut in a southerly direction. In order to determine the termination point of the baseline, observations were made from an established known base. The survey station X5412 was used as the orientation point (backsight) and the instrument setup under survey station point X15915. The endpoint of the baseline was determined to be at an existing hangingwall survey station X17354. The total length of the baseline was 638.7metres. A target was levelled and centred on a tripod directly under this point to serve as the endpoint of the baseline. As it was decided to reduce the risk exposure in the operation, the instrument was roughly centred under the survey station using the plumbing rods and then accurately aligned using the laser plummet of the instrument. This method seems to be very effective if the hangingwall is low enough to access with the extendable plumbing rods normally used to suspend targets from reference objects. One further disadvantage of using hangingwall survey stations is that the wooden plugs used to hold the brass spad in position can deteriorate over time and can easily be pulled out of the roof by placing too much tension on the plumb bob string or if a too heavy plumbing rod is used. Such an incident would mean that the survey station has been damaged to a point where it has to be re-surveyed from a stable base further back in the tunnel.

The point of origin of the test baseline was surveyed in the conventional hangingwall survey method and the co-ordinates, obtained from the observations and reductions made by the instrument, were used to compare the accuracy of the freestation points. It is argued that should it be found that this freestation closure with the check survey co-ordinates within the prescribed MHSA standards of accuracy, the freestation method could be considered a viable alternative to the conventional hangingwall survey station survey method. The co-ordinates of the baseline stations are listed in Table .

Table . Co-ordinates of baseline stations

In order to establish the first set of sidewall stations, a set of four wall stations were installed at a convenient distance from the instrument and co-ordinated using a single observation to determine distance and direction. The positioning of the sidewall stations were configured to be on the grade line and located within approximately 10metres of each other with opposite stations placed in pairs. This configuration was chosen to replicate the grade-peg layout used on most South African mines and used to control the vertical positioning of a tunnel. From the origin point wall stations WS01, WS02, WS03 and WS04 were installed. The position of the wall stations were indicated to the survey crew using the laser from the instrument to identify visible points in the general area where the network was to be established. The survey crew would then drill a 10mm hole with a battery operated drill and hammer the tapered brass plug in place, refer to Figure

Figure . Brass Plug and Bayonet attachment



Photograph by H Grobler

It was found that the taper of the plug required the drill operator to ream the hole slightly larger at the collar of the hole in order to prevent the plug from protruding from the sidewall. As a result of the prevailing amount of copper theft on the mines, low cost plastic alternatives to these brass plugs are being tested on some mining operations. Once the plug had been installed, a bayonet prism adaptor was screwed into the hole and a large circular prism attached to the adaptor. The point was then painted and numbered using a fluorescent paint.

Figure . Sidewall Station

Photograph by H Grobler

2. Description of the observation methodology

The newly installed point number was communicated to the surveyor to record on the instrument using a two-way radio. The crew would then illuminate the prism to be surveyed in order to identify the correct survey station as well as to make it easier for the surveyor to observe. The hangingwall survey station was surveyed using a standard mini-prism with a -17.5mm prism constant and standard large round prisms with a 0mm constant was used for the sidewall stations. It was found that in the case of points close to the instrument, the exact optical point of the prism could be orientated. Illumination of the prisms proved difficult in some cases due to communication difficulty between the crew members as a result of drilling noise drowning out radio communication. It was found to be a highly effective practice to use the red-laser to identify the point that needed to be illuminated by the survey crew. However the sighting of a target while the laser was visible proved to place excessive strain on the observer’s eye. It was found that it was easier to slightly illuminate the side of the telescope with the cap lamp which would then reflect in the prism and make the prism more visible. [146]

As with the sidewall station experiment at the University campus survey stations in the sidewall were surveyed from the existing hangingwall control network by observing the horizontal and vertical angle as well as the slope distance to each point. The sequence of control point installation was set up under a known survey station (X15915) and from this orientation the sidewall stations as illustrated in Figure were installed:


Figure . Installation of sidewall stations.

The software of the instrument used in the study limited the number of observations taken. It is recommended standard observation practice on this mine to take at least one “face-left” and one “face-right” reading to each point. Once the new wall stations had been co-ordinated and an additional check observation made to the termination point X17354, the instrument was switched off and moved to a new, random setup position between the newly installed set of wall stations. A copy of the raw field observations is included in Appendix 12. Freestation RS01 was established at a random point between wall stations WS01, WS02, WS03 and WS04 at a distance of 20.7metres from the point of origin of the baseline as illustrated in Figure .

Figure . Freestation setup from sidewall stations

The time taken to establish the points and position was 33 minutes according to the on-board fieldbook. The length of observation can be explained by the fact that this was the first time that the survey crew had to install sidewall stations. From freestation RS01 the following sidewall points were established: WS05, WS06, WS07 and WS08. An illustration of the sidewall peg installation is depicted in Figure .

Figure . Idealized freestation setup of Sidewall stations in tunnel
Photograph by H Grobler

In order to check on the progress of the accuracy of the freestation method the target at the termination point (X17354) was observed from each consecutive freestation setup and visually compared to the conventional hanging wall traverse co-ordinates of the point X17354. Each freestation was indicated by a RS prefix in the observation file of the instrument. In this manner sidewall stations were installed from the new freestation points up to a point where the last set of wall stations were installed around the foresight point X17354.

The time duration for peg installation during the initial phase of learning the method was 28 minutes on average for each setup. The speed of installation improved to 13 minutes once the crew had become familiar with the process and a manageable method of communication was agreed upon. The survey of points already previously installed averaged 9 minutes. Table provides a summary of the total time taken to install new sidewall stations and co-ordinate the same. Time saving in the setting up of the instrument, orientating the instrument and the installation and observation of new sidewall stations could be the result of some of the following factors

• 
  No working-at-heights risk assessment required;
  
• 
  No additional safety harnesses required;
  
• 
  No need to locate ladder and carry it between the backsight, station and foresight;
  
• 
  No need to place a ladder to install a machine bob for instrument setup under a survey station;
  
• 
  No additional time required to level and centre the instrument under the roof station;
  
• 
  No need to use a ladder to install foresight points;
  
• 
  More freedom in placing the new stations in areas with competent rock;
  
• 
  No additional time required to point out the exact position for the new survey station position;
  
• 
  No need to use a ladder to hang a backsight target from the hangingwall;
  
• 
  No need to erect tripod over railway lines in the centre of the tunnel.
  

Table . Time study of setups

3. Method of evaluating the accuracy of the Sidewall Survey stations

The method used to evaluate the accuracy of the sidewall stations was to compare the final results of the last resection RS10 with the co-ordinate of the Foresight point X17354 obtained by observations from station X15915. It is argued that the one observation along a long baseline would determine the final Foresight point with greater accuracy than if a number of short traverses were made to obtain the same co-ordinates, as this would possibly introduce additional errors into the results of the closure point. At the final freestation setup, the instrument was placed directly under the final point and levelled and centred in the same manner as a conventional hangingwall survey. The original co-ordinates for the end point of the baseline were ignored as it was unclear whether a check survey had been made on this level. In order to evaluate the accuracy between the two methods, it was decided to use the co-ordinates as determined during the experiment.

4. Sidewall station network results

The solution of final co-ordinates of the freestation position of the hangingwall survey stations were calculated using the on-board resection software of the Leica TPS1201+ in the same manner as during the initial studies conducted on campus. The principle behind the “freestation” program includes calculating the provisional plan co-ordinates using a computation of the station co-ordinates using each possible unique solution including the solution of triangles, each possible combination of resections and Helmert transformation. For each solution, the result of the computed value is compared with the observed value. These results with the smallest number of observed minus calculated values are used to calculate the median of the values which in turn will provide the final result for the freestation position. [112]

The position of the point RS10 was determined from the last set of sidewall stations installed around the hangingwall station X17354 for this purpose. A table of the original- and freestation co-ordinates are listed in Table . The term “estimated” indicates a point surveyed by freestation and the term “measured” indicates a point determined by traverse.
Table . Comparison of freestation co-ordinate closure

5. Closure obtained at the breakthrough point

The distance of tunnel surveyed, from peg X151915 to peg X17354, using the freestation method was determined to be 638.679metres. Using the formula for minimum standards of accuracy required by the MHSA , the following parameters were calculated in Table .

Table . Calculated Minimum Standard of Accuracy for the survey

The minimum standard of accuracy between the Provisional and Final co-ordinate of a survey station is defined as the vector distance calculated from the difference in the Y co-ordinate and the X co-ordinate, where “s” is the distance of the total traverse from the starting survey station to the “closure” survey station in the following manner:

Table . Error vector of freestation closure

According to the minimum standard of accuracy as defined by Chapter 17 of the Regulations of the Mine Health and Safety Act, 1996 (Act No. 29 of 1996), Government Gazette No 34308, 27 May 2011, the closure is deemed to be within the minimum standards of accuracy required to classify the network as a Class “A” network in both the horizontal and vertical planes of the survey. The Main Cross-cut used in this project can therefore be classified as a Class “B” secondary survey network as the regulation states that: “…any survey carried out for the purpose of fixing main or access development, mine boundaries…” [17] would be classified as a Class “B” survey. It is has been observed that the mining property is adjacent to other mining properties and that therefore, the determination of the boundary would be determined from this network as well as the location of possible connections could be made to other shafts and therefore a Class “A” minimum standard of accuracy would be recommended .

During the test phase discussed in Chapter 5, it was found that the instrument software could not cope with the inverted instrument and target heights normally used in conventional hangingwall surveying. This problem was overcome by simply ignoring both the signs of the instrument and target heights in order to provide correct elevation results. The closure on elevation, although not as good as on the horizontal plane was still within the prescribed minimum standards of accuracy.

The bearing error calculated from a Join between the original point and the resected point was compared to the baseline direction and listed in Table .

Table . Bearing error comparison


It is standard practice on mines to check the direction of a survey network every 500 - 1000 metres using a gyroscope. The data proves that with sufficient reference objects and careful observations, the bearing error propagation can be controlled and provide results within the required MHSA standards of accuracy.

6. The two-point freestation method

Upon completion of the four- sidewall station method over a distance of 630m, the direction of the survey was reversed and a quick method using only two reference points as described in the Australian context [96] were used. The “Australian method” using only two reference points as described by Jaroz and Shepard [96] was used to make a closure on the origin point of the hangingwall baseline. It was decided to use sidewall stations in the same groups as before but to use only one station on each sidewall as illustrated in Figure .

Figure . Two point sidewall station method


A comparison of the sidewall station co-ordinates surveyed initially and resurveyed using the two-point method was made. The results of the points resurveyed as well as the closure is listed in Table . From the results it can be seen that a Class “A” closure could be possible for distances up to 180m before the accuracy starts deteriorating to a Class “C” type survey.

Table . Freestation co-ordinate comparison

The bearing error calculated from a Join between the RS10 the freestation point located at the position of survey station X17354, and the last resected point located at X15915, was compared to the baseline direction and listed in Table .

Table . Closure error

The MHSA states that an error of 2 minutes of arc between consecutive stations is required after a check survey has been completed. This “two point” network was not check surveyed but at the moment the error in bearing are within the prescribed minimum standards of accuracy.

Table . Calculated Minimum standards of Accuracy

Using the bearing error and distance between the two points, a closure error of 0.139m could be expected over the distance. This expected closure distance compares well to the 0.109 calculated error vector obtained from the observations and provides evidence that the error propagation in bearing has a larger impact during sidewall station surveying.

The closure of the two-point survey method was found not to be within the prescribed minimum standard of accuracy, with quite a large closure in the Y co-ordinate. Due to the North-South orientation of the tunnel, it seems that the error in bearing increased far more rapidly than in that of distance. It is suggested that the error propagation of the survey is as a result of the increased distance as well as the reduced number of observed points. For a distance less than 180metres in this case, the two-point method of surveying will meet the accuracy of a Class “A” survey after which the accuracy deteriorates very quickly. The method has therefore sufficient accuracy to be used for fast “reconnaissance” type surveys used to establish control in a tunnel and for measuring, but the network will have to be strengthened by additional control points and survey observations during a follow-up or check survey before the network would be acceptable for use as a primary survey network. Reconnaissance surveys such as that used to access old workings would benefit from this technique.
Under the stated conditions in this project, this method would give a closure of Class “C” and would be acceptable under certain conditions for measuring and tacheometry purposes. This would mean that although the method would not be suitable for primary survey networks where great accuracy is required, the method would still have merit for localized surveys where required.

7. A Comparison between the two methods

It is held that using four reference points will provide the accuracy required to meet a Class A survey accuracy. In the case of the second method, the “two point method”, it is maintained that the accuracies obtained will not consistently provide the accuracy required for Class A or B type surveys for development surveying over distances exceeding 180 metres, but would be within the closure limits required for localized surveys such as for measuring from an existing network for measuring and offsetting purposes. However the accuracies obtained using this method can be improved if the network is supported by additional points and checked with follow-up surveys. Notably only a relatively small time saving in observation and installation could be observed between the two methods. It is proposed that the four station method will provide consistent accuracy with less error propagation over a longer distance.

Not all possible geometrical combinations were considered, but possibly longer distances between points would increase the accuracy. In the “grade-peg” layout it seems that the required standard of accuracy of a Class “A” survey cannot be obtained when using only two reference points. Accuracy could only be maintained for two sets of resection points which in this case was a distance of 180metres before the propagated error increased to a point where it was no longer in the Class A minimum standards of accuracy refer to Table . The distance used to calculate the limit of error was determined to be the distance between the two freestations.
Table . Distance between resection points

Table . Minimum standard of accuracy for RS10 to RS12


A table comparing the bearing and horizontal distance results obtained from the two methods are included here as Table :
Table . A comparison of relative accuracies obtained.

Table . A compassion of accuracies in co-ordinates

Table 59 details an analysis was made on the determination of the breakthrough point from each of the freestations that was obtained from a four-peg setup. It is noted that the closure in the horizontal and vertical planes are within the minimum standards of accuracy for a Class “A” survey, with a standard deviation in closure of 26mm on the y co-ordinate and 18mm on the elevation.

8. Conclusion

From the results listed in the tables above the conclusion can be reached that the sidewall station method used for the establishment of a survey network is plausible under certain circumstances. It follows that the four-point sidewall survey station provides more accurate results with less error propagation than the results obtained from a two-point setup method.

9. Does the sidewall station method meet MHSA standards?

The four point sidewall station survey method demonstrates that under the described circumstances it will provide accuracies that are within the minimum standards of accuracy of a Class “A” survey as prescribed by the Mine Health and Safety Act. The two point sidewall station method provides less accuracy over longer distances but lends itself well to Class “C” type surveys for reconnaissance and measuring purposes. The second method appears to have a greater probability of severe bearing error propagation over distances in excess of 180 metres and would require regular check surveys in order to strengthen the network.

10. Is the sidewall station method a safer method of surveying?

Both sidewall survey station methods proved to save on time of installation and a reduction in risk exposure for the survey crew, when compared to traditional hangingwall surveys, refer to the tabulation on page 180. The reduced time exposure at a setup reduces the immediate risk exposure of the survey crew. The added benefit of shorter setup times is that the flow of production is interrupted for a shorter period of time. The need for ladders and the related exposure to working-at-heights can be almost completely eliminated.

The advantage of being able to do a “random” position setup means that the surveyor is able to move the instrument out of “harms way” by setting up out of the line of travel of locomotives in the case of conventional mining or out of the route of trackless tramming equipment. In the case where poor ground conditions exist, the surveyor has the option of moving the setup to a safer location. As ladders are not required, the hazard of aluminium ladders in a fiery mine can be avoided. A risk that requires to be mitigated is the stability of the sidewall where new survey points are installed. It has been observed that a number of fatal injuries in South African Mines have resulted from poor sidewall conditions and in deeper mines resulting pressure bursts. In coal mines the condition of the sidewalls are not always monitored as it would be barricaded off from normal workers. In the case where points have to be installed in these conditions extra care must be taken and a proper risk assessment must be performed before any work commences. During installation of the brass plugs a rubber hammer has to be used to prevent damage to the thread of the plug. The rubber hammer has a violent rebound and care has to be taken not to injure the person installing the plug.

11. Does the sidewall station method provide a faster method of surveying?

From the time study results it is postulated that the initial training and familiarization stage of surveying with sidewall stations will take longer than the hangingwall survey method. As the crew and surveyor become familiar with the method significant time saving can result from the freestation method. The time saving is mostly realized during the setup and installation phase as the need to install plumb bobs and targets under the hangingwall points is not required and no time is wasted looking for ladders or other approved means of accessing the hangingwall for these purposes. The times recorded during the time study included the installation of four new points. In a standard hangingwall survey only one survey station would have been installed.

12. Issues encountered during the survey study:

• 
  The possible duplication in the observation of points but observation points numbered differently in the electronic recorder.
  
• 
  The incorrect identification of reference points both when surveyed initially and when used as reference objects during a freestation setup.
  
• 
  Installing the brass plugs in certain conditions proved difficult. Using a hammer to firmly place the plug can result in damage to the thread. Using a rubber hammer may lead to injury due to the violent rebound of the hammer.
  
• 
  The Software observation method used does not allow new points to be observed in the face left and face right mode. This may lead to errors due to incorrect observation procedures as well as when instruments with a larger angular tolerance such as a 7 second instrument are used instead of a 1 second instrument.
  
• 
  Incorrectly sighting or measuring points that are in the same line of sight, including the deflection of the measuring ray to the wrong prism.
  
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  Incorrect prism constants used if more than one type of prism is used between points.
  
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  The training of the survey crew to understand new methodology.
  
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  Communication between the surveyor and crew when a point is numbered, installed and observed with specific reference to the noise and poor visibility in working tunnels.
  
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  The incorrect sighting of the optical centre of a prism as a result of very close observation distances.
  
• 
  The effect of heat and humidity on both instrument and target causing the instrument to “fog up” making accurate sighting virtually impossible until the instrument and prisms have acclimatized to the environmental conditions.
  

13. Perceived advantages of the sidewall station method:

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  No measurements of instrument-or target height are required as a zero constant can be used.
  
• 
  A reduced opportunity for error in observations due to the observation of four reference points instead of the traditional one backsight.
  
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  A reduced risk exposure profile for working-at-heights and setting up the instrument in a safe location.
  
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  Increased speed of observation and installation.
  
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  Improved accuracy as a result of increased redundancy in observations.
  
• 
  No centring error as a result of not having to set up under a peg and being influenced by ventilation and incorrect alignment practices.
  
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  A reduced effect of refraction as the setup can be placed near the centre of the excavation where the maximum number of points can be observed in the most optimum configuration.
  
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  Movement can be detected in sidewall or damage to the reference points as incorrect point fixture can be identified by the software and the user warned thereof.
  
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  Compromised points can be removed from the observation in order to improve the accuracy of the position fix.
  
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  Redundancy of points as the number of available reference points are increased to four pegs not just one, allowing multiple bases of reference to become available.
  
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  Combinations of reference points can be used in order to improve the robustness of the network.
  
• 
  No limit of reference points that can be observed.
  

14. Perceived disadvantages of the sidewall station method:

• 
  Any error in freestation positions will be transferred and perpetuated by the installation of new sidewall stations installed from this setup and lead to error propagation.
  
• 
  Incorrect identification of points may still provide a “fix” but will be incorrect.
  
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  It will require change management to all levels of surveyors in order for the sidewall station method to become an acceptable alternative to conventional surveying.
  
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  The manual checking of the freestation calculation is difficult without least squares adjustment. It has been found that using all the measured information will not provide a satisfactory answer, leading surveyors to distrust the method.
  
• 
  It has been found that very few surveyors actually understand how the limit of error prescribed by the act needs to be calculated for a conventional hangingwall traverse. In the case where the sidewall station method will be used this confusion may be compounded.
  

15. Suggestions and recommendations for using the sidewall station method would include:

• 
  The modification of the freestation program to include the allowance for face left³⁷ and face right readings combined with measuring the distance in each face.
  
• 
  Points to be installed on the grade line to serve as the gradient control.
  
• 
  It is advisable that radio communication between surveyor and crew be used if the use of radios is allowed by mine standards. In the case of a very noisy environment the survey crew will have to make use of pre-arranged hand and light signals to communicate. The correct numbering and identification of pegs are crucial to this method of surveying, it would therefore be advised that the surveyor personally oversees the installation and numbering process. The use of a skilled survey crew is essential to the success of this method of surveying.
  
• 
  The illumination of targets in the correct numbering order is critical. It was found that using the instrument’s red laser provided a good method of signalling the intention of the surveyor to the crew as far as which target should be illuminated.
  
• 
  If survey stations are installed on the grade line it is advisable that only one or two prisms are used and the prisms moved as the surveyor completes his orientation, in that manner measurements cannot be deflected or made to the incorrect target.
  
• 
  Observation protocol must include strict numbering and observation discipline.
  
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  The maximum distance before a check survey is done should be 750m.
  
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  Bearing checks by gyroscope are essential to prevent bearing error propagation.
  
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  Working-at-heights standards should be modified.
  
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  Inspection of sidewall conditions must be rigorously enforced as sidewall collapse in deep mines can happen.
  
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  Closures between networks should be made wherever possible.
  
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  The observation geometry including the maximum distance of observation, minimum distance from or between pegs must be determined.
  
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  The limit of error in the MHSA should define the error vector in 3D.
  
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  the focal length of the Total station constrains the optimal spacing of the sidewall survey stations from the freestation position, in most cases this would be a minimum of 1.7metres.
  
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  On-board software of the instrument must be adjusted for underground methods. Currently the best practice is to ignore the signs of the instrument and target heights.
  
• 
  In order to prevent dirt from contaminating the exposed screw thread in the tapered brass plug, the plug should sealed with commercial re-useable putty or blasting protection.
  
• 
  Ensure that the correct prism constant is used.
  
• 
  Co-ordinates of sidewall stations are “pseudo” positions dictated by the size of the prism and the offset of the prism centre which is at an offset from the true sidewall position.