Mobile scanning for stockpile volume reporting Surveying by André Oberholzer, EPA Survey The versatility and speed of mobile lidar scanning as well as its ability to survey large and difficult areas has seen the technology being used extensively on mines for stockpile volume reporting. This article compares mobile laser scanning with conventional methods and highlights the advantages of employing a mobile lidar system for volume reporting on mines. Laser technology has been around for nearly four decades now. Today we trust the technology to provide us with topographical maps of large areas acquired from the air. The airborne laser or lidar, as we like to refer to it, has established itself as a tried and tested method for obtaining large quantities of information quickly and accurately. Static scanners were once seen as a must have tool for every survey department. These scanners promised to deliver accurate survey results, at record speeds. Scanners were purchased and hopes were high, soon to be crushed by complicated software, stringent setup and scanning procedures, and long waiting periods to deliver data. Despite having much higher data capture rates and accuracies, the longer range time-of-flight static scanners in most survey departments today have become expensive wall ornaments, and are labelled as complicated, tedious and onerous to employ effectively. Most mine surveyors therefore opt for traditional survey technologies which are quicker and easier to use. Shorter range phase shift static scanners, as with airborne lidar, have found their niche with companies specialising in 3D laser scanning and modelling in the processing industry. A relative newcomer to the survey industry though is the mobile ground based lidar system. Mobile ground based lidar systems This is a revolutionary vehiclemounted concept with great potential as long as the necessary time and financial investment is put into the development and implementation of the software and its technological applications. A deterrent, though, of this type of technology is often the very steep and long learning curve to delivery of accurate survey results. Despite this mobile lidar technology brings with it increased flexibility and new opportunities. Scanning work previously requiring multiple static scanner setups, can now be done in a simple drive around. As the system is fitted with many high grade positional technologies, fewer control points are required to achieve the necessary accuracy requirements. Software Airborne lidar and the static laser scanner paved the way in terms of the development of software for the effective application of mobile lidar systems. Airborne lidar processing suites developed complex algorithms to correct and analyse moving positional data. In the process, software developers learnt how to handle large projects with specific routines in order to distinguish features from featureless uniform points. Static laser software developers also found intuitive ways to display multiple millions of points in a meaningful manner, to manage memory and to optimise the way computer hardware supports the new demand for speed. During the early developmental stages of mobile lidar, huge advances were made when developing the software needed to process and make sense of the captured information. The mobile lidar system collected more points than had ever been collected before. Today laser observations are now captured at millions of points per second, with point spacing of mere millimetres. Mobile lidar in the field Mobile lidar scanners are quickly becoming the alternative to expensive survey operations. It is a non-invasive Fig. 1: Conventional method for obtaining Fig. 2: A cone does not necessarily represent the true shape and volume of an actual stockpile. PositionIT Jan/Feb 2014 19
Conventional m 3 Lidar m 3 Variance m 3 % 6802,97 7393,74 590,77 8 Table 1: Comparison of obtained stockpile Fig. 3: A typical model of a stockpile as represented by a conventional survey. survey method, able to achieve centimetre accuracies, and more and more consumers are seeing first-hand the powerful data sets that these systems can deliver. Using a mobile lidar scanner provides the following advantages: Cost: The need for a large survey team and multiple surveys are eliminated as a single collect will produce a complete survey. Time: With the mobile scanner able to survey at driving speed, field time is cut down to an absolute minimum and stoppages or disruptions due to survey are minimised, and in some cases even eliminated completely. Complete data: The point cloud density is far superior to any alternative surveying method with the laser able to survey spaces that are normally hard to reach. Safety: People are taken out of the field, minimising exposure to hazardous risks and environmental elements. Confidence: By having a full 3D representation of the surveyed area by virtue of the dense point cloud, any disputes normally associated with survey can be settled with confidence as human interpretation of the environment is taken out of the equation. Mobile lidar scanning for volume reporting Mobile lidar scanning is used extensively on mines. Due to the versatility and speed that these systems operate at they are able to survey large and difficult areas. With the stringent safety requirements for surveyors in hostile mining environments it is often difficult to achieve the desired quality for a survey. In these situations the mobile lidar scanner provides a cost effective alternative. Due to the point density of the scan, actual surfaces are represented in the data and this results in higher levels of accuracy in terms of the measurement of stockpile Fig. 4: The point cloud of the stockpile represented in Fig. 3. Name Conventional m 3 Lidar m 3 Variance m 3 % (of Conventional) SP1 16 573,83 16 079,39-494,44-2,98 SP2 33 441,54 32 483,26-958,28-2,87 SP3 16 110,5 15 114,18-966,32-6,00 SP4 13 781,4 13 366,53-414,87-3,01 SP5 23 314,65 22 089,26-1225,39-5,26 Total 103 221,92 99 162,62-4059,30 Table 2: Comparison of the calculated results. SP5 SP5 SP4 SP4 SP2 SP2 SP3 SP3 SP1 SP1 1098 Points 130 803 Points Fig. 5: Comparison of the collected points. RTK Mobile lidar Fieldwork time 5,5 hours 2 hours Field team 4 persons 1 persons Officework time 3 hours 4 hours Original survey 3070 points 321 217 032 points Final survey 3070 points 108 740 points Table 3: Summary of the project statistics. 20 PositionIT Jan/Feb 2014
Fig. 6: Top view of complex stockpiles. Name Conventional m 3 Lidar m 3 Variance m 3 % 1 311 553 361 755 50 202 16,11 2 104 152 124 702 20 550 19,73 3 21 845 23 327 1482 6,78 4 23 937 24 486 549 2,29 5 34 768 34 670-98 -0,28 6 80 725 71 594-9131 -11,31 7 104 385 93 824-10 561-10,12 8 251 770 256 837 5067 2,01 9 587 202 583 393-3809 -0,65 10 20 238 15 867-4371 -21,60 11 6916 6532-384 -5,55 12 45 314 44 464-850 -1,88 13 12 432 11 699-733 -5,90 14 17 149 19 757 2608 15,21 15 2570 2331-239 -9,30 16 11 564 12 094 530 4,58 17 18 760 19 450 690 3,68 18 32 497 33 756 1259 3,87 19 5642 6158 516 9,15 20 1233 1525 292 23,68 21 46 725 45 373-1352 -2,89 1 741 377 1 793 594 52 217 Table 4: Summary of reported (Red indicates the lidar scanner reporting a lower volume. Green indicates the lidar scanner reporting a larger volume.) indicates that the mobile laser method is 8% more accurate than conventional surveying methods when conducting stockpile volume measurements. Consider a stockyard that employs stacking and reclaiming equipment. Compare the illustration seen in Fig. 3 to the actual point cloud of the same stockpile seen in Fig. 4. Fig. 3 illustrates how the stockpile seen in Fig. 4 might be represented by a conventional GPS survey. When comparing the calculated results, Table 2 represents the difference between the conventional survey using modern day GPS total stations and the mobile lidar system. It should be noted, however, that the stockpiles illustrated in Fig. 5 were very neat and very little interpretation of the survey positions was necessary. (Generally bigger differences are found in stockpiles with irregular and untidy shapes.) Despite the neatness of the stockpiles though, there was still a considerable difference in Conventional methods vs. mobile lidar Conventional methods for obtaining volumes rely a great deal on human interpretation, personal preferences and attention to detail. These elements contribute to the resultant volumes reported. When looking at the conventional method for obtaining volumes, the solution is as indicated in Fig. 1. For measuring a stockpile under a belt, observations are taken on the toe-line and tip resulting in a cone shape for calculation. Looking at the stockpile in actuality, the cone does not necessarily represent the true shape and volume (see Fig. 2). Table 1 compares the obtained volumes using both conventional methods and mobile lidar technology. The percentage variance in Table 1 Table 3 represents a survey conducted of ROM coal stockpiles using both conventional and mobile lidar survey systems, and Table 4 provides a summary of the reported Figs. 6 and 7 illustrate a plan of some of the surveyed stockpiles. These stockpiles have complex shapes with many hills and valleys. The complexity of the stockpile often influences volumes reported. PositionIT Jan/Feb 2014 21
a) Top view of potential stockpile b) Side view Fig. 9: Top view of a potential stockpile (a) and side view of a potential stockpile (b). Fig. 7: Plan of surveyed stockpiles. Fig. 8: Looking at a single isolated stockpile, the lidar survey of the stockpile thinned and shaded at elevation on the left, and a conventional survey of the same stockpile on the right shaded to elevation. Factors impacting conventional data collection Personal interpretation, survey habits and fatigue, as well as the condition of survey equipment, the equipment use and setup, and attention to detail can play a large role in the volumes obtained from conventional survey methods. Fig. 8 indicates where the potential for these differences can arise. Personal preference, habit and fatigue Take the example of a stockpile that has not been smoothed over yet and shows only loads tipped from a truck (see Fig. 9). The orange line showing on the top view of a potential stockpile (see Fig. 9a) is an example of a walking path that a surveyor would consider to cover the entire stockpile. The orange dots in the side view (see Fig. 9b) represent observed GPS points. The variations that can be achieved based on a poor walking path are very high. And as fatique sets in it becomes difficult for a surveyor to maintain a good walking method when climbing and observing every hill end every valley. Equipment condition, equipment setup and attention to detail The condition of equipment being used to measure volumes, the set-up and the user s attention to detail play an important role in the eventual accuracy of the collected data. Fig. 10 illustrates some of the factors that can affect accuracy. These include: Worn brackets Worn bags GPS antenna hanging skew Antenna height incorrectly measured Bending while climbing obstacles can affect antenna height from the ground Soft material on peaks results in the surveyor sinking into the material while walking. When measuring with a pole, the pole is often pressured into the material when walking. The variables mentioned above may seem subtle and unimportant, but tests have confirmed the impact on reported On a larger scale and over a period of time these small amounts could result in large discrepancies, usually associated with large sums of money. 22 PositionIT Jan/Feb 2014
Worn brackets. The GPS antenna has significant play. Bag worn. GPS antenna hanging skew. Antenna height incorrectly measured Fig. 10: Factors affecting conventional survey. With mobile lidar almost all of the above mentioned factors are eliminated. The mobile scanner comprises navigational instruments calibrated and placed in fixed positions. These instruments have precise offsets and references Bending while climbing obstacles affects antenna height from ground Soft material on peaks allow surveyor to sink while walking. When measuring with a pole, the pole is often pressured into the material when walking between them. The two laser scanners are positioned on a fixed angle providing optimal coverage collecting observations of up to 1-million per second. The operator is in charge of driving the vehicle and starting and stopping the scanning system. No personal interpretation or preference plays a role in the collection process. Because of the enormous volume of collected points, a complete threedimensional cloud is available after the survey to guarantee quality data. Conclusion Surveyors are required to measure and represent the physical world to the nearest degree of accuracy. This almost impossible task has been pursued by surveyors and hardware manufacturers for decades, with the result being the continuous improvement of accuracies. Mobile lidar technology has come a long way since its inception and is consistently improving. By utilising this tool to assist surveyors in overcoming the limitations of traditional methods, large advances have been made towards the delivery of safer, quicker, more reliable and much more accurate measurements. Contact André Oberholzer, EPA Survey, Tel 013 243-5864, andre@epasurvey.co.za Leica Viva GS14 GPS system CS15 The Leica SmartWorx Viva software operates with the GS14 GPS system and is designed with 2 goals in mind. Simplicity and Productivity. GS14 Scanning, Monitoring, Imaging and Surveying T: +2711 312 7450 F: +2786 770 0924 info@aciel-geomatics.com www.aciel-geomatics.com PositionIT Jan/Feb 2014 23