I-SiTE Product Development White Paper Written by Simon Ratcliffe and Andrew Myers
Title:
Laser Scanning in the Open Pit Mining Environment A Comparison with Photogrammetry
Created:
July 1, 2006
Introduction In recent years the terrestrial laser scanner has developed from a high technology gadget to a user-friendly instrument that is fast becoming an essential measurement tool for the surveyor. Our technical services experience and sales figures support this statement nowhere better than in production open pit mining environments. It is here that the operational realities of time, safety and multiple competing uses for measurement services place extra demands on measurement professionals and equipment. No rival technology at present comes close to matching the utility of a laser scanner, such as the I-SiTE 4400LR, for performing a large portion of the measurement tasks required in this environment. By comparison, digital photogrammetry using automated software point matching, represents a great advance in the state of the art in the field of photo-metrology. It offers a realistic alternative measurement technique to laser scanning that is capable of producing similar deliverables to the “end user�, such as the geologist or mining engineer. For a cheaper initial outlay, it can offer an alternative for obtaining photomapped face models in 3D for geological interpretation and occasionally data for volumetric analysis. However in both these respects it is not an equivalent alternative to laser scanning. We do not believe it to be a cost-effective alternative either once operational considerations are taken into account. We argue here that a purpose-built laser scanning instrument such as the I-SiTE 4400LR is a superior choice over even the best photogrammetry system for getting the job done on the measurement tasks most frequently required on a large open pit mine site. Photogrammetry systems may be reliable for the same tasks, and should be considered on some sites, but are not easily employed. This view is founded on key operational and performance differences that are highlighted when it comes to working with professionals in mining operations around the world. In a production mining environment, where data acquisition is just one of many tasks undertaken by geologists, surveyors and engineers, the ease of use of any system is of paramount importance to quickly and effectively gather data. We point out some of the operational differences between using the two technologies for gathering data. We briefly explain how laser scanning works and make various comparisons between the measurements obtained via scanning compared with photogrammetry. It is also important to understand that the I-SiTE 4400LR is unique among similar laser scanning products in that it integrates the now familiar laser scanning for 3D data generation with the high resolution imagery available through digital CCD photography to create a combined 3D surface 1 . So in addition to all the uses to which 1
The factory integrated combination of laser scanning and linear CCD imaging is the subject of a patent we hold.
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laser scanning can be applied in a mining environment, it is the only device capable of generating a correctly geo-located image-mapped 3D surface directly from a single point of acquisition, which is suitable for detailed geological mapping.
Laser Scanning – Summary of Key Advantages
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The laser scanner is a survey instrument. It is tightly integrated with surveyors’ existing workflows, whereas photogrammetry systems require different training and a different mind-set. Surveyors experience operational advantages with laser scanning that are difficult to match by even the best photogrammetry systems: Only an hour or two training time on site is required to fully become familiar with using a laser scanner correctly and precisely. Concepts such as setting up and backsighting to known points are already second nature to professional measurement personnel. Rapid set-up and take down is routine with no requirement for in-situ (or on-site) calibration. Geo-located data is obtained automatically from the measurement process. Faces, stockpiles and failures do not require marking with paint or reflectors for accurate geo-location. This is a major safety advantage, especially in a mining environment. The measurement process is based on simple physics that we all understand.
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The laser scanner is an active self-contained measurement technology. It generates its own laser light for the measurement process. On the other hand photogrammetry is passive and intrinsically reliant on (1) the lighting conditions present in the environment and (2) multiple set-ups for all measurement tasks. Current systems are also substantially reliant on postprocessing of the data to obtain results. Photogrammetry systems are therefore no match when it comes to: The ability to obtain measurements under any lighting conditions, including times and places of high differential lighting. 2 The ability to measure dense point coverage irrespective of visible features in strata or stockpiled material. 3 The number of set-ups required. 4 Instant checking of data in the field to guarantee one field trip only, every time.
Oblique sunlight with features casting shadows is typically a troublesome scenario for photogrammetry systems, particularly when shadows change between shots due to time delay or differential cloud cover. In very low light or night time the task becomes impossible. Laser scanning can operate at any time of day or night provided atmospheric conditions are free of large quantities of dust or precipitation. Photogrammetry on a featureless face or a completely homogeneous stockpile is equivalent to doing photogrammetry in the dark – no matching stereo points can be obtained. In practice this can lead to extremely sparse data in certain applications and introduce uncertainty into the measurement result, but more importantly the measurement task. “Are you sure we can get that?” becomes the question always asked of photogrammetry in poor lighting or on material lacking sharp features. Because of the 360-degree imaging capability of the laser scanner and the fact that large overlap continuity is not required between set-ups, typically less than half the number is required than with a photogrammetry system in a complex pit environment for a typical survey.
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Laser Scanning – Measurement Theory Laser scanner measurements rely on two basic physical principles: • •
Laser light travels in straight lines. It does so with constant speed.
These two principles lead to the direct measurement of distances by timing the passage of a pulse of laser light to and from a surface or target in the measurement environment. These principles underpin much of modern survey practice because they are also employed in the rangefinders used in survey total-stations. Laser scanners, like total stations, must also measure the angles at which the beam is sent out of the instrument in order to accurately resolve distance measurements to locations in space. This is achieved through measurement of angles internal to the instrument and is factory-set – no post-analysis or software reconstruction of the measurement geometry is required. A laser scanner differs from a total station in that instead of locating individually specified points to high accuracy, an automated dense coverage of hundreds of thousands of points is recovered from a scene by steering the laser in two axes of rotation while rapidly sampling. Each point does not correspond to a feature but rather a spatial sample of the position of the surface at that point. The size of the surface patch that is averaged to formulate the position of this point is a function of size of the laser spot footprint, which is a function of range. Figure 1 illustrates the scanning principle. The key point is that a dense coverage of data regardless of specific features arises from the measurement process.
Figure 1: Uniform automated dual axis rotation of the rangefinder in a laser scanner yields uniform angular point coverage of points rather than precise location of selected features.
Multi-path returns can be an issue with pools of water and retro-reflectors when laser scanning. False points not corresponding to a true surface can result when the laser spot illuminates two surfaces, one in the foreground and one in the background,
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giving a point somewhere between the two. The presence of falling rain, snow or substantial airborne dust can also yield point measurements not on physical surfaces. In our experience, mitigation of all of these factors is readily achieved in an open pit mining environment and only poses rare operational delays and even rarer abandonment of the field measurement process. Figure 2 is an illustrated summary of the problems to avoid. 5
Out of range Unwanted data
Dust scattering
Structural occlusion
Poor reflectivity material
Reflected points
Temporary occlusion
Figure 2: Problem areas for obtaining good laser scan data in a pit.
As a result, the vast majority of data points lie within tight, known error tolerances of the true physical surface at the measurement location, leading to excellent confidence and reproducibility of measurement results with laser scanning. This is most notable with the measurement and location of linear, area and volume features within the scene. Alternative technologies typically outperform laser scanners on individual point accuracy, but beware of comparisons that fail to account for the overall accuracy gains achieved in modelling extended features from laser scan data. Length, area and volume measurements obtained from laser scanner data benefit from the thorough uniform point coverage. To take a simple example, the position of a plane in terms of its distance from a datum may have an uncertainty of +/- 50mm with a single point measured on it with a scanner. However, once a plane of best fit is used to average 10000 points scanned on the plane, each with an independent uncertainty of +/- 50mm, the position of extended feature in space can be resolved far better from a laser scan – to 0.5mm in fact. 6 Absence of data from particular surfaces is often observed in laser scans when the reflectivity of the material is below the threshold of detection at a given range or incidence angle – this effect can be seen at the range extremes on most scans but can also show up as “black holes” on road surfaces, coal faces and similar absorptive material at only modest ranges in some scans. The onset of this is a function of laser instrument power and detection sensitivity. If dense data is important 5
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Avoiding reflections and dust is not always possible but false data arising from them is easy to identify and remove with software filtering operations after measurement when necessary. Uncertainty improves as √N where N is the number of independent measurements. So in this case,
σ plane =
σ po int N
=
50mm 10000
= 0.5mm
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from that location, then an alternative set-up must be chosen closer or more normal to the surface in question. A quick inspection of the scan on the instrument controller in the field can reveal this, and an alternative set-up can be chosen then-and-there if required. It does not take long to build a base of experience where repeatable setups for known jobs lead to a highly repeatable, hassle free, routine measurement process.
Laser Scanning and Photogrammetry – Operational Comparison There are several operational differences to take into account when comparing laser scanning and photogrammetry. Any particular measurement job can be broken down into a number of stages. A planning stage is where the areas to measure and data collection locations are selected. This is followed by an acquisition stage where the data is gathered and finally a post-processing stage where the final 3D surface and image is made available for further down-stream processing. The planning stage for photogrammetry involves a number of steps, all of which have considerable bearing on the quality of the final product. The planning stage for laser scanning is considerably less involved. In order to measure a given area using photogrammetric techniques, camera lenses must be selected with a particular focal length and a suitable baseline determined to give the required accuracy and sufficient overlap of images. At least two acquisition points are required for any photogrammetric measurement, whereas the inherent 3D measurement process of the laser scanner can generate a 3D image from a single setup point. This simplifies the planning stage where there may only be limited site access to obtain views of the surface to be measured. The camera lenses must be calibrated to take into account distortion, and calibrations must be done for various focal lengths, aperture settings and focusing. By comparison the laser scanner is a calibrated survey instrument, so the device can be located at any distance from the surface, within the range limits, without any changes in its configuration. The complexity of the surface to be measured also has a bearing on the planning stage. Mapping a long, straight highwall may simply involve a number of images taken from the opposite wall. However for more complicated surfaces where the distance from the camera to the surface can vary greatly (from 10s to 100s of metres) more thought needs to be given to set-up locations. For a given scan versus photo pair, the laser scanner measures a wider range of distances. Coverage is also a full 360 degrees around the device. A similar task with photogrammetry would require a number of images from a single location to be merged. A typical scenario is presented for comparison in Figure 3. An extreme example encountered in small steep pits is illustrated in Figure 4. In order to geo-reference the data to a mine coordinate system, either the locations of the instruments must be known or there must be identifiable markers on the surface whose position can be surveyed. With the I-SiTE 4400LR scanner, locating the scanner over a known location and backsighting to a second point is sufficient to fully geo-locate the data because of its built-in level compensation. With photogrammetry the camera locations could be measured, but with reduced accuracy if the optical centre of the camera is unknown. Even with the position of two cameras on the baseline known, without the ability to level the cameras, there is still a requirement for additional control points. Generally this involves placing markers,
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either painted on rock faces or using survey pegs on the surfaces to be measured. In many cases however, due to safety reasons it is not possible to do this.
B
B A
B B
B A
Figure 3: Even simple open pit operations involve geometrical complexity in set-ups. A realistic comparison of 2 set-ups (symbols marked A) required with a laser scanner vs 5 setups plus multiple shots (B) with photogrammetry. The differential lighting potentially affecting photogrammetry deployment, but having no effect on laser scanning, is illustrated also.
Figure 4: A more extreme yet typical example encountered in general pit survey when a laser scan of the bottom bench and walls is required to keep a pit model up-to-date. Obviously larger pits may involve several scans if range becomes an issue, but we’ve never seen more
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than half the number of scans required than camera set-ups when comparing the two technologies on a practical job.
Due to the passive nature of the photogrammetric process, the best results will be obtained when the surfaces are well lit and when that lighting does not vary during the acquisition process. This may place some impositions on the times at which particular regions can be measured. As already discussed, laser scanning is an active sensing technology and does not suffer from these limitations. In general, laser scanning can be used to measure a wider variety of surfaces without any significant planning prior to acquiring the data. Once the planning for the photogrammetric measurement is done, the data acquisition is straightforward. Best results would be obtained using a tripod set up on a known location. In most cases there would be more camera locations required compared to laser scanning, but the image capture is a fairly quick process. Setting up the I-SiTE 4400LR requires a little more care in locating the scanner over a known point, levelling and backsighting for geo-referencing, but no more so than other surveying instruments. The actual scanning may take several minutes 7 , but this requires no further operator involvement. Once the laser scanner has acquired the data, there is little need for further processing to obtain the 3D imaged surface. The data can be inspected in the field to ensure correct coverage. A single 3DP file 8 contains the 3D surface data and a correctly registered photograph of the scene. The photogrammetric process now requires post-processing steps to obtain the 3D data points. It is not until this process is completed back in the office that it will be determined if the correct result has been obtained. The photogrammetric post-processing steps involve loading the images into the software package. User input is required to select several correspondences in the various photographs in order to automatically extract 3D points from the images. A degree of quality control is also required on the part of the user to ensure that the automatic matching process locates points to within the expected tolerances. The required software algorithms to maximise point density and minimise errors in depth determination over the full dataset – known as “auto point matching” and “bundle adjustment” respectively – are slow and computationally intensive. As output from a self-contained measurement device, the laser scanner points are all guaranteed to lie on the real surface within a fixed uncertainty 9 . In addition, the scanner provides a dense set of data points regardless of the lighting conditions on the surface. So at the end of the planning, acquisition and post-processing stages, both photogrammetric and 4400LR laser scanning approaches yield 3D surfaces with coordinated imagery, which can be used for various tasks such as geotechnical analysis, volumetric measurement and reconciliation, and geological studies. It would be argued, however, for day-to-day measurement tasks which are required on an ongoing basis, as opposed to one-off specialised projects, laser scanning offers a more general and effective solution.
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A 360° scan at standard resolution takes 5 minutes to complete with the I-SiTE 4400LR. A 3DP is a “three dimensional picture” file. 9 This statement applies to an unambiguous measurement surface with no foreground partial occlusions and in clear air. 8
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Laser Scanning – Multi-spectral Geological Interpretation Contrast characteristics between ore and waste in certain rock types, for example oil in some oil sands deposits, can be superior in the wavelength of light used by the scanner than in the visible spectrum. The ability of the scanner to map and see in the infrared spectrum can thus sometimes be used to great additional advantage, especially if photography is only available under less than optimal lighting conditions. Obtaining a photo-mapped model of a real-world scene is available using both technologies but obtaining an infrared reflectivity map to add a multi-spectral component to data interpretation is only available using the laser scanner.
Laser Scanning and Photogrammetry – Point Accuracy Comparison For any given measurement scenario using photogrammetric techniques, the required point accuracy can be predetermined and achieved given a suitable baseline separation and lens configuration. This provides the flexibility to measure a wide range of distances, theoretically from microns to interstellar distances, and a configuration can always be proffered that will exceed a given laser scanner’s distance and planimetric accuracy (and be capable of making measurements at greater range). However, there is more to the story. The accuracy of a photogrammetry system configuration is a complex function of the set-up geometry 10 . The associated errors for laser scanning remain far less influenced by the measurement set up. For all time-of-flight based laser scanners, the distance accuracy is almost constant with range 11 and planimetric accuracy of the surface sample is a function of the beam divergence of the rangefinder and the direction determination of the mechanical scanning system – typically a few factors worse than the resolution achievable with focal plane devices such as CCD cameras. However the 37 Mpixel camera of the 4400LR scanner samples pixels at a higher resolution 12 than the range points, meaning that more details are obtained than by using the range points alone. For example, a single point from an I-SiTE 4400LR laser scan at a distance of 300 m would have a range accuracy of 50 mm. With a beam divergence of 1.4 mrad this measurement would correspond to the average distance to the surface patch lying under a footprint of diameter 420 mm at this range. A 0.04º angular uncertainty in the beam direction gives a planimetric accuracy of 210 mm. But at the same time, and in the same 3D image, there may also be points at 50 m range with a 30 mm uncertainty and planimetric accuracy of just 35 mm. Photogrammetric techniques could not capture the same range of distances in one task due to both the need to calibrate the set-up for a specific distance and the inherent inability to cover the entire region using a pair of photographs.
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It is a strong function of distance. Stereo pair matching is essentially a triangulation technique, the accuracy of which degrades with the square of distance. Distance uncertainty weakly increases with range – only because of the noise performance of the receiver system. Typical values for the I-SiTE 4400LR rangefinder uncertainty are 30mm at 50m, growing to the specified 50mm at 300m. The camera image is five (5) times the horizontal and three (3) times the vertical resolution of the range image when scanning in the highest detail.
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Figures 5 and 6 show images of a highwall displayed directly out of an I-SiTE 4400 13 scanner with zero post-processing, revealing the 3D detail achieved when higher resolution photography is automatically combined with lower resolution laser scan geometry.
Figure 5: Photography at 15 times higher resolution than the laser scan points is shown automatically mapped onto a highwall face scanned at 80 m. Note that the camera position shown here is artificial because the data has been rotated and zoomed in the 3D software environment to show the detail – the laser scanner was located some 80 m to the right of picture when the scan was taken. See Figure 6.
Ultimately the choice of measurement technique may come down to what degree of accuracy is required to complete the task, whether it be mapping a highwall, determining the volume of a stockpile or carrying out a topographic survey. If the accuracy requirements of the work being undertaken demand better than that offered by the I-SiTE 4400LR, and this should be investigated and determined by trials, then other measurement instruments and other technologies can be adopted. However the I-SiTE 4400LR has been designed from the ground up as a general purpose, self-contained 3D imaging system, and has been put to effective use in mining environments worldwide.
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The current 4400LR has 40% better angular resolving power than this earlier model.
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Figure 6: The same camera position as Figure 5 is shown with only the wire frame 3D geometry from the laser scan process shown to facilitate comparison with the gain in resolution obtained by the 37Mpixel photography offered by the I-SiTE 4400LR scanner.
Conclusions Laser scanning and photogrammetry are both technologies that are solving 3D measurement problems more effectively and easier than ever before. Both may legitimately be put side-by-side for comparison, particularly in geological mapping applications where they can offer a comparable end result. However, from practical considerations of familiarity, training, deployment, location geometry, environmental considerations and safety in the context of an open pit mining environment, the case is clear cut in favour of laser scanning, particularly when it comes to an integrated survey instrument such as the I-SiTE 4400LR. Additional support for laser scanning as a superior technique also comes from looking at accuracy considerations in some detail and the fundamental physics behind the alternative measurement principles involved. A full cost/benefit analysis is beyond the scope of this document and is dependent on site and personnel specific factors, but it is hoped that the reader will now be in a position to debate this with a fuller understanding of the real world technical and practical considerations. In our experience these factors dominate any analysis that sets out to arrive at meaningful predictions and conclusions when faced with a choice about which technique to invest in.
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