Quarry Surveying

21 April, 2015

First published in the April 2015 issue of Quarry Management

An examination of quarry surveying techniques and technologies – past, present and future, highlighting some of the global challenges

By Simon Briggs, managing director, Geodime Ltd

Over the course of the last 50 years, the development of survey technology has been dramatic, often at the cutting edge of worldwide technological development. The unique nature of quarries and opencast mines has posed problems for the accurate three-dimensional mapping of complex and inaccessible areas. The challenge has now shifted from data collection to ‘big data’ handling; from collecting 300 measurements a day to manipulating 300 million.

Survey instrumentation available in the mid-20th century relied on excellent optics. Trigonometry was the foundation of surveying, using the measurement of both horizontal and vertical angles to give an angular difference between points and to derive a distance. Recording data, even into the 1980s, was done by hand and plans were drawn manually. Instruments were built to last. The iconic Wild T2 went into production in 1924 and continued to be made until 1991. Surveying was exciting and pioneering; concerned with building Bilby towers to get above the jungle canopy and astro-navigation to fix a position in the desert.

The development of EDM (electronic distance measurement) was revolutionary. The transmission of an infrared beam and its reflection back by a prism to give a direct measurement of distance was initially an add-on module. However, when it became fully integrated into the theodolite the term ‘total station’ was born. At the same time, onboard logging allowed the reliable collection of more data and relieved the surveyor of the elusive hunt for transposed numbers.

During the 1990s total stations became robotic, the prism being automatically tracked and controlled from a remote radio on the pole. The assistant, who formerly served an apprenticeship behind the instrument, was suddenly freed to work productively and learn alongside the surveyor. The integration of a laser meant that, finally, inaccessible areas could be surveyed remotely.

Global Positioning System

An enduring problem for the surveyor was how to fit the survey into real-world co-ordinates. In the mining and quarrying industries, arbitrary co-ordinate systems were not an option as everything had to be fitted to the National Grid. To traverse in from trig pillars was the ultimate solution, however in most cases, fit to OS was achieved by picking up and best fitting (graphically) points of local detail. In rural areas this would often mean fence corners, and although the OS is comprehensive, inaccuracies in the order of meters were common.

The development of the Global Positioning System (GPS) solved this key issue. Early systems were huge, heavy and required many hours of observation time to get one (post-processed) co-ordinate. GPS receivers calculating a position essentially by measuring accurate distances to satellites in an orbit 20,200km from the earth are very impressive but struggle with ionospheric distortion of the signals. This is resolvable by using a second receiver fairly close by to collect the same signals and transmit the corrections to the surveyor. Hence, differential GPS to centimetric accuracy.

At first the surveyor would have his own differential system, consisting of two GPS receivers acting as a base and rover. The base, if set up over a known point, would allow surveying directly to the chosen co-ordinate system, hence Real-Time Kinematic (RTK) surveying was born. If the corrections could be broadcast over the phone system, a local base station would no longer be necessary, freeing surveyors to work without constraint. However, a trial by the OS in the 1980s generated a massive phone bill and a lot of red faces. For many years progress had to wait for low-cost mobile phones and the Internet to catch up.

Today, surveyors around the world are able to pick up, via a mobile phone, corrections from national virtual GPS networks over the Internet; straight out of the car and immediately working on a National Grid. GPS remains and will continue to remain the primary means of positioning on the world’s surface. The network solutions are reliable, stable and very accurate. No markers are required on site and survey control is simple. GPS did, however, give quarry surveyors a headache around the issue of scale factor.

In the days of total stations and small quarries, many surveyors based their mapping on a simple, ‘earth is flat’ system, but GPS forced everyone to accept that it is, in fact, a sphere (or very nearly a sphere) and mapping has to accommodate this (in the UK, via the Transverse Mercator Projection). Close to the central meridian this is not too much of a problem, but further east and west of Greenwich, and with quarries well over 1km in length, distances on the ground could be markedly different from those on plan and many companies used this as an opportunity to shift and stretch their mapping to standardize on the OS co-ordinate system.

The headache for quarry surveyors was nothing compared to that now faced by the OS themselves, who knew the earth was spherical but suddenly found that there were errors in the primary control network on which all national mapping was based, sometimes in the order of tens of metres. Hence, the 1990s rectification programme. Now, with everything sorted, beloved trig pillars and benchmarks finally became redundant.

Point clouds

The emergence of high-definition scanning initiated the challenges that surveyors still face today. It is now possible to take thousands of readings per second. As surveyors, our role now is to filter and manipulate these data so that they are suitable for the multiple disciplines that require spatial data for the design and management of quarries. Around the turn of the century, led by the RICS, surveyors’ job titles changed. We were no longer land surveyors (collecting spatial data) but geomatics professionals responsible for the collection, manipulation and presentation of spatial data (albeit still with muddy boots).

The last few years have seen a huge refocus on photogrammetry. Age-old technology has been given new life by high-resolution, yet inexpensive, digital cameras teamed with powerful software that can recognize and match identical points in overlapping photographs to derive an accurate point in 3D space with an attached RGB value. The resultant point cloud, although similar to LiDAR data, is presented in full colour.

The new kid on the block, of course, is the unmanned aerial vehicle (UAV). The largest annual world exhibition for geomatics (Intergeo in Germany) has been packed full for years as UAVs explode on to the market. A recent report published by the House of Lords cited a prediction of the creation of 150,000 new jobs within the European UAV sector by 2050. This is certainly achievable if the current UAV innovation, production and application continues to grow with its current force. What is, perhaps, most interesting is the investment and commitment of survey equipment manufacturers to the most professional (and expensive) commercial systems on the market, alongside UAV manufacturers recognizing that survey is a key mainstream market rather than a niche specialism. Leading the way are Trimble with their acquisition of Gatewing, and Hexagon with Aibotix. Both systems sell for around €50,000. The BBC, by way of comparison, uses £800 UAVs for filming.

A UAV is, of course, just a platform for putting a sensor in the sky. For quarrying, this is crucial, allowing the sensor to ‘see’ all of the detail the operator wants to survey, and doing it remotely, thereby addressing one of the key health and safety issues of the industry; removing personnel from working areas.

The UAV industry is driving developments in sensors that will bring benefits to the mining and quarrying sectors. It is already possible to equip UAVs with LiDAR, GPR, multi- and hyper-spectral sensors (with potential applications for geotechnics). From a photogrammetric viewpoint, the UAV is able to fly with an accurately planned overlap and able to geotag the photography with an increasingly accurate camera position. 3D accuracy for the point cloud is now better than GPS.

Alongside UAVs is mobile mapping; a vehicle-based system that integrates GPS, high-definition scanning, inertial systems and, increasingly, photogrammetry, for extremely rapid data collection and processing at high vehicle speeds.

All of this exciting technology is about collecting point clouds and using them to generate accurate, rendered, three-dimensional models and ortho-rectified photographs. The problem is that the quarrying industry is used to, and far more comfortable with, conventional data sets, for example, polylines in CAD systems that act as break-lines in 3D models, and tadpoles on slopes. There is software that can auto-extract lines from clouds, but whilst this works well with building elevations and even trees, quarries have few really well defined geometrical lines. Faces do not have sharp edges but roll over, and this is very difficult to cope with.

The future

The future will require us all to work on and realize the potential applications for the rich, accurate and comprehensive virtual reality that surveyors today are capturing. As a profession, surveyors have always been frustrated about holding back a large part of the spatial data they have collected, from the time when levels were taken off paper plans to prevent overwriting. Finally, the days are coming when we can share it all.

A glimpse of the future can be seen in the strategic vision of companies such as Hexagon Mining, who see constant measurement and monitoring as the key to the most efficient and cost-effective use of mobile plant in particular, massively reducing their fuel costs. At the heart of their vision is a real-time, accurate, virtual 3D model of the mine or quarry. Perhaps the spatial data will be collected and processed live by the site’s own mobile plant or fully automated intelligent UAVs, but there will still be a role for the land surveyor in there somewhere.