• In [minute] detail

    • Part of Durban as seen from the air by 60-megapixal camera.
    • The Lidar data that"â„¢s collected can be used to generate a side view of the area surveyed "“ for instance, the bridge on the dam pictured.
    Date:31 December 2009 Tags:

    Digital maps like you’ve never seen them before.

    It’s being called Google Earth on steroids. High-tech airborne lasers with the kind of X-ray vision Superman would kill for – they penetrate all but the thickest vegetation – are enabling a Johannesburg company to create striking, precise maps, accurate to within 10 cm, that have revolutionised the way planners tackle everything from infrastructure to agriculture.

    The groundbreaking digital mapmaking system was refined by Southern Mapping Company, one of the top in its field globally. It combines Lidar (light detection and ranging), a 60-megapixel camera and hyperspectral remote sensing.

    In short, the technology makes it possible to map an accurate ground surface without cutting down forests. “If Joe Bloggs wants to build a road through there, we can survey it and the engineer can design it long before they’ve even cut anything. That can save years,” says technical director Jim Vaughan. In the DRC, with 60-metre-high tropical jungles, they were able to penetrate to the ground comfortably.

    The centrepiece is the newly arrived $1,1 million (about R8 million) Orion M200 airborne laser mapping system, which supplements the firm’s existing 3100EA unit. The high-precision system from Canada, the first on the African continent, is so compact, and so precise, that the US State department regards it as a “controlled item”… its potential in the missile guidance field has clearly not gone unnoticed.

    The Orion forms part of a suite of equipment put together by SMC and able to provide hitherto undreamt-of mapping accuracy. Besides the 3-D terrain models created after post-processing, it’s also possible to identify individual species of plants, gases and aerosols, minerals and soil types by means of hyperspectral remote sensing, measuring objects’ reflected electromagnetic energy and making use of the fact that everything has a unique reflectance “fingerprint”. This technology has become a vital tool in precision farming, where it helps determine soil properties, plant health, and even crop yield.

    It’s the new laser equipment that has got SMC really excited, though. Small enough to fit into a helicopter – currently, fairly big aircraft have to be used, restricting their ability to fly below about 500 metres – the Orion M200 will without doubt improve accuracy. Although much of the post-processing is automated, manual checking plays an important part in correcting outliers, from birds to leaves.

    “Say we fly over the building, but this building has skylights,” says SMC technical director Jim Vaughan. “Your laser shots go through the glass and you end up with this beautiful structure (showing the outline of a roof), but there are points 20 or 30 metres down there on some workshop floor.”

    In Chile, they surveyed one of the world’s steepest valleys, with a river 500 metres below.

    “The geological structure had a whole series of caves in it. We (mapped) caves that were below ground level. They weren’t big, just several metres, but they were not the norm.”

    Playing the what-if game
    SMC grew out of a group of surveyors working for the Eskom transmission Group. They were tasked with modelling the state of power lines in given meteorological conditions and with a given load flow – a process called ampacity. Combinations of power through the lines and resultant heat, combined with ambient temperatures, cause the line to sag and eventually to fail and collapse. For economic operation, power utility companies want to run their lines with the minimum ground clearance permitted, while putting the maximum power through them.

    With survey data at their disposal, utility companies run “what if” scenarios, juggling power line load, tension and ground clearance.

    “Two power lines that we surveyed for Eskom both gave a 50 per cent increase in power with relatively minimal mechanical work done to the power line,” Vaughan says. They avoided having to build a 50 km power line. “Now this was approximately 10 years ago, when building a new power line cost a million rand a kilometre.”

    In 2006 Vaughan, SMC chief executive Peter Moir, Jannie Engelke and Norman Banks left to start SMC. Today they’re a 28-strong team of land surveyors and GIS professionals whose clientele ranges from municipalities to farmers.

    “We can go over an area and the engineer can say, if I lift that dam wall up by 6 metres how much volume am I going to get? What’s the evaporation rate going to be? By the way, how many families am I going to displace? How much vegetation is in the flooded areas?” Vaughan explains.

    There’s more to come. Combining the exploration industry’s traditional geophysical measurements with the digital orthophotos may shed new light on what lies underground, for instance. Says Vaughan: “If we can get this into the hands of the requisite infrastructure designers, technicians, exploration geologists, whoever it might be, they are going to have a data set that they probably will never have seen before.” For more information see www.southernmapping.com

    How it works: Airborne Laser Terrain Mapping (ALTM)
    Lidar (light detection and ranging) calculates distance by emitting laser pulses and measuring the time it takes for them to be reflected. It’s based on a similar principle to radar (radio detection and ranging), which uses non-visible radio waves.

    Up in the aircraft, the Orion M200’s laser pulses at 200 000 times a second, hitting a rotating mirror that projects downwards a powerful 18-watt beam shaped like a disc. A reflected pulse indicates an absolute 3D co-ordinate in space; as many as four reflections can be measured from each individual disc-shaped pulse.

    At the same time, the 60 megapixel Trimble Rollei Metric digital camera would be snapping away. According to technical director Jim Vaughan, in 1997 SMC’s personnel were the first in the world to marry digital cameras with Lidar technology.

    To ensure positional accuracy, before the aerial survey takes place, surveyors on the ground establish known reference points. In flight, a GPS measuring at 10 times a second works in tandem with an inertial navigation system (INS), whose 3 ring-laser gyros and accelerometers determine the aircraft’s position.

    Once back on the ground, the Lidar point cloud is calculated and uploaded to computers for post-processing. Irrelevant or spurious information is stripped out, and what’s left is analysed to derive an accurate ground surface and digital orthophoto – an accurate aerial photographic map that can be used for taking measurements.

    Hyperspectral sensors – cameras that operate in the non-visible spectrum – provide additional information. The 400 nm to 1 000 nm band is primarily used to identify vegetation and its health, and the 1 000 to 2 500 nm SWIR (shortwave infra-red) band picks up evidence of minerals on the surface.

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