Predicting and monitoring rock falls at El Capitan, Yosemite National Park, California.
Down to Earth
26 October 2012
Remote sensing of our environment used to happen - well - from a distance. Now it seems you can pop some kit on your car and off you go. Nick Tate tells us about some rapid developments in this important technology.
Science and technology have always been intimately linked - technology makes scientific observation and experimentation possible. Environmental science in particular often uses technology that indirectly senses physical properties - often from afar - and Terrestrial Laser Scanning (TLS), also known as ground-based LiDAR, is the focus of this article.
LiDAR (which stands for Light Detection And Ranging) has been around in some form for several decades, but the size and cost of the equipment has put TLS out of reach of field-based scientists. Now things are different. The size and cost of the equipment have reduced dramatically and we're seeing an explosion of applications and technological development.
The technology is similar in many ways to its better known cousin, RADAR. Both systems determine distance from the time it takes for an electromagnetic signal to reach, and be reflected back from, objects of interest; but while RADAR uses radio waves, LiDAR uses light waves from a low-energy laser.
It's already very clever, but recent developments are making the whole process even smarter.
LiDAR has historically been used on platforms like aircraft or satellites, to survey the landscape below. For example, NASA's Geoscience Laser Altimeter System (GLAS), which operated from a satellite called ICESat, could measure surface elevations down to tens of centimetres. The measurements are used to produce surface models which can then be further processed to remove things like trees and buildings, depending on what the model is needed for.
It's already very clever, but recent developments are making the whole process even smarter. LiDAR systems have been getting smaller, lighter and faster. The latest models can be used on cars and helicopters, and TLS is light enough to be mounted on a tripod. Today's TLS systems can be as light as 5kg and capture an amazing 1,000,000 measurements every second. The ability to obtain such fast and millimetre-accurate 3D surface models from these systems has produced an explosion of applications, from measuring landslides and cliff erosion to modelling forest canopy and tree geometries, and the characterisation of surfaces like river banks and beds. Often these features have characteristics which can only be captured from an oblique scanning viewpoint which isn't possible from aircraft and satellites. For example, car-mounted LiDAR systems are increasingly used in cities to capture building frontages. This data can then be fused with roof information gathered from airborne LiDAR to produce very accurate 3D models of cities.
Preparing to survey a lava dome using ground-based LiDAR.
The hardware is getting more specialised all the time, and one of the most exciting recent developments is full waveform TLS. Instead of building up a 3D model from a collection of points sampled from a signal, a full waveform scanner records the full waves of reflected laser energy. The great benefit of this technique is that the extra detail it provides can help us understand complex scenes, for example the mix of leaf, bark, sky and ground you are likely to encounter in a vegetation canopy.
TLS is not without its problems, mostly down to the amount of power the systems need and the large volumes of data they have to deal with. Terabytes of data make serious demands on data storage hardware and processing software.
Sharing knowledge and expertise is crucial for solving these problems and getting the most out of technological advances. To that end, in 2010 we set up LiDARnet, to bring together the users, suppliers and developers of TLS technology. The network is part of a wider initiative called the Earth Observation Technology Cluster, which helps academics and industry work together to exploit remote sensing technology.
The network has really helped shape the way forward, identifying training needs, software and data-storage developments, and highlighting the potential for both commercial and non-commercial applications.
Thanks to TLS, we have the best quality 3D data for modelling and testing scientific theories about our environment. And the technology develops apace - an exciting future beckons.
The Earth Observation Technology Cluster involves the Remote Sensing and Photogrammetry Society, the National Centre for Earth Observation, the British Association of Remote Sensing Companies, and the International Society for Photogrammetry and Remote Sensing.
Dr Nick Tate is a senior lecturer in geographic information at the University of Leicester.
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