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By Paul Wilkinson
September 11, 2017
The construction trade press may be buzzing about virtual reality and augmented reality (VR and AR respectively), but, for many projects, capturing ‘actual reality’ is a more basic and pressing need. Photogrammetry can provide a low-cost alternative to laser-scanning.
As outlined previously at Jobsite (‘Location Intelligence’ - Why Construction Needs GIS, and vice versa), we have long been able to capture details of sites for new projects by survey methods. A site layout might be created from maps, perhaps augmented by hand-held and aerial photographs, as well as satellite imagery. Preliminary survey work might be undertaken using measuring chains and theodolites. Surveyors might also use total stations so that they can accurately capture and process data for further use by computer-aided design (CAD) or building information modeling (BIM).
We also described laser surveys, through which a tripod-mounted laser scanner can capture thousands of densely-scanned points very rapidly, creating a ‘point cloud’ that can be used to generate 3D imagery for use in CAD and BIM. Such instruments can measure around a million points per second at great accuracy (+ or – 2mm per 100m), while vehicle-mounted scanners used in conjunction with GPS can rapidly survey linear infrastructure such as rail tracks and highways.
However, sometimes a laser scan may not be possible. Tripod-based laser scanners are expensive high-precision pieces of equipment, often hired for short duration use on site by specialist operators. Also, they may not always be able to scan in very confined spaces. While some hardware vendors have developed hand-held scanners, they too are often expensive to hire and may be unavailable at short notice. Moreover, the cost of a millimetre-accurate laser survey may not always be justified; staff may want to quickly grab a loose approximation of a particular feature, or they may be involved in activities – such as master-planning – where a wide contextual view of an area is more important than detailed measurements within it.
This is where digital photography and, in particular, photogrammetry, comes in. Much as optical theodolites and total stations are being replaced by laser scanners, traditional film cameras have now been almost totally replaced by digital cameras. As the image resolution of digital cameras has also dramatically increased in recent years, high resolution digital sensors have dropped in size and price. Many smartphones now incorporate sensors capable of broadcast-quality, high resolution photographic outputs.
Photogrammetry is the science of taking measurements from photographs. The practice dates from the mid-19th century. Essentially, it allows a user, if they know the scale of the image, to measure the distance between two points. Today’s sophisticated photogrammetry uses multiple, overlapping digital images to not only capture and measure distances, but to build an increasingly accurate 3D visualisation of the subject matter, with computer software using camera’s specifications, its lens, and its relative position, and then comparing common visual elements shared across multiple photographs.
Solutions include Catch in Autodesk’s 123D suite, PhotoScan, RealityCapture, and Bentley’s ContextCapture (formerly Acute3D's Smart3DCapture). To give an example of the potential of such tools, Bentley’s application was used to create a photo-realistic model of large areas of central Philadelphia so that authorities could plan for the Pope’s visit to the city in 2015. A ‘reality mesh’ was captured based on 28,000 digital photographs, including imagery taken by helicopter. A 3D model was then populated with maps and designs, helping communicate the details of over 56,000 temporary structures, 33 miles of security barriers, and other safety and transportation provisions.
Compare to laser scanning, photogrammetry has lower equipment costs; the equipment is often more readily available, and the technique is more easily learned. However, both approaches require use of specialist software to combine the data into useful imagery, and the resulting outputs are often large datasets (Bentley’s Philadelphia dataset was 28GB) that may need sophisticated hardware to process, manipulate and store.
A related field is 3D or stereoscopic photography where the end result is similar to that viewed by human eyes. It is achieved by taking two photographs from different horizontal positions, using separate side-by-side cameras or a stereo camera with two or more side-by-side lenses. Like photogrammetry, 3D photography can be traced back to the 19th century; the mid-20th century saw film-makers delivering 3D films, and more recently we have had 3D television. Today, 3D digital cameras have become a popular consumer item, but there are also professional grade 3D devices, such as the Matterport. It can be used to take multiple images of a space, which, when processed and combined online, can create an immersive VR visualization – popular with hotels and property letting agents as well as for construction purposes.
Some of this functionality is even available on a smartphone: the Lenovo Phab 2 Pro is (so far) the only phone to incorporate Google’s Tango technology (launched in 2014) and able to interact with Matterport’s Scene app. The phone’s array of cameras and sensors enable 3D motion tracking; depth perception (using an infrared projector) can be used to create 3D point clouds, while ‘area learning’ works like an indoor GPS.
Digital hardware advances are rapidly changing how we capture the world around us. They are providing us with new ways to visualize anything from pipework in a plant-room to Papal progress around Philadelphia. The skill will be in knowing when, where, and how to utilise photogrammetry or 3D photography technologies so that they aid decision-making – and not just create interesting or pretty pictures for no practical purpose.
If you liked this article, here are a few webinars you might enjoy:
Leveraging Virtual Reality for Construction Execution
Augmented and Virtual Reality
The Virtual Jobsite: Advantages, Pitfalls, Best Practices & the Newest Tools for Tracking Job Progress
Augmented and Virtual Reality: The Latest Job Site Technology
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