Topographic land surveys are fundamental to construction projects, giving architects and engineers an understanding of the land’s current elevation and contours before beginning development.
Traditionally these surveys have been conducted by ground crews going out on foot and plotting topography points. But as remote sensing technologies have become increasingly practical, aerial survey approaches like photogrammetry and LiDAR are seeing a dramatic increase in utilization. This is giving construction managers not only more detailed data, but data in places where surveying would have previously been impossible altogether.
LiDAR is a technology that uses pulsed laser light and its resulting reflections to calculate distance much in the way a bat uses sound reflections to locate its prey in the dark. The result is a highly accurate 3d map of the target.
Traditional LiDAR scanning via manned aerial vehicles can be resource-intensive to arrange and is too costly to be practically applied to smaller plots of land.
As LiDAR units have become smaller, lighter and financially accessible, manufacturers have begun mounting them to UAVs. While the market has grown accustomed to LiDAR units that carry price tags from $70,000 to $300,000 and weigh several kilograms, cutting edge units may weigh less than 600 grams and some can be acquired for as little as $10,000 or below.
This increase in availability and practicality has resulted in surveyors rapidly acquiring and utilizing the technology on a daily basis.
One example of a UAV-mounted LiDAR is the Phoenix Ranger System which is capable of capturing an extremely dense point cloud map by flying low over the target area and capturing hundreds of thousands of shots per second. The result is a 3d map with over 500 data points per square meter vs a typical output of 50-80 ppm from a manned aerial vehicle flying at a higher elevation.
This method penetrates tree canopies extremely effectively, revealing underlying elevation points that are otherwise hidden from the eye even with high resolution imagery or photogrammetry.
The resulting implications are huge: previously impassable terrain can now be surveyed in just a couple of passes with a UAV system. In areas with impenetrable vegetation, irregular elevations or otherwise suboptimal conditions for human ground crews to go by foot or vehicle, drones can be deployed to gather extremely accurate and information-dense results. What’s more, it is becoming cost-effective to implement this method on plots of land as small as an acre. Previously, creating a LiDAR-generated map for such a limited area would have been impractical.
Photogrammetry is a more common form of digital elevation mapping and is more affordable than LiDAR.
Photogrammetry operates by processing regular two dimensional photos into 3d cartometric models. The processing required to do this is resource intensive and the results are dependent on ambient light revealing an accurate picture for the base images. For this reason, photogrammetry cannot be conducted in the dark nor can it identify the elevation of ground that is covered by a tree canopy. Elevation data is less accurate than LiDAR point clouds as distance is judged by comparing closely overlapping images much the same way that the human eye judges distance. This means that several flyovers may be required to collect all of the images required. The result is a map that is intuitive to interpret but doesn’t have the same density of data.
On the upside, the technology is more accessible as it uses conventional cameras instead of expensive LiDAR sensors. The maps are also more intuitive to interpret by the human eye because they contain accurate color values as opposed to LiDAR, which outputs a monochromatic image unless color is added in post-processing. For projects that don’t require a great deal of precision, photogrammetry remains one of the best ways to generate a 3d map.
Surveying the future of digital elevation modeling
Since the cost of creating sophisticated digital elevation models is falling rapidly, a new generation of surveyors are able to affordably map the earth at a high level of precision. Both photogrammetry and LiDAR are poised to play key roles for the future of construction as well as cartography as a whole.