Creating Digital Elevation Models
By Ken Whitehead
Unmanned aerial vehicles provide accurate surveying results.
By Ken Whitehead
British Columbia-based Accuas Services Inc. has emerged in the new field
of Unmanned Aerial Surveys™ (UAS™). Accuas is credited as the world’s
first commercial UAV surveying company,
British Columbia-based Accuas Services Inc. has emerged in the new field of Unmanned Aerial Surveys™ (UAS™). Accuas is credited as the world’s first commercial UAV surveying company, and is continuing to grow this new industry. With crews located in western Canada and Ontario, the company carries out surveys throughout Canada using unmanned aerial vehicles (UAVs).
|A digital elevation model created using the Accuas technology.|
Similar in size and appearance to the hobby aircraft used by radio control enthusiasts, the UAVs are used to carry out detailed low altitude aerial photographic surveys of small and medium-sized sites from 20 hectares up to 30 square kilometres. The flight characteristics of UAV platforms make them ideal for producing high-resolution orthophotos and Digital Elevation Models (DEMs), which can be used to carry out on-demand volumetric surveys and accurate site surveys.
Traditionally, users requiring detailed site surveys have had to choose between ground surveys carried out by surveyors, or manned aerial surveys. While ground surveys can deliver high accuracy for specific points, they are unsuited to larger sites and tend to have low-density, irregular point coverage. Traditional aerial surveys are very expensive, and scheduling is usually determined by the aircraft operator, rather than the end user. UAV surveys offer a cost-effective solution for small and medium-sized sites of 30 square kilometres or less. High-accuracy surveys can be carried out on demand, at a price lower than would be possible for a conventional aerial survey. The point density of such surveys is considerably greater than can be achieved by ground survey methods, and therefore provides a better representation of the ground surface, especially for inaccessible areas such as the top of stockpiles, and for dangerous and inaccessible terrain.
The technology behind UAV surveys mirrors that of conventional aerial surveys in many ways. However, payload capacity is a limiting factor. The Areohawk platform that Accuas uses weighs approximately five kilograms and is officially classified as a mini UAV. The weight limitation precludes the use of a traditional aerial camera. Instead a lightweight micro 4:3 format camera is used to acquire the photography. Although the geometry and resolution is not in the same class as that of a conventional aerial camera, this is compensated for by the lower flying heights from which photos are obtained. UAV surveys are normally carried out at altitudes of less than 1,000 feet, giving a pixel size of around six centimetres on the ground, considerably better than is normally achieved using a conventional aerial survey.
In a UAV survey, the aircraft operates autonomously, acquiring photography according to a predetermined flight plan. Planning is carried out using proprietary software, which uses the site boundary, desired flying height, and desired overlap between photos as inputs. The program then produces a flight plan, which includes GPS waypoints, as well as the location of projected photo centres. This flight plan is uploaded to the onboard autopilot, which then guides the aircraft through the flight. Except for takeoff, the entire flight is autonomous, with the operator only intervening in unforeseen circumstances. Landings are performed by an automatic parachute deployment. Following the flight, the photographs, GPS flight log and data are downloaded for further processing.
Processing is carried out in house using photogrammetric software. The logfile is used to provide initial estimates of camera positions and orientations for each photo. Survey grade ground control points are used to refine the estimate, with final adjusted camera positions and orientations being produced using a rigorous block adjustment procedure. Once the acquisition geometry has been successfully recreated, detailed 3-D elevation models can be created, typically with a grid spacing of one metre. The digital elevation models are then used to produce a high-resolution orthophoto mosaic covering the site with a spatial resolution of 10 centimetres or better.
The elevation models can also be used to create accurate volumetric estimates, providing the aggregates industry with the timely inventory information it requires. According to Accuas, after carrying out a number of project accuracy assessments, they determined that typical horizontal accuracies are in the 10- to 15-centimetre range, with vertical accuracies in the 15- to 20-centimetre range. Survey accuracies usually fall within the American Society for Photogrammetry and Remote Sensing (ASPRS) level-1 accuracy specifications for horizontal and vertical1: 1,000 scale topographic mapping.
At present the main hurdles preventing widespread use of UAV surveys are regulatory. Transport Canada currently allows commercial UAV flights, subject to a number of restrictions. Although commercial flights in the United States are currently prohibited, the FAA Modernization and Reform Act of 2012 will change this, allowing commercial operation of UAVs within U.S. airspace within the next two years. As regulatory limitations become less of an issue, it is likely that this new industry will continue to take flight.
Ken Whitehead is a Photogrammetry / Remote Sensing specialist with Accuas.