Surveying Using Drones - Basic Concepts
During the past 50 years, surveying and engineering measurement technology has made five quantum leaps: the Electronic Distance Meter (EDM), the Total Station (TS), GPS, Robotic Total Station and Laser Scanner. Unmanned Aircraft Systems (UAS's) or Drones (also known as Unmanned Aerial Vehicles or UAV's) will be the sixth quantum leap in technology. Although drones have been around for a while, the technology has not yet been widely used in the surveying and remote sensing professions. But it soon will be, thanks to the advent of practical, lightweight lithium polymer batteries, low-cost drone technology, lightweight digital cameras and advances in close-range oblique aerial photography - all of which make the future of drones in land surveying exciting.
There are two basic types of drones - fixed-wing units with a brushless electric motor and a rotor type that has between three and eight or more brushless electric motors (commonly referred to as multirotors). The rotor units enable drones to hover at a precise altitude and position and operate in confined airspace. The technology for both systems relies on the featherweight lithium polymer battery that stores an incredible amount of power and the brushless motors that have a large variable speed range with good power at all speeds, lightweight airframes and a simple, hobby-type airplane radio controller. GPS and altimeters enhance their capabilities. Small, on-board video cameras with micro-video transmitters enable pilots on the ground wearing digital video headsets to “fly” as though they were sitting in the pilot’s seat.
A basic, eight-engine, multirotor drone consists of a lightweight airframe that resembles a spider with the motors at the end of each leg. The “body” of the spider contains a lithium-polymer battery that sends power to small devices called Electronic Speed Controllers (ESC's). Each motor has an ESC. The unit has a control system that regulates the speed each ESC tells each motor to rotate. The control system is operated by a chipset equipped with an electronic gyroscope that is about the size of a pencil eraser and by a similarly small accelerometer chipset.
The operation is logical. For example, if the drone is hovering and one side needs power to level the craft, then the sensors detect the tilt and the control system automatically adds power to the engines on the low side. The “pilot” on the ground can override the ESC's manually with a radio to control flight. Half the motors spin clockwise, and the others spin counterclockwise. Yaw is accomplished by changing the speeds of opposing motors to create a very small torsion perpendicular to the vertical axis without creating differential lift.
The control accuracy of a multirotor drone is extremely precise. Drones can fly at incredible speeds horizontally or vertically, reach 400 feet in a few seconds, hover and stop on a dime.
Applications
There are several reasons why land surveyors are increasingly adding drones to their portfolio of instruments.
1. Firstly, using a drone can vastly reduce the time spent collecting accurate data. By acquiring raster data from the sky - in the form of geo-referenced digital aerial images, with resolutions as sharp as 1.5 cm (0.6 in) per pixel – you can gather millions of data points in one short flight.
2. More time still can be saved by using a survey-grade drone such as the eBee RTK. Such GNSS/RTK receiver systems are effectively flying rovers, capable of receiving data corrections streamed from a base station or via VRS to achieve absolute X, Y, Z accuracy of down to 3 cm (1.2 in) - without needing Ground Control Points (GCP's).
3. With collection made so simple, one can focus the energy on using and analysing data, rather than working out how to gather it.
4. With such a large increase in the amount of physical data being collected, this does mean an increase in office time spent processing and utilising this data. However, this expansion is cancelled out many times over by the huge time savings a drone produces out in the field.
5. Last but not least, less time spent on the ground means staff safety is improved by minimising risk to surveying teams when measuring sites such as mines, unstable slopes and transport routes. Simply choose take-off and landing locations that are out of harm’s way.
DRONE SURVEYING WORKFLOW
1. Flight planning
i). Choose/import base map.
ii). Highlight coverage area (rectangle/polygon).
iii). Set desired Ground Sampling Distance (i.e. 5 cm (2 in) / pixel).
- Flight altitude defined automatically as a result (e.g. 5 cm/pixel = 162 m altitude using default eBee WX camera).
- This altitude determines maximum single-flight coverage possible.
- Automatic definition of flight lines & image capture points.
iv). Set image overlap.
- Necessary for stereo coverage.
v). Define safe landing zone.
2. Setting of on-site GCPs
i). For absolute X,Y, Z accuracy of down to 3 cm / 5 cm (1.2 in / 2 in).
ii). No GCP's required, to achieve similar accuracy, if using eBee RTK.
iii). Optimal size & shape of GCP targets defined by GSD of imagery.
3. Flight
i). Autonomous flight
ii). Monitor progress/change flight plan via flight control software.
iii). Automated landing as per defined landing zone.
4. Import images
i). On-board SD card contains images and flight log (.bbx file).
ii). Images geo-tagged according to flight log during importation.
iii). Generate Quality Report on site, in Postflight Terra 3D, to verify quality and coverage.
5. Generation of orthomosaics and 3D point clouds
i). Using supplied post-flight photogrammetry software.
ii). Relative orthomosaic/3D model accuracy: 1-3x GSD.
6. Analysis/production of deliverables
i). Creation of break lines, reference points, digital elevation models, contour lines.
ii). Calculation & analysis of volumes and stockpiles.
iii). Export of output files (geoTIFF, obj, dxf, shape, LAS, KML tiles etc.) to third-party software as required (such as Quick Terrain Modeler, Autodesk, Benteley etc.) .
7. Final report/deliverable creation in third party software.
Limitations
The combination of low cost and easy assimilation of multiple technologies provides significant opportunities for development, including by surveyors and consulting engineers. An excellent GPS-controlled drone with a camera can be developed for less than $6,000. A quick scan of YouTube videos reveals stunning drone technologies, such as groups of drones flying in formation. Possibilities include real-time 3D imaging and an almost unlimited list of others.
However, drones must first overcome the problems associated with safety, privacy and homeland security. Drones penetrate federal airspace, which starts at ground level. According to the Federal Aviation Administration’s (FAA) interpretation, no rules have been developed for drones, so their use is generally illegal unless it falls under the recreational use exemption. The FAA is developing new regulations to license drones, but finalization will be time-consuming.
Regulatory restrictions for safety, although necessary, are the biggest threat to the development of drone technology. Most low-cost drones are not nearly as reliable as manned aircraft and have a much higher crash rate. Videos of drones flying over populated areas are widespread. A small bolt or camera falling from a drone at 200 feet can reach a speed of 70 miles per hour. Overcoming these challenges will require a concerted effort by everyone who has an interest in moving this technology forward.
Once UAS's are cleared for takeoff, surveyors and engineers will have the ability to deploy low-cost drones that will fly over a site and collect photography or other remote-sensing data that will improve our ability to measure the surface of the Earth. Keep your eyes on the skies; this remarkable new technology has the potential to transform our profession.
Also See : Use of Total Station in Surveying
Surveying Using GPS
Robotic Mapping
Also See : Use of Total Station in Surveying
Surveying Using GPS
Robotic Mapping
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