This guide is Part One of the two-part guide Conducting a UAV Survey,and suggests good practice in taking near vertical photographs using a small Unmanned Aerial Vehicle (UAV) as part of an archaeological survey. Part two of this guide is published by the Archaeology Data Service and deals with data handling, after capture.
Both parts have been created in response to the high degree of interest in the potential application of UAV technology for creating 3D maps.
Although aimed at archaeology, elements of this guide will be of interest to anyone intending to use a UAV for surveying purposes. This guide focuses on the methodology of data capture and subsequent data managment, rather than data processing. Low-cost solutions and open technologies are given priority where they exist and are suitably mature.
The principle behind this guide is not accuracy but disclosure. Accuracy is a noble goal but one which can prove prohibitively expensive and is often entirely superfluous to requirements. Disclosure, on the other hand, can be free and is never superfluous. When making use of an unproven technique such as the low-altitude aerial survey we must be particularly careful to record and disclose our data collection and processing methods, the base error rates of our equipment and the provenance of our data. This guidance is intended to aid readers to achieve this.
The Civil Aviation Authority (CAA) is the body in the UK which oversees aviation. Unmanned aircraft with an operating mass of 20kg or less are defined in CAA legislation as ‘Small Unmanned Aircraft’ and, when carrying a camera, are considered to be 'equipped to undertake...surveillance or data acquisition'. Such aircraft are exempt from the majority of the regulations that will normally apply to manned aircraft under CAA rules.
If the operator is not to be paid for conducting ‘aerial work’ i.e. the operator will not get ‘valuable consideration’ (payment) for undertaking the flight, then no special permission from the CAA is required to conduct a survey. However, these common sense rules do still apply;
If an operator will be paid for conducting ‘aerial work’ then permission must be sought from the CAA. The process of gaining ‘permission’ is fairly straightforward and involves paying a modest, annual payment to the CAA.
Importantly though, the paid UAV operator must demonstrate competence before being granted permission by the CAA. They must do this by undertaking appropriate training such as that delivered by either EuroUSC (their assessment is called the Basic National Unmanned Aircraft Systems Certificate-Small or BNUC-S) or Resource UAS (their assessment is called the ‘Remote Pilot Qualification-Small or RPQ-S). Where UAV operation is to become a paid activity, such training is a required investment.
All UAV operators, paid or unpaid, should ensure that sufficient third party insurance is in place before undertaking a flight.
It is beyond the remit of this guide to describe in detail each UAV platform suited to aerial surveying. The information provided below is intended only as an outline of the major types of UAV platforms which are currently available.
Many UAVs are based upon a multirotor aircraft design, such as a quadcopter (four rotors) or a hexacopter (six rotors). These are relatively simple to construct with minimal engineering knowledge, or are available to buy ready-to-fly. The multirotor has vertical take-off and landing capabilities but high power requirements, making for short flights; multirotor flights greater than 20 minutes are still uncommon.
Another popular UAV platform is the radio-controlled, electric aeroplane. Constructed of foam (typically expanded polystyrene or expanded polypropylene) or fiberglass, the aeroplane has the advantage of longer flight times and so greater range, but requires a wide, open area for take-off and landing.
Automation has not yet advanced so far in the control of electric aeroplanes as for multirotors; the anonymous landing of an aeroplane, in particular, requires a fine-grained automated control which is impossible for many low-priced flight controllers to achieve. This means that manual aeroplane landings are the norm.
An alternative to both multirotors and aeroplanes is the radio-controlled, helium filled blimp. Blimps do not withstand the wind well and inflation in the field can prove impractical, however they can have very long flight times, as electrical power is not required to produce lift.
The evolution of UAV airframes is a rapidly developing area, selecting a platform should be informed by:
Battery technology is advancing quickly. The de facto standard for UAV work is now the lithium polymer (lipo) battery. However, flight times of more than 20 minutes for multirotors or 50 minutes for an aeroplane are still unusual. When photographing a large area, a number of battery changes may be required.
The flight controller is perhaps the single most important component of any UAV. It interprets commands given by the UAV operator and controls the aircraft appropriately; it is essentially an on-board computer.
Commands can be given in real time (e.g. ‘do this now’) or, where the flight controller is capable of autonomous flight, pre-prepared and uploaded in advance (i.e. 'When I flick a switch, go from Point A to Point B, proceed to Point C then return to Point A’ where each point is specified in terms of its latitude, longitude and altitude).
In order to fly autonomously a flight controller must be used with a GPS. UAV GPS units are often small, plug-in modules. Differential GPS units suitable for UAV use are emerging but they still uncommon and can cost more than the rest of the system combined.
Flight controllers often also make use of other types of sensor to help achieve stable flight including a gyroscope, accelerometer, electronic compass, barometer and, increasingly, sonar.
Many UAV platforms are inherently unstable (such as multirotors) and so require a flight controller of some description to achieve flight. Even where this is not the case (e.g. aeroplanes) there are benefits to having a flight controller capable of autonomous flight.
Firstly, a pre-prepared ‘flight plan’ (the set of pre-prepared instructions interpreted by the flight controller) can be repeated with iterative improvements. If an area of interest is missed in an initial survey the flight path can be amended to include the area in subsequent flights.
Secondly, many autonomous flight controllers provide a GPS ‘flight logging’ function which allows us to say, with a known degree of accuracy, not just where a UAV should have been at any given moment (as specified in the flight plan) but where it actually was. This is invaluable information for the later georeferencing of photographs.
All autonomous flight controllers have some way to input a flight plan. This is often done by connecting the flight controller to a laptop computer via a USB cable. Some controllers though, such as the open source, APM can be used in conjunction with a telemetry radio and a laptop or tablet computer to provide a two-way wireless datalink, even while the UAV is airborne. This offers far greater flexibility.
A UAV requires a radio control system in order to issue and receive manual commands. In the UK, the 2.4GHz band is often used. Where a wireless telemetry link is also used, for example to issue commands to an airborne UAV or to monitor a UAVs current performance in detail, a second frequency is required which does not conflict with the first. 433MHz in commonly used in the UK.
It is unlikely that any equipment within the survey area will interfere with either of these frequencies as modern radio control equipment employs technologies to prevent this. However, in the unlikely event that this does occur the source of the interfering signal should be identified and isolated for the duration for the survey.
Modern point-and-shoot cameras are generally adequate for aerial survey work carried out at low altitude. Resolution of images is no longer a limiting feature, with most point-and-shoot cameras providing at least ten megapixels of resolution. Digital Single Lens Reflex (DSLR) cameras can give far greater resolution but their greater weight and cost means they are not popular for UAV survey work.
Whichever camera is selected, it is important that it has a small number of very specific features. Firstly, the camera must offer manual control of settings such as focus (which should be set to ‘infinity’), resolution (set to highest), ISO, white balance, shutter speed and aperture.
Perhaps the most important feature though, is the capability to take photographs automatically. There are two common ways to achieve this, either by using an intervalometer function or by attaching the camera to the flight controller which issues ‘trigger shutter now’ commands to the camera during a flight.
Such features are still somewhat unusual in point-and-shoot cameras. Here, Canon cameras have a distinct advantage. The third-party CHDK (Canon Hacker’s Development Kit) can be used to add additional functionality to Canon cameras such as; the ability to shoot in .raw format, manual control over camera settings and, crucially, operation of the shutter via either an intervalometer script or a command issued by the flight controller. A Canon Powershot camera running CHDK software can give very reliable results.
The software used to create a flight plan will depend on the flight controller to be used, and more specifically the operating system of the flight controller. The Arduino-based APM flight controller is often used in association with Mission Planner software (PC, Mac OS) or AndroPilot (Android tablet). Using a laptop or tablet computer allows a flight plan to be modified in the field.
A flight plan should ensure the area of interest is covered in its entirety and also that 60-80% overlap will exist between photographs. A standard approach is to fly a raking ‘lawn-mower’ pattern over a site. The distance between passes is determined based upon altitude. Lower altitude requires more passes to ensure sufficient overlap between photographs, greater altitude requires fewer passes and fewer photographs but gives less detailed results.
Given the area to be surveyed and camera specifications, some software (e.g. Mission Planner) can automatically generate an appropriate flight plan.
A Ground Control Point (GCP) is a point of reference with a known and recorded position. The use of GCPs facilitates the later georeferencing of captured images.
Here, the UAV operator has a distinct advantage over the aerial surveyor operating a manned vehicle at far greater altitude. A marker the size of a saucer can be visible to a point-and-shoot camera attached to a UAV at an altitude of 50m. As long as it will not blow away and is sufficiently different from other markers nearby (e.g. has a clearly visible and unique number or letter code), any object can be used as a GCP.
It is important to establish the positing of each GCP as accurately as possible and record these before a flight commences. The degree of accuracy required, and so equipment selected for this task, will depend on the purpose of the survey. For example, in ideal conditions, a hand-held GPS unit such as the Garmin GLO employs WAAS (Wide Area Augmentation System) technology to provide an accuracy of 3 meters. A Differential Code-Phase GPS or Carrier-Phase GPS might be used if greater accuracy is required.
Using a UAV it is possible to conduct an aerial survey of a site of around 1km square in less than an hour, and for a total cost of only a few hundred pounds (including the purchase of all hardware and software). The results can be of sufficient accuracy for many purposes.
A typical UAV surveying procedure is described below. This assumes the use of a flight controller capable of autonomous flight, a suitable airframe and a camera with an intervalometer function.
The nature of the site is assessed, based on existing maps (e.g. Google Earth) and field visits. Initial questions should include: can the survey be carried out safely? For example; aircraft should never be flown in close proximity to crowds or indeed to any individuals who are not under the direct control of the UAV operator. Overhead power lines should entirely preclude the use of a UAV.
Can the survey be carried out effectively? Factors such as dense foliage or strong coastal winds will have a negative impact on survey results. However, partial tree cover can still allow for useful results and a heavier UAV can withstand moderate winds.
In order to comply with the CAA regulations, outlined above, a UAV must not be operated within 50m of a (man-made) structure. Sites of archaeological interest may be included within this definition and so 50m should be considered the minimum altitude when conducting a survey.
Once finalised, a flight plan is uploaded to the UAV’s flight controller (either wirelessly or via a USB connection).
GPCs are placed at the edges of a site to be surveyed and also within the site itself. More GCPs offer more accuracy but add to time spent surveying on the ground. Ten meters intervals are adequate for many purposes.
A location should be selected for take-off and landing which is as flat as possible and which is at least 30 meters from any person not under the control of the UAV operator. For safety reasons batteries should not be attached until flight is ready to commence. Final, pre-flight checks involve establishing that the UAV has power and is receiving commands. A camera with an intervalometer function is started before take-off.
Take-off will typically be done manually, operating the UAV as a radio-controlled model. Once the operator is satisfied that the vehicle is behaving as expected, ‘automatic’ mode is engaged. The autonomous function of the flight controller then takes over.
The survey phase is largely automated. However, the operator must maintain sight of the survey vehicle at all times. If unexpected behaviour occurs the operator will engage ‘manual control’ mode, and so end the autonomous flight.
Upon completion of a flight plan, some aircraft require the operator to take over manual control once again and perform a landing; others can be made to land autonomously at the site of take-off.
For safety reasons, batteries should be removed as soon as a flight is concluded. A redundant copy of all captured images should be made as soon as possible on landing, ideally in the field using a laptop computer.
If properly collected and standardised, the data collected via a low-altitude UAV survey can be visualised in many different ways. Two common procedures are the stitching together of orthorectified images into a highly detailed map layer and the generation of 3D meshes or point clouds via the process of photogrammetry.
Photogrammetry is the extraction of 3D data from a series of 2D photographs, via a process of comparison. Each image is compared to the others, the relative positions of the camera are calculated and the 3D form of features within the photographs is inferred. The technique is highly specialised but fortunately several software programmes now exists which simplify the workflow.
Many archaeologists have historically relied on commercially available aerial image sets. This has often proved to be less than ideal. The growing realisation that data collected using a UAV and an inexpensive digital camera can indeed be fit-for-purpose has opened up an exciting new possibility; even a very minor archaeological site can now benefit from a detailed aerial survey.
However, we must ensure that resulting datasets are well-described and are in appropriate formats if we are to overcome the risks of data inaccuracy, incompleteness and incompatibility.
For more on standardising data after capture, sharing data and the long term preservation of data see Part Two of this guide, published by the Archaeology Data Service.