*A UAV photo, used to follow construction progress of a new road layout*

What is an Unmanned Aerial Vehicle (UAV) Survey?

There are many different names and acronyms used to refer to unmanned aerial vehicles, ‘drone’ being the one most commonly used by the media. Other less inflammatory terms include Unmanned Aircraft (UA) and Unmanned Aerial Vehicle (UAV).

We first carried out aerial surveys in 2014 on the A380 Kingsteignton By-Pass, at which time UAV was a new phenomenon, so we at Kemp have stuck with this since.

UAVs, used in combination with photogrammetry, have since 2014 become an established technology, providing an innovative platform for low-level aerial imagery acquisition. Easy to mobilise and flexible in operation, UAVs provide users with the ability to create Digital Surface and Terrain Models (DSM/DTM) and orthophotos from aerial imagery, for mid-sized areas previously only accessible at higher costs and with longer planning cycles.

UAVs are used in a variety of applications, including preliminary surveys for corridors and rights-of-way, volumetric/earthworks surveys, topographic surveys, boundary surveys, inspection/condition surveys and much more.

How Does It Work?

The typical UAV system consists of an airframe containing integrated GNSS (Global Navigation Satellite Systems), inertial system, battery, radio and a large megapixel digital camera.

A portable computer (known as the Ground Control Station or GCS) is used to operate the UAV and by pre-defining the area of interest on the GCS, the UAV can use its on-board navigation system to fly automatically from launch to landing. The terrain can then be photogrammetrically mapped through capturing parallel strips of consecutive camera shots that overlap in both forward and side directions (screen grab required to show flight path). The flight plan can be adjusted on site to suit site conditions. UAVs can also be flown manually for image collection if necessary and these can be used for progress photographs or videos. The photos are normally stored on board the UAV’s camera and log files, relating to position and height, generated by on-board GNSS observations, are transferred back to the tablet computer via radio communication in real-time.

Once the flight or flights are complete, the images are retrieved from the camera and processed in specialist photogrammetric-based software which delivers the DSM/DTM and geo-referenced orthophotos. With forward planning and spare batteries, multiple flights can be carried out in a day to cover large areas.

What Is the Workflow for the Creation of Survey Data?

When using a UAV there are three main steps to the image processing: pre-processing, registration and DSM generation. For accurate survey data however pre-flight ground markers and check observations will be installed and carried out by the surveyor to cover the site. The ground markers and their locations are paramount to obtaining quality data sets, even when RTK GNSS is employed on the UAV system.

In the pre-processing phase, any poor quality or redundant images are removed. In a parallel step, RAW format images are processed to remove distortion using a camera calibration profile. This profile is generated by calibrating the camera on a regular basis.

Although post-processing can be undertaken without calibration it will lead to a reduced accuracy of 3D data due to the fact that the compact cameras typically installed on the UAV have small lenses that are prone to distortion.

In the registration phase image position and orientation data is generated in a user-specified coordinate system and referenced to the ground markers. This data, including the undistorted images, constitute the core information needed to create the DSM and derived products, such as the orthophotos.

In the DSM generation phase, the data created from the previous two phases can be used to create a triangular network of 3D points across the surveyed area. This is generated using photogrammetric based software that analyses the multiple overlap areas across adjacent image sets. As well as determining the position of the camera as it was ‘in flight’, individual 3D points can then be generated by automatically correlating similar pixels that appear across each image. Once created, the 3D points can then be interconnected to form a mesh of the surface at differing resolutions.

What are the Operational regulations?

To operate a UAV commercially in the United Kingdom (UK), the company and UAV operators must hold a Civil Aviation Authority (CAA) ‘Permission to Operate’ licence. In order for this to be granted the company must submit a UAV operations manual and a UAV technical manual to the CAA.

Commercial UAV operators (pilots) in the UK must also sit a CAA ground school course and exam followed by a flight test on the UAV they are to operate. Once these exams are passed the operator is awarded a Certification of Competence on a specific UAV type.

The final hurdle is aviation insurance. Premiums vary and more insurers are moving into the market as the technology advances. Some factors that affect insurance premiums are:

• Experience of the company as a UAV commercial operator

• Experience of UAV operators (hours and types flown)

• Hours to be flown per year

• Type of environment the system is to be flown in.

Regulations for the use of UAVs in the UK are constantly changing as the technology develops. Visit www.caa.co.uk/uas for the latest regulations.

What are the Limitations of UAVs in Surveying?

Current regulations allow for small UAVs to be employed as a commercial tool in UK airspace. Given the weight limitations imposed by the CAA, heavy surveying equipment cannot be carried. Developments in LiDAR technology is such that this traditionally heavy piece of equipment can now be employed to a degree. We at Kemp have employed LiDAR on UAVs to cover densely vegetated sites allowing for the extraction of the terrain below the canopy line.

However, given the cost of LiDAR, photogrammetry, in its multi-image/Structure-from-Motion form, is by far the most popular technique employed currently. The data outputs are currently limited to orthophoto mosaics and DSMs, in both point and meshed forms.

Operational limitations with photogrammetry include:

• No operations over highly urbanised areas

• Ground measurements are limited by dense vegetation and tree cover (less so by LiDAR)

• Limited to 500 metre line of sight operation.

• Limited to 122m/400ft flying height.

• Weather – principally wind speed and rain.

• Poor quality camera images can be detrimental to the final results.

What Level of Accuracy can be Achieved?

Accuracy can be the most important factor to an engineering professional using a survey model or drawing.

Whatever the reason for the survey, in most cases, professionals will be making decisions based on the results. Whether the tolerance of the project is millimetres or metres, it is important the client communicates this to the surveyor.

The surveyor using the UAV will be able to plan the survey to produce the results closest to the project requirements, or may say that the UAV survey option is not suitable for the accuracy required. In all cases the surveyor should deliver a report, together with the UAV survey data, that describes the methods used. The report should also contain a good sample of ground proofing checks, emanating from other survey techniques to give the client confidence in the dataset. This is the only real way that the accuracy of the UAV survey can be assessed, as the survey platform and instrumentation can perform variably given differing factors including, weather, flight planning, GNSS ambiguities, operator, survey control, processing techniques, altitude and point filtering.

As a rule of thumb the precision, (or relative accuracy) of the UAV survey after processing, is known to be within the order of 1-2 pixels in plan and 2-3 pixels in height. The absolute accuracy of a UAV survey will depend on the georeferencing that is derived from the GNSS and/or Ground Control Points (GCP). By using geodetic GNSS and a suitable number of GCPs in the processing, the accuracy of the survey can be derived from the average Ground Sampling Distance (GSD) (the distance on the ground between pixel centres measured on the photograph). Using this principle, if the GSD of a survey is 40mm the accuracy could be assumed to be approximately 40-80mm in plan and 80-120mm in height.

What are the Dangers in Using UAVs?

A UAV is an airborne projectile with mass that can fail and descend rapidly to the ground. With this in mind, the CAA have imposed restrictions on where flights can take place and under what circumstances a flight can be made. The CAA have categorised UAVs by weight to create strict boundaries as to who can fly them and where – the heavier class UAVs need more experienced operators and come under a different set of regulations – this is clearly because they are more dangerous. At Kemp our operations and pilots are licenced and comply with CAA regulations. Risk assessments and method statements ensure that risk is minimised.

What are the Advantages of UAVs in Surveying?

The principle advantage in using any UAV is its ability to provide a remotely controlled platform for acquiring low-level aerial imagery, often from unique vantage points. Like most surveying equipment, a UAV can be carried in the back of a car or van. A UAV adds value in its ability to cover large areas quickly and operate when manned aerial surveys cannot be flown or it is difficult or dangerous to do a ground survey.

Sometimes the imagery of an area of interest is too outdated or simply non-existent and satellite imagery is not always available when you need it and often lacks the required resolution. Here is a list of some of the advantages that a UAV survey can offer:

• Survey difficult to reach, sensitive or dangerous areas e.g. survey a quarry without interruption of mining operations.

• Fly at lower altitudes than manned aircraft.

• High resolution mapping 3cm GSD is possible.

• Fast and flexible.

• Collection of imagery to enable interrogation by others.

Uses of Photogrammetric Data

There are many uses for photogrammetric data, but generally any application that requires a volumetric or three-dimensional set of data or topographic vector information can be considered.

Typical applications include:

• Topographic surveying

• Infrastructure modelling

• Asset inventory

• 2D and 3D vector mapping

• Visualisation/animation

• Volumetric analysis

• Forest monitoring

• Flood plain surveys

• Vegetation surveys

• Survey reconnaissance

• Environmental monitoring

• Site change detection.

*A Topographical Survey overlaid a UAV photo*

Survey Specification

It is important that the client specifies their requirements in detail and should include the following information when requesting a quotation:

1. The area to be surveyed.

2. The level of detail required.

3. The accuracy required.

4. The grid and datum to which the survey is to be related.

5. Access restrictions.

6. Flying restrictions.

7. How the data is to be presented.

At Kemp we are very experienced in producing survey data and for us a UAV is another instrument that we can employ. Depending upon the details specified above we will advise whether a UAV survey is the most suitable tool to employ and what we the optimum solution is for obtaining your survey data.

UAV surveys are a cost-effective alternative to more traditional surveys, given certain criteria. They open up the feasibility of surveying some projects by their ability to cover large areas of terrain in a short period of time using relatively low-cost equipment. If being undertaken on a commercial basis it is important to use a fully qualified, insured operator with experience in data capture by this method. Another important factor is the required accuracy of the survey results. Accuracy should be established before commencement of the survey and checked by sampling areas using more traditional survey methods with the results being published in the report.

UAV surveys have become a recognised surveying technique which is constantly developing as technology and the law regarding air space changes. If you are considering commissioning a survey of this nature then please get in touch and we will be pleased to discuss your requirements.

What is an Aerial Survey used for?