This article was originally published in Geomatics World.
Drone technology is becoming widespread. On suitable projects drones enhance the speed and safety of site work. Once the imagery is captured, the deliverables that can be produced photogrammetrically are exceptionally versatile and easily fall into existing CAD, GIS, BIM and Virtual Reality workflows. Surveyors play a crucial role in embedding new technology and techniques into established ‘best practice’ ways of working. This article describes a recent project to demonstrate how the UAV was incorporated into a survey and the significant time, safety and cost value it added to the client and the contractor.
Satellites, Drones and Lasers
In 2017, SNC-Lavalin’s Atkins business undertook a survey of a disused gas holder site for National Grid. The client required a 1:200 CAD topographic map and an ortho-photograph mosaic of the 2 hectare site. They also needed a site video showing the condition of the gas holders themselves. The site was in a major city centre and all site work was completed within four hours.
In simple terms the site work had three major components. Firstly, there was the need to install ground control in order to reference the project to British National Grid and meet the accuracy requirements of the project. Secondly, there was the need to capture the aerial imagery in a suitable format for stereo-digitisation; thereby, enabling the accurate production of 3D CAD topographic mapping. Finally, there was the task of visually inspecting and videoing the gas holders (which were between 20 and 40m high) with the drone.
To install control, a combination of methodologies was adopted. Network RTK methodology was used according to the best practice standards of the RICS for control establishment. This involved amongst other things re-measuring the control stations with a new satellite configuration. The first step was to measure control on the ground. Once the control network was in, a total station in reflectorless mode was used to coordinate elevation points on the top of the gas holder. This was done whilst waiting for the satellites to move into their new positions. It was important to measure control at various elevations since the accuracy of the aerial triangulation is especially sensitive to changes in height and these need to be quantified to ensure an accurate and robust triangulation result; the final accuracy of the mapping is profoundly dependent on accurate triangulation.
Whilst ground control was measured using GNSS and Total Station techniques, the drone pilot commenced the video survey of the site and the gas holders. Capturing ultra-high-resolution video; be it, 4K, 2K or HD provides tremendous clarity. When only visual information is required, the drone is often able to offer a welcome substitute to a person working at height. The drone team comprised a pilot and camera operator. This enabled the pilot to focus on the safe flying of the drone platform and the camera operator to focus on getting the imagery and videos required.
Once the ground control points had been re-measured it was time for the drone to undertake the nadir photography of the site. The drone carried a full frame (35mm) DSLR sized camera and can cover 8ha in 20 minutes in one flight. This enabled the image acquisition for the mapping data to be completed very quickly. The drone has the audio footprint of a small electric mower and when operating at 100m above ground level, the aerial flight was a very discreet and rapid process.
Measuring Detail from the Office
Once in the office, production was undertaken following the businesses’ long established photogrammetric workflow - an ISO9001 process which encompasses quality assurance, image enhancement, GNSS analysis, aerial triangulation and deliverable production. One of the strengths of our delivery pipeline is our ability to digitise from the imagery in stereo. This involves delineating assets and features directly from the imagery in 3D. This is our preferred approach to mapping because each pixel sits adjacent to the next and each pixel contains information which influences the geometric fidelity of the survey, whereas with point data from aerial derived point clouds, no information exists between points i.e. there are gaps in data. This becomes evident when one zooms into a point cloud to delineate a hard edge such as a kerb or road edge. When doing this the points appear to spread out making the task less precise; however, it should be noted that a very dense point cloud can go a long way towards mitigating this issue. By drafting the CAD mapping straight from the ultra-high-resolution imagery in 3D, it enabled us to meet a very high standard of accuracy efficiently during the digitisation process.
Synergy in Methodologies
Utilising the drone in tandem with GNSS and the Total Station enabled the project accuracy requirements to be met and independently checked. Without the GNSS, it would not have been possible to install control at the required accuracy in the time available. The Total Station also enabled indirect measuring of critical control points in hard to reach areas that would have been nigh on impossible to reach directly. Once the control was in, the drone enabled the whole site to be easily imaged within an hour.
In deliverable production, photogrammetry permitted detail from hazardous and hard to reach areas to be digitised from the comfort of the office; essentially permitting one to substitute time spent in the field to time spent in the office. This generates time and cost savings, as well as reduced exposure to the health and safety hazards of working onsite. Meanwhile, the ability to generate an orthophoto mosaic from the imagery added significant contextual information to the CAD drawing, making it more user-friendly and intuitive to interpret.
Further benefits were realised from the drone video footage of the site and the gas holder structures. This added significant value to the client by enabling them to visually assess the condition of the site and structures without the need to physically travel to the site.
This project highlighted the importance of the surveyor in orchestrating and directing technology. Knowledge of how to utilise Total Stations, GNSS and photogrammetry was critical to the success of the project. It was the combination of approaches used rather than any one tool in the box that enabled the client’s expectations to be met in a minimally-disruptive, data-rich and time-efficient manner.
Drones are becoming increasingly ubiquitous across construction and infrastructure projects. The health and safety advantages, speed of survey and the diversity of captured data; be it, high resolution imagery or video are a few of the game changing benefits enabled by drone technology. Incorporating their use when appropriate into our existing and emerging survey workflows enables surveyors to bring to bear the latest technology for the mutual benefit of our clients and ourselves. Ultimately, it is for professional surveyors to champion and embed new technology and techniques into established ‘best practice’ ways of working.
This article was published in Geomatics World March/April 2018
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