Author: Paudie Barry, managing director, Baseline Surveys Ltd In January, Baseline Surveys was awarded a contract to survey approximately 1km squared of Dun Laoghaire town centre and harbour area for the purpose of providing a topographic study for a detailed engineering redesign of the existing port. The specific survey information required was a comprehensive topographical land-side survey. The site is located in Dun Laoghaire and was in marine harbour and maritime recreational use with live road traffic and with vessel sailings for the duration of the survey.

• The client was the Dun Laoghaire Harbour Company;
• Survey work next to tidal water was necessary to include revetment and all waterside features down to low water;
• Entry to individual buildings was not envisaged, however spot levels to define building heights were required;
• Survey work outside the confines of the harbour on public and private roadways with live traffic was specified;
• Contour mapping of open areas at 0.1m intervals was required;
• The final output data format was to be in AutoCAD dwg format.

A list of typical features in the specification to be surveyed included cover level and location of underground services, overhead services, all street furniture, kerb lines, fences, road markings, buildings with heights all to be located within an accuracy of +/- 25mm. The harbour area was occupied by individual stakeholders, clubs and statutory bodies. Entry to these sites, and their timing, was subject to individual consent outside the Harbour Company’s direct control. Although every effort would have been made by harbour staff to facilitate the survey in advance, the final consents were a matter for liaison by the successful surveyor. Each required a minimum notice of two weeks to facilitate issue of any entry permits. This was an important aspect of the survey. While the specification is typical of such projects, how we carried out the survey was quite atypical. As far as we are aware, we are the first in the world to use a remotely piloted aircraft system (RPAS) for this type of survey task. PRE-FLIGHT PLANNING [caption id="attachment_12267" align="alignright" width="5866"] Click to enlarge[/caption] Had this survey been carried out conventionally, it would have been labour intensive, hazardous and would have involved a lot of organisation in terms of organising access onto private property. It also would have required a traffic-safety management system to be implemented, which would have caused substantial disruption to traffic for the duration of the survey. A special track-side permit would have had to be obtained from Irish Rail and special safety consideration would have had to be allowed for in order to ensure the safety of surveyors working at sea-front locations. Typically, a project such as this one would have taken a ground survey crew three-to-four months to complete. Using RPAS on the project eliminated all this. The data capture took less than one hour for the entire site and the final survey was produced within 10 days, while achieving all the requirements as set out in the client’s survey specification.

Attaining a permit for this work was a challenge as normally, RPAS are not allowed to fly over congested areas and must fly within a visual line of site of 500m. Our unmanned aircraft required a take-off and landing area of 100m x 100m clear area. The nearest suitable take-off and landing area was a soccer pitch, which was a maximum distance of 1200m from our flight location, directly over a congested town centre. We prepared a detailed operational risk assessment and mitigation matrix on hazard identification in the target and surrounding area, weather minima, an outline of our crews experience to date and operational altitude and location information in the aerial works application. The Irish Aviation Authority was very helpful in assisting us in preparing the operational risk assessment and granted us the permit to execute the works. A flight plan was generated so that that an 80% overlap and a 80% sidelap and at an altitude of 120m to provide an expected GSD of 2cm. Flight direction was plotted at 90 degrees to the actual wind direction to maintain a constant ground speed. DATA ACQUISITION SYSTEM [caption id="attachment_12154" align="alignright" width="6000"] Click to enlarge[/caption] The data acquisition system we used on this project was a C-Astral Bramor RPAS. We used the Real-Time Kinematic (RTK) Global Positioning System (GPS) to establish Irish transverse mercator co-ordinates on our specifically designed ground markers to provide photo control. The GPS unit has a spatial accuracy in the region of 10-25mm both horizontally and vertically. The C-Astral Bramor Unmanned Aircraft is a blended wing constructed of Kevlar and carbon fibre and weighs 4kg. It is catapult launched, has extremely steady flight characteristics and advanced safety features afforded by its Lockheed Martin autopilot including a parachute deployment system for emergency and routine landing procedures. It carries a Sony Nex-7 24MP RGB sensor, which is oriented in portrait which allows for more forward overlap at a slower triggering interval. Our ground-control markers were carefully designed so the most exact centre of the marker could be determined with a very high degree of accuracy from the photography at the processing stage. The prevailing weather conditions on the day of our flight were overcast with a wind speed at our flying altitude of 120m above ground level (AGL) was a maximum of 7m/s. The topography of the site had a 17m variation in level and had buildings, church spires and tower cranes to 40m in height, which would all contribute to turbulence at our flying altitude. DATA PROCESSING Firstly, ten ground markers, which were surveyed by Network RTK GPS in Irenet95 co-ordinates, were downloaded into Geosite office 5.1 and exported to AutoCADlt 2014.

"The expansive, multi-format and digital nature of this survey will allow us to utilise it as a daily management, time-line and planning tool for many years to come. I'd strongly recommend this approach to any entity with a similar requirement. Tim Ryan CEng CEnv, operations manager, Dun Laoghaire Harbour Company

We downloaded 2,800 photographs from the Bramor RPAS along with the flight log file. We imported the photos and the logfile into Agisoft photoscan and, using the software, we eliminated superfluous photographs by deleting photos with high roll values, which occurred at turns. The refined 2,000 photographs were then used for the photo-alignment stage. We then imported the ten control stations into photoscan and identified the centre of each control point marker on each photo and attached it with its appropriate co-ordinate value.

We then processed the data into a 3D model for subsequent orthophoto and Digital Surface and Bare Earth model output. The orthophoto was outputted at a resolution of 20mm pixel size and the digital elevation model was outputted at 50mm pixel size. The resulting geo-referenced orthophoto was imported into AutoCAD for the process of heads up digitising, which accurately defined the photographic features in computer-aided design (CAD). The ground points were isolated in Agisoft photoscan and exported as a bare earth surface into global mapper, where we generated 10cm contour file and spot levels for export to CAD. The outputs were:

• AutoCAD Topographic Survey in 2D or 3D
• Orthophoto for CAD/GIS underlay
• Digital surface models and digital terrain modelling geotiff
• LIDAR Point Cloud