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Coming ashore

By [email protected] - 21st August 2018 - 16:36

For eight months in 2017, life was literally a beach for the survey crews of Bristol-based AG Surveys. Armed with stacks of control targets, GNSS receivers and Unmanned Aerial System (UAS) technology, teams raced against the tides and weather to survey 80 different beaches along England’s southwest coast.

“Each tidal range lasted for up to seven days and we tried to survey five to seven beaches within that tidal window,” explains Andreas Garbe, managing director of Bristol-based AG Surveys. “Often we had just an hour in which to collect a significant amount of data before the tide chased us away. And we needed robust, photogrammetric software to process and deliver the measurements within two weeks of each beach survey. It was a relentless cycle of time crunches, surveying, processing and delivering data.”

This intensity was all part of a national coastal monitoring programme funded by the Department for Environment, Food & Rural Affairs (Defra) and managed by the Environment Agency (EA) to collect topographic beach surveys from around the country.

Compounding the stressful nature of the project was AG Survey’s decision to use a UAS––an unproven technology for beach surveys––to measure a large percentage of those beaches.

“Historically, all of the topographic beach surveys for the programme had been done with GPS technology,” says Garbe. “Together with specialised UAS processing software, we wanted to prove that a UAS-based approach could be a better solution for a number of challenging beaches.”

It was a successful gamble. In acquiring a UAS for the project, AG Surveys became the first company in England to officially use one to survey beaches and prove the viability of the technology for coastal monitoring. And, by pairing the aerial technology with an intelligent and rapid data processing software package, the new solution gave them the results and expertise to open up new business opportunities.

Sensing an opportunity

For decades, England’s roughly 5,670 km of coastline was assessed and monitored through regional and local programmes. In 2011, the EA took six existing regional coastal monitoring programs and integrated them to form the National Network of Regional Coastal Monitoring Programme (NNRCMP). Its intention: to provide a national, homogenised umbrella for regional monitoring efforts.

After the first five-year phase, the EA issued tenders in 2016 for all six regions for the 2017-2021 campaign. Having successfully surveyed a number of beaches in North Cornwall for the initial monitoring program, AG Surveys was awarded three of the four designated areas in the programme’s southwest region.

The project area stretched from the tip of St Austell on the south coast to the small village of Aust, 16 km north of Bristol. Each of the 80 beaches required one baseline survey –– a full coverage from one side to the other and from the back of the beach to the lowest waterline at Mean Low Water (MLW) tide. The survey had to be based on a five-metre grid and achieve a data height accuracy of three cm. Each also required a series of profile line surveys. The latter follow pre-defined lines set every 50 m from the back of the beach to the MLW line, with GNSS measurements taken within 10 cm of each side of the line.

“During the initial monitoring programme, we did all the beach surveys with GPS, which was quite labour-intensive,” says Garbe. “A UAS-based system could provide the same height model as GPS but would enable us to cover larger areas faster and produce better data detail. Equally important, however, was having a software solution that could readily handle the volume, spectral richness and spatial density of the UAS imagery.”

After researching options, AG Surveys chose Trimble’s UX5HP Multispectral UAS (now Delair) and Trimble Inpho UASMaster photogrammetry software for its aerial beach surveys. They conducted the surveys between January and September 2017.

Rocking it

To devise a survey plan, the company first consulted Google Earth and identified 22 beaches that would be too laborious to survey with GNSS or would pose health and safety risks to crews.

In general, Garbe and his crew of three or four could cover three square kilometers a day, flying four to five UX5 flights at an altitude between 100-120 m, speeds of 80 kph, and a lateral overlap of 80 percent. To ensure the reliability and accuracy of the UAS data, they also used a minimum of 10 ground control points (GCPs) for each flight block and measured each target’s position with a GNSS receiver.

One particularly challenging beach they had to survey was aptly named Black Rock, a large, rocky beach about three miles south of Bude and which is twinned with Cornwall’s Widemouth Bay Beach. Since Widemouth Bay and Black Rock sit adjacent to each other, AG Survey’s crew surveyed both simultaneously.

Arriving in the early morning for low tide, they first located suitable positions for the 50 GCPs they needed for the 2.5 km x 1 km area of interest (AOI). To work around the rock-laden shorelines, they found flat spots on rocks and marked them with biodegradable paint.

Garbe used Trimble’s Aerial Imaging Flight Planning software to establish three flight paths; one at the back of the beach, one at the waterline, and one in the middle. Once the GCP locations were measured with GNSS, Garbe flew the first survey along the back of the beach at an altitude of 120 m for 30 minutes. Once the UAS landed, the ground crew laid GCPs for the next flight over the low-water shoreline, and then repeated the process a third time for the last flight over the middle block. After a six-hour day, they collected a total of 1,745 images covering the whole AOI at a ground sample distance (GSD) of 1.65 cm.

All of the imagery was downloaded into their integrated Trimble Business Center (TBC) and UASMaster processing chain –– the system workhorse and the engine that created the data products that were needed to prove the viability of the UAS approach.

Digital proof

Similar to all of the UAS-derived beach surveys, Garbe and his team first imported into TBC the flight and GNSS data plus the base station data from the combined Widemouth Bay and Black Rock survey. Using both the UX5’s on-board GNSS positioning data and the ground control data, they processed precise, short baselines between the base station and each photo point. After ensuring the sharpness and brightness of each image, all 1,745 images were integrated into UASMaster to create a dense point cloud and a seamless orthophoto of the entire AOI.

“One of the reasons we use UASMaster is for its photogrammetry power,” says Garbe. “It’s incredibly good at automatic aerial triangulation and tie-point matching for point cloud creation and subsequent orthophotos. The matching capabilities are particularly important for beaches that are often homogenous and low-textured.”

The UX5 captured this fly-by image of Coverack Harbour, a small, unspoilt, fishing village on the east side of Cornwall’s Lizard peninsula

Because the imagery was pre-processed in TBC, UASMaster already has a foundational geographic reference for the location of each photo in relation to its neighbours. To produce the point cloud, the program uses the coordinates from the ground control and each photo centre to calculate each image’s interior orientation and exterior orientation. It then looks for pixels with the same or similar gray-scale values in neighbouring images. When the software finds matches, it uses triangulation to calculate the 3D coordinate of those points, a process that results in dense 3D height models. The program also automatically colour codes each of the points to indicate what they are –– sand, boulders, rocks.

Unique to UASMaster is the ability to produce seamless, georeferenced orthophotos directly from the point cloud. The program uses the height model of the point cloud, adjusts the 3D point sizes to fit the ortho pixel size, and stretches the images over the 3D model to orthorectify it. The entire project is then integrated back into TBC for final editing.

Rapid results

It took two days for UASMaster to produce both a three-cm point cloud and orthophoto of Widemouth Bay and Black Rock, a process that would have taken over a week to complete with manual photogrammetric techniques. And, Garbe says, the UAS provides significantly more visual information than a traditional aerial photo or GNSS survey.

“We produced an amazing, color-coded, five-cm grid point cloud in which you can zoom around and view the entire beach, including cliff faces and edges, from different angles,” says Garbe. “I don’t think you could get that kind of detail and visualisation any other way.”

Since the baseline surveys are only performed once every five years, AG Surveys will need to wait to see if their UAS approach will be adopted for the next campaign. But their success has already led to other topographic beach surveys, and Natural England has expressed interest in testing the UAS system for vegetation and habitat mapping along the coast.

At this rate, the crews at AG Surveys may be finding sand in their hair for quite some time.

Mary Jo Wagner is a writing and editing consultant and contractor, based inVancouver, Canada

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