After the cathedral of Christchurch, New Zealand, was hit by an earthquake, drones and advanced photogrammetry software supported the rebuilding work.
Surveying a damaged building can be dangerous. Mapping, using drones, reduces risk to staff and costs to the community. This case study of the iconic cathedral of Christchurch, severely hit by an earthquake, demonstrates how drones and advanced photogrammetry software delivered the orthomosaic map, enabling the accurate spatial planning needed to rebuild the cathedral.
In February 2011, New Zealand’s second most populous city was rocked by an earthquake. The iconic Christchurch Cathedral was shattered in the shake, and the clean-up is still ongoing. Christchurch's central city and eastern suburbs were badly affected.
Before the cathedral could be rebuilt and the 'red zone' surrounding it rejuvenated, a full survey was required. Christchurch City Council’s surveyor Jed Clement, licensed cadastral surveyor, stepped up to the task with the help of drones and Pix4Dmapper.
Locals describe Cathedral Square as "the heart of the city" and as being "key to Christchurch". It sits in the centre of Christchurch, both metaphorically and literally. The garden city, with its large urban parks bisected by the sleepy Ōtākaro Avon river, is known for agriculture, for being the gateway to Antarctica and, more recently, for earthquakes.
New Zealand is stretched across a fault line and earthquakes are common, although seldom as severe as the 2011 quake. By August 2012, the area had experienced more than 11,000 aftershocks of magnitude two or higher. Aftershocks were felt up to 300 kilometres away from the epicentre.
Strict building standards no doubt saved many lives. Scientists estimate that the shake that damaged the cathedral would have “totally flattened” most cities around the globe. The timber-framed homes favoured in New Zealand are relatively resistant to earthquakes, and most damage was sustained in poorly-designed buildings, or stone structures like the Christchurch Cathedral.
Prior to the 2011 earthquake, the cathedral had been damaged by earthquakes in 1881, 1888, 1901, 1922 and 2010. However, the greatest damage occurred in 2011. The first shake destroyed the spire and part of the tower, and left the rest of the building severely damaged. Aftershocks collapsed the west wall of the cathedral, and what was left of the tower had to be demolished in 2012.
Rebuilding After the Earthquake
As the city and the nation debated whether the Cathedral should be rebuilt at all, worshippers gathered in a temporary ‘Cardboard Cathedral’ made out of comfortingly earthquake-resistant materials – including cardboard. But now the cathedral is being rebuilt and commercial development in Cathedral Square and the surrounding area encouraged. Access to the area has been limited due to quake damage, and it’s hoped the development will revitalize the area.
To assist with the rebuild, the Christchurch City Council team launched a drone flight to capture ground levels and provide an up-to-date orthomosaic drone map of Cathedral Square to allow for accurate spatial planning.
The Benefits of Drones in Dangerous Situations
Much of Cathedral Square is open, but there were areas of the Square that were impossible to access due to the risks relating to construction as well as the damaged buildings, including the cathedral. Aerial photogrammetry was therefore the best choice for capturing data in these areas.
The drone could fly inside the restricted perimeter fences without risk to the operator. Just as importantly, Pix4D’s algorithms allowed for the optimal capture of imagery to render a high-quality 3D model that the surveyors could use to take precise measurements – all without entering the site.
Mapping an Inaccessible Area with Drones
The flight team met in Cathedral Square at 8am on a Sunday morning. The early start meant fewer people around the square and fewer vehicles on the road. While this caused less disruption to the public, it also had advantages for the team. Moving objects (like cars and people) may appear in the orthomosaic as transparent artefacts. While it is possible to remove these 'ghosts' and improve the appearance of the orthomosaic, the early start allowed the team to avoid capturing them in the first place.
The Christchurch City Council survey team’s drone pilots licence is pending, and they currently operate under New Zealand’s CAA Part 101 operating rules, which also regulate balloons and kites. The aerial mapping flight was approved by the city’s Roading Authority and the cathedral trust.
The team hoped for overcast weather, and got it. “We were concerned about the surface being quite reflective, which would mean losing detail in the final outputs,” says Clement. “But the morning of the flight could not have been better, being overcast and with no wind.” A total of four flights were completed: two oblique and two grid nadir to capture as much information as possible.
“Unfortunately, we had an issue with the connection to the drone on one of the flights,” says Clement. “That meant we were missing one set of oblique images over most of the square, which resulted in missed detail on the cathedral and surrounding buildings.”
Despite this issue, the team was able to reconstruct the 3D drone model in less than 23 hours in Pix4Dmapper aerial photogrammetry software.
Before take-off, eight ground control points (GCPs) were levelled to a 5mm accuracy. A further 12 checkpoints were added during processing, giving the mapping project an average ground sampling distance (GSD) of 1.38cm. “Quality ground surfaces and reporting – plus ease of use – is why we chose Pix4D,” added Clement.
Modelling a Moment in History
The model gave the team the certainty they needed to begin the detailed design phase of the southern portion of the Cathedral Square rebuild. This part of the rebuild is to coincide with the commercial development that is underway on the southern perimeter of the square, and is due to begin opening in late 2019.
Regenerate Christchurch notes that: “Redevelopment will acknowledge the past and the events that have shaped the city, while reflecting the best of the new… This is an opportunity to breathe life back into Cathedral Square and re-establish it as the heart of the city.”
The model of the square and broken cathedral is not only a useful tool, but the aerial photography is a snapshot of a moment in the city’s history.
The original version of this article was published on Pix4D.com. Last updated: 24/06/2020
Point Cloud From Drones
Point Cloud What Is It? Point Cloud is a series of data points that have been calculated via set methods and assigned torelative locations in space using a specific coordinate system, these data points can be utilised for several processes including 3D imaging for CAD Models, which may eventually be used to manufacture spare parts. Or for BIM (BuildingInformation Modeling) used for the development of new infrastructure. Point cloud is even used in land survey, digitalelevation models and topographical outputs.
Point Cloud, How Is It Achieved? Point cloud (s) can be achieved using various techniques and processed traditionallyvia photogrammetric methods which is the science of capturing images of a specific subject at different angles, whilstusing a high overlap method of up to 90%. The software recognises the common areas across the multiple images andassigns common data points to each resulting in a fluid point cloud.
Historically point cloud has been processed via the use of terrestrial active remote sensing scanners such as laser or radarwhich measure points on a subject surface resulting in deliverable 3D point cloud files. Drone technology adopts the traditional photogrammetry method when considering point cloud outputs, integratingspecific programs that allow to drone pilot to properly plan a flight allowing for sufficient front lap and side lap. whilsttaking into consideration flight height, speed of data collection, whether or not the drone should stop to capture eachimage and various other variables. All of this can affect the density, resolution, and detail within the point cloud which inturn affects any other deliverable formats derived from the point cloud process.
Point Cloud Via Drone, How Accurate Is It? Point cloud accuracy via photogrammetric techniques can be extremelyaccurate when rectified to its respective grid in the case of UK OSGB36 (Ordinance Survey Great Britain 1936) we havecarried out Geospatial survey and processed point clouds that have been within 5mm. It all depends on the quality andaccuracy of the data acquisition and position and accuracy of the assigned ground control points.
GCP’s or Ground Control Points are essential to line up the model to its respective grid and are achieved by choosing orlaying down high contrast tiles and gathering the coordinates X,Y and Z via use of a GPS rover or similar device. Thecoordinates are inputted into the chosen software and the project is then optimised resulting in absolute accuracy. IfGCP’s are not used the point cloud is still accurate although relatively within its own model.
Point Cloud and other 3D Deliverables… The benefits of adopting drone technology to either assist with your 3Dproject or as a standalone option can pay dividends and can help capitalize on specific parts of project budget. Thebenefits as always with drone technology are cost saving and time efficiency.
Deliverables that can derive from the drone processed 3D deliverables are all topographical outputs such as digital surfacemodel (DSM), Digital Elevation Model (DEM), Digital Terrain Model (DTM), Orthomosaic (Orthorectified Imagepresented as a Geotiff), Contour Files, Mesh detailing and all can be merged with terrestrial data.
Drones provide construction teams with an overhead view of jobsites, materials, machinery and people. Contractors are using the autonomous flying machines to record images and videos that help optimize everything from grading plans and operations to identifying differences between as-designed and as-built site plans. Their usefulness can be enhanced with thermal cameras and other add-ons like mapping tools and GPS units.
Also called unmanned aerial vehicles (UAVs), drones can be fully or partially autonomous but are usually controlled remotely by a human pilot on the ground.
“Drones have become the go-to tool for construction firms to track, map, survey, inspect, and manage worksites more efficiently and safely,” said Dan Burton, founder of DroneBase, a drone pilot network that provides support for construction companies. “Through aerial imagery and data, builders can map projects, report progress updates and gain insights through advanced analytics to make better, faster and more reliable decisions.”
Construction applications
Because UAVs provide a bird’s eye view of a jobsite, they offer builders a range of benefits that can’t be matched by any other technology, and the top applications in commercial construction are for logistics, progress monitoring, as-built surveying and marketing, said Dustin Williams, CEO and founder of FlywheelAEC, a reality capture service provider based in San Francisco. Among other projects, his company provides weekly drone flights over the Raiders’ Allegiant Stadium jobsite in Las Vegas.
Denver-based PCL Construction has utilized drones for more than three years on nearly all of its major projects to improve jobsite communication, perform volumetric analysis, overlay design documents with installed work for visual verification, verify grades and provide historical documentation.
"The old adage that a picture is worth a thousand words has never been more true," Bill Bennington, PCL's national quality manager, told Construction Dive.
New England contractor Windover Construction uses drones to establish a 3D model of jobsites, which managers import into the company’s BIM workflow.
Drone provider Skycatch is recording all construction activity on Microsoft’s Redmond campus renovation project, feeding data into more than 100 models that contain almost 3 million 3D building components.
For marketing purposes or for client updates, drones can capture detailed, close-up images that are not possible with a photographer on the ground or even in a helicopter, Burton told Construction Dive. “It’s also a much safer solution than taking pictures from a crane and more affordable than a helicopter flight,” he said.
UAVs are also useful tools for accident prevention, according to Drone Enthusiast. Overhead images showing erection sequences, crane locations or perimeter security can help pinpoint where projects get congested and even predict where hazards could pop up. The improved transparency and collaboration decreases inefficiencies that often lead to a reduction in change orders, Williams said.
More drone-related advancements are on the horizon. Burton said that drones with thermal sensors and ground control points (GCPs) will gain more traction this year, enabling construction managers to identify problems like water leaks or concrete cracks more quickly. Unpiloted UAV systems such as the University of Michigan’s autonomous system that nails shingles to a roof, are also upping drones' usefulness for construction pros.
What you should know
Before implementing a drone program, it's important to select the right model for the job, starting with the choice of fixed-wing or rotary. With an airplane-like design, fixed-wing drones can glide on a set path and reach higher altitudes, making them efficient for mapping topography and surveying greater distances, according to Big Rentz. However, fixed drones are only able to fly forward.
For closer aerial inspections and photography, rotary drones are the better option since they are easy to control and their rotating blades allow them to hover and remain stable.
Contractors also need to know that despite the perceived low barrier to entry due to the affordable cost of consumer drones, many specialty technical skills are required, as well as professional-grade hardware and software, said Williams. There are dozens of apps that help control and program drones; a recent JBKnowledge report found that the top three apps were DJI Go, Drone Deploy and Pix4D.
“Having a drone program is becoming essential; but having it implemented internally is not practical for everyone,” Williams said. “If implementing internally, contractors should budget money and time appropriately to go from R&D through implementation.”
That said, he added, many contractors would benefit from partnering with a specialized drone service subcontractor. These specialists can help decide the best products and approach for the specific project and ensure that local and federal regulations regarding the use of airspace are followed. They also have trained and FAA-licensed operators.
A newly proposed identification system for commercial drones will help expand usage in the United States for everything from drone-delivered packages to emergency medical supplies. The FAA's Remote ID system, announced late last month, will require the vast majority of drones in the U.S. to broadcast tracking signals that will help authorities to identify their operators and locate them, according to a press statement.
“Remote ID technologies will enhance safety and security by allowing the FAA, law enforcement, and federal security agencies to identify drones flying in their jurisdiction,” Transportation Secretary Elaine Chao said in the statement.
When the new regulations hit in a few years, according to the Washington Post, FAA compliance for drone operators will be more important than ever, Burton said. "Companies will need to make sure they have proper clearance, insurance and licenses before flying," he said.
Roadblocks
Drone technology rapidly continues to evolve and bring new applications and benefits every month, but the quick pace of innovation can also be a drawback for contractors who are trying to keep up with the changes, Williams said.
A few other challenges include:
Getting buy-in from construction company leaders. Because there are costs associated with starting a drone program, whether in-house or through a third party, some firms are reluctant to make the investment.
Deciding whether to invest in hardware and training for an in-house pilot or soliciting a drone service provider. The choice between DIY or using a contractor comes down to how spread out the need is, Burton said. If firms are working on multiple sites across the country, a drone operations company can be helpful so companies don’t have to worry about buying hardware, getting insurance or training operators.
Finding a drone service provider that understands the needs of the AEC industry, such as how to coordinate flights around active construction such as cranes or how to integrate drone data into the BIM/VDC process. "While drones are invaluable tools on construction sites, like anything else, they need to be a cost-effective and scalable solution in order for the program to be successful," said Burton.
Ensuring secure transmission of data. The U.S. Department of Homeland Security issued an alert last year citing “strong concerns” that Chinese-made drones could be sending sensitive flight data to manufacturers and potentially to the Chinese government. The drones “contain components that can compromise your data and share your information on a server accessed beyond the company itself,” said DHS’s Cybersecurity and Infrastructure Security Agency memo. DHS cautioned users to be wary of purchasing Chinese-made drones and to take precautions such as turning off a unit's internet connection and removing secure digital cards.
Author: Jenn Goodman
Published: Jan. 8, 2020