A Picture Perfect Plant
February 25, 2011
When an industrial plant in Africa needed to develop 3D site models for information management and future construction, the plant turned to African Consulting Surveyors. The scope of work called for 3D terrestrial scans of approximately 70 percent of all the process structures along with the capture of high-resolution imagery and LiDAR data for topographical purposes. The site covered about 1,730 acres (700 hectares), 494 acres (200 hectares) of which were built up with process plants and associated structures. Some structures reached heights of up to 328 feet (100 meters). It was considered the largest as-built documentation project in Africa.
The primary objective was to create accurate models and drawings of the plant with 2D to 3D environment migration. “Our goal for the project was to create a 3D virtual plant that was accurate enough to create as-built data and engineering drawings, yet can be a versatile tool to track personnel, manage assets and essentially become a plant information and management system,” explains Willie Timmerman, project manager and African Consulting Surveyors' director of technology. “This, in turn, would facilitate an increase in productivity and therefore profit.”
Three methods of data collection were used for the project: a LiDAR system, a mobile mapping system and a terrestrial scanner. The generated data layers were combined to form the point cloud for the entire plant. “The LiDAR data formed an important part of the whole plant layout from an external viewpoint, whereas the terrestrial scanner focused on the internal structures of the buildings and the plant processes inside,” Timmerman explains.
Spirit-leveled, post-processed GPS control was used to correlate the various sources of data. Conventional survey methods were incorporated to control and quality check the point clouds generated by the terrestrial scanner inside the various buildings. The GPS control points on the tar roads were demarcated with white paint so that they could easily be identified on the LiDAR and mobile mapper scans.
The surveyors began by establishing 10 permanent pillar beacons and 50 ground control aerial marks across the plant. The horizontal coordinates of the beacons were fixed by post processing, with 30 minutes of data collected at 0.5 second epoch intervals. The verticals were fixed by implementing a precise double-leveling circuit. The ground control points were used by both the LiDAR and mobile mapping teams to apply the necessary network adjustments.
The control beacons were then used to transfer control (reflective target stickers) into the various plants by using total stations with reflectorless capabilities. These targets were used to fix the scan targets, which averaged +/-5 targets per floor. Closed traverses were then introduced to effect quality checks.
Once ground control was established, five LiDAR passes were flown over the plant to increase the point resolution from 15 to 75 points per square meter. This allowed for a higher density on structures and high-lying features such as overhead cables and pipes as well as other lower-priority detail such as slime dams and open fields. The LiDAR data acquisition was completed in a single day.
To capture the next level of detail, the crew used a StreetMapper 360 mobile mapping system--the first commercial deployment of such a system in the region. The vehicle-mounted mobile scanner was driven between buildings to gather data that was omitted by the LiDAR scanner. The point cloud density of this scanner was 8,000 points per square meter, and two runs of the plant were driven to get the accuracy of the point cloud within the required tolerances. The relative accuracy between points was less than 0.01 meter and the positional accuracy less than 0.03 meter using the real-time GPS and IMU as part of the mobile scanning system. The surveyed road markings were used for the final reductions of the mobile scanning and LiDAR scanning point clouds to correlate and increase the positional accuracy of the data. The mobile mapping was also completed in a single day.
A Leica HDS6100 terrestrial scanner was used for all the building interiors and continues to be used to capture plant-specific detail. The scanner is of a higher order with positional accuracies of 5 millimeters up to 25 meters and a decimated unified point cloud resolution of 4 millimeters across the plant. resulting data will be used for future design work, reverse engineering and retrofitting.
A scan of such a large area produced a point cloud containing billions of data points--2 terabytes in all. The data were broken into manageable sections, and each plant structure was allocated its own dedicated file. The coordinate system had large constants that had to be removed and brought as close to zero as possible to avoid creating rendering issues. The team used Cyclone, AutoCAD Architecture, Navisworks and 3Ds Max software to process the data.
Despite the different technologies used to gather the data, the point clouds of the three orders of scanning all agreed well with each other. According to Timmerman, combining the technologies proved a highly efficient way to meet the client’s needs. “The fact that the LiDAR scan and mobile scan each took one day to complete in the field meant that the client saved a lot of time and money for such a large volume of data,” he says, noting that the terrestrial scanning continues due to internal and external modifications and extensions to the plant.
The scans provide information of sufficient detail, accuracy and density to populate a comprehensive plant information management system. With applications including asset and plant management, design and engineering support, and safety and security, the laser scanned data will support decision making, reduce costs and improve safety. Integration with other systems and data, such as wireless sensors, vehicle and/or personnel tracking and onsite access control systems, will further assist with the day-to-day management of the plant. Says Timmerman: “The accuracies and benefits of laser scanning have the payback effect of increasing productivity and can save companies money over the long term.”
Sidebar: Technology OverviewLiDAR: Leica ALS50 (Phase II) and Leica RCD Camera
LiDAR distance measurement systems use aircraft-mounted laser technology to determine accurate heights of land and buildings. As the plane flies over a site, the laser is beamed to the surface, and the time it takes for the beam to bounce back is recorded. Using the position of the aircraft from the onboard satellite positioning equipment along with the return time for the laser beam and the known value of the speed of light, the distance between the aircraft and ground can be calculated. In order to increase the detail of the height model, mirrors can be used to deflect the laser beam and create multiple measurements from a single pass of the aircraft. The Leica ALS50-II allows data capture at pulse rates up to 150 kHz and accuracy of 11 centimeters (including GPS errors) at all pulse rates. Combined with Leica’s integrated 39-megapixel RCD medium-format digital aerial camera system, the technology allows users to provide digital aerial photography along with the LiDAR data.
Mobile Mapping System: StreetMapper 360
Mobile mapping systems (MMS) use laser scanning technology combined with a navigation system to scan highways, waterways and buildings from a moving vehicle. MMS can be mounted on various types of vehicles, including automobiles, boats and trains, to allow for numerous applications including highway mapping, coastal, river and canal surveying, city modeling and flood mapping, to name just a few. By employing the latest laser scanning technology for improved field performance and accuracy along with precision navigation--including a solution for reduced GPS coverage in urban areas--the StreetMapper 360 can achieve high-precision mapping to a range of 984 feet (300 meters), a capacity of 300,000 measurements per second per sensor and recorded accuracies in independent real-world projects of better than 1 centimeter.
Terrestrial Scanning: Leica HDS6100
3D laser scanning captures 3D images by performing a number of independent laser distance measurements in different but well-defined angular directions. This distance data together with the associated angles form the basis of the 3D images, also commonly referred to as a point cloud. When combined with a high-resolution digital camera, every scan point can be assigned a color using the image data. The Leica HDS6100 is a high-speed phase-based 3D laser scanner that scans up to 508,000 points per second and has a full field-of-view (360 x 310 degrees).