February 1, 2012 by Richard Rybka
You may not be familiar with mobile mapping systems, but the fact is you probably use the information they produce every day. Each time you access the Internet to map a driving route or find a restaurant near your destination, you are using data that was generated by a mobile mapping system. Do these systems have a place in construction? The answer is definitely YES.
Mobile mapping systems are rapidly gaining acceptance as the new standard for data collection. These technologically advanced systems quickly collect visually dynamic information in diverse environments such as highways, urban streets, railways and pipeline routes. More importantly, they have created a new class of data (digital information) that provides a totally immersive 3D view of objects and surroundings.
Changing data environments
Data is the backbone of the design/construction/facilities management workflow. Today, the acquisition and consumption of data is generally managed in 2D environments. Base data for planning and design is collected by mapping discrete points, one-by-one, in the field. Two-dimensional paper plans are generated for the bidding, construction, and inspection phases of the project. As-built drawings are created on the original paper plans. After construction, the owner receives reams of paper plans and record documents for managing the facility.
Data produced by mobile mapping systems provides a better replication of reality than 2D data sources. There is less correlation work for the mind to do. Imagine trying to visualize the conditions on a site, or plan a project, using traditional discrete point data and a stack of individual photographs. We go back and forth between the two data sources, trying to match up what we see in the picture to what we interpret from the point data. We are viewing in 2D while our mind is trying to envision in 3D.
Now compare this to opening a 3D dataset on your computer screen. Rotate, pan, and zoom 360 degrees from the centre of an image set. The images provide instant cognitive recognition of the setting and features-we immediately see and comprehend exactly what we are looking at. We can navigate freely through the project as if we were standing on site. Dense clouds of geospatial points enable us to measure and locate features to a high degree of accuracy.
How mobile mapping systems work
The operation of mobile mapping systems is best explained as the integration of two subsystems: vehicle positioning components and information sensors.
To establish the location and orientation of a vehicle as it moves along the surface of the earth, three positioning technologies are used:
Global Navigation Satellite System (GNSS) receiver and antenna
GNSS signals provide information to establish the x, y (horizontal) position and z (vertical) position of a vehicle on the earth.
Inertial Measurement Unit (IMU)
As a vehicle moves along the earth’s surface, it also has motion similar to a plane traveling through the air-roll, pitch and altitude. While we hardly notice these motions when driving, this information is required for the proper alignment and registration of data from sensors.
Distance Measurement Instrument (DMI)
DMI’s are attached to each of the rear wheels of a vehicle. These devices measure revolutions that are used to track velocity and distance. Direction can also be extrapolated from DMI’s since the inside wheel revolves slower than the outside wheel when turning.
We can count on the IMU and DMI’s to provide continuous, uninterrupted information to the system as the vehicle travels. This is not the case for GNSS. Sky view obstructions such as tall buildings, tree canopy, overpasses and tunnels can block satellite signals. This results in a temporary loss or degradation of the vehicle’s x, y, z position on the earth. But the two other redundant positioning components provide supplemental information that contributes to the processing of an accurate vehicle trajectory (path).
Two categories of information sensors are typically used in a mobile mapping system:
Laser scanners operate on a principle known as LiDAR (Light Detection and Ranging). Emitters scan an area with focused light beams of a certain frequency. The light beams are reflected back to a sensor after striking objects. The time it takes for the beams to return is recorded and used to compute distances to the objects. Laser scanners produce millions of geospatial points with horizontal (x, y) and vertical (z) values. Groups of these points are known as a “point cloud”.
Digital single-frame and video cameras are typically used with mobile mapping systems. Single-frame cameras can be mounted in an array to cover targeted viewing angles, usually to the front and sides of the vehicle.
For example: one manufacturer offers a mobile mapping solution that uses an imaging device with six cameras in one enclosure. The capture rate of images can be set electronically for time or distance intervals. After the data is collected, processing software stitches the individual frames to create a 360-degree panoramic or spherical image.
Data collected during a mobile mapping mission is typically processed in an office environment with specialized software. GNSS data from the mobile mapping system can be post-processed with satellite data logged with an on-site receiver or obtained from a reference station network to improve accuracy.
After processing, the resultant data products consist of an accurate vehicle trajectory, a 3D point cloud comprised of millions of geospatial points, and 360-degree spherical images.
Mobile mapping is an emerging technology and the benefits are just beginning to become well known. These systems, compared to conventional data collection methodologies, offer tremendous benefits. Reports from contractors using mobile mapping systems indicate a 50 per cent reduction in data collection costs and a fourfold increase in productivity. Data can be collected at normal roadway travel speeds. Safety is greatly improved-mapping is accomplished securely from the interior of a vehicle, eliminating the risks of placing personnel in or near traffic corridors.
Construction applications for mobile mapping systems are almost endless. Consider a highway expansion project as an example. At the planning stage, the corridor for the project can be quickly mapped to obtain important engineering and cost information. Existing features, such as light poles and trees, can be located and identified. Using this information along with the images, a determination can be made as to whether these objects will need to be removed, relocated, or replaced if in poor condition. A surface model can be derived from the point cloud using CAD software to study drainage patterns.
During the construction phase of a project, the site is “inspected” by periodic passes with a mobile mapping system. Topographic models of the roadbed can be created from point clouds. By comparing models of current and prior surfaces, earthwork volumes can be calculated. A continuous as-built may be recorded as features and pay items are installed. A complete visual record of the project at different points in time is created with high-quality images, providing a documented history for future reference.
Large scale contractors often have several shops, equipment yards, asphalt plants, and other facilities at different geographic locations. Physical management of these remote locations is often challenging. By collecting a mobile mapping dataset of each site, managers can access 3D views of roads, buildings and yard inventory at their desktop computers. Costly site visits to determine conditions or iden
tify problem areas at each location are eliminated. Changes are quickly documented by re-driving a site. Decision making and management processes are streamlined and expedited.
Case Study-Rapid Survey Solutions
Brad Chambers and Nick Nolan, two men with experience in construction surveying and management, came together to form RSS -Rapid Survey Solutions. This Australia-based company is using a mobile mapping system for a widening spectrum of construction survey and engineering projects.
Earlier this year, RSS was commissioned to report on the quality and accuracy of grade restoration along a pipeline corridor leading to the Victorian Desalination Project. Chambers and Nolan mapped the 10-kilometre long by 40-metre wide route. The project would have taken weeks using conventional survey methods. Using the mobile mapping system, they completed the mapping in four hours and delivered their report to the client on the following day.
On another project, RSS completed a full-feature survey of a proposed route for an 18-inch diameter water line. Deliverables to the client included DOT-coded CAD drawings and DTM’s. RSS covered eight kilometres of suburban streets in metropolitan Melbourne in one day. A project analysis concluded that mobile mapping resulted in a 30 to 40 per cent cost reduction in field survey costs over traditional methods. It also enabled a quicker turn-around of deliverables with full photographic records of the route conditions.
Mobile Mapping’s Future in Construction
Based on the tremendous benefits of mobile mapping systems, it’s clear that this emerging technology will soon be used for numerous applications in construction surveying, engineering, and management. Successful contractors are resourceful and always seeking methods and technologies to improve productivity and efficiency. Mobile mapping systems are helping them to achieve these goals.
Richard Rybka is a mobile mapping specialist with Topcon Positioning Systems. Send comments to firstname.lastname@example.org.