A major gap in the construction process is the unavailability of reliable information about the location of underground utilities for civil engineering construction projects such as highways, railways and pipelines. Mapping underground infrastructure is benefiting from accelerating innovations in technology in detecting, locating, capturing and sharing subsurface utility location data. Mobile + cloud solutions have been developed that replace the traditional error-prone and slow paper-based workflows with near real time digital processes that result in complete, accurate and up to date records of the location of underground utility infrastructure. Mobile + cloud solutions have been adopted by a variety of stakeholders in the construction industry including engineering firms, contractors, professional locators, and government transportation agencies.
Surveying exposed utilities
Capturing survey-grade location data for underground utility infrastructure requires that the utilities be exposed (corresponding to QL A of the ASCE 32 standard.) For newly installed utilities this requires the survey to be conducted while the trench is open. For utilities exposed as as part of a SUE survey (Colorado and Pennsylvania mandate subsurface utility engineering surveys (SUE) during pre-construction), each facility must be surveyed as it is exposed. Similarly for facilities exposed during excavation. For utilities targeted for relocation or other treatment, the facility must be surveyed after it has been relocated and before the trench is filled. For a large construction project there may be a survey crew on site that would be available to conduct these surveys, but in general, calling a survey crew for these types of surveys is uneconomical.
A key feature of mobile + cloud solutions is the ability to capture survey-grade location information about newly installed utilities, utility infrastructure exposed as part of a SUE survey or utility facilities exposed during excavation in real time. Instead of calling a survey crew a widely used alternative made possible by the availability of modern GPS/GNSS technology is to have a technician in the field use a GNSS RTK receiver to capture the location of utility equipment. GNSS receivers are already an established method of precise positioning, but they have limitations. They cannot be used in areas where facilities are inaccessible or metallic pipe is exposed. The development of GNSS rovers with integrated inertial measurement units (IMUs) that are self-leveling, not affected by magnetic disturbance, and don’t require calibration has broadened the range of application for these devices. The first GNSS rovers required that the receiver, typically attached to a pole, be positioned vertically over the facility whose location is being measured. The next generation of RTK receivers supported tilt, making it possible to record a survey-grade location when the pole with the RTK receiver was not vertical. However, the pole has to be in proximity to the facility being surveyed. Facilities that are not physically accessible or are too hazardous to access cannot be surveyed with this technique.
Visual Positioning
At the Canadian Underground Forum, Simon Pedley of Leica Geosystems described a remarkable GNSS RTK rover that makes it possible to capture accurate points from a distance with images. The new GNSS RTK rover (GS 18 I) expands the sensors in the device to include a camera. The camera makes it possible to do visual positioning - capturing hundreds of measurements at survey-grade accuracy, simply by taking images of the area of interest. With this device a technician can simply record images while walking alongside an open trench or take a photo of an exposed facility during a SUE survey. Later in the office survey grade location measurements can be made from the images captured in the field. Since imagery captured in the field represents a permanent record, return visits to a site can be avoided simply by making additional measurements from the captured images.
The ability to capture accurate points from a distance means that whether near an open trench or at the location of an exposed asset a field worker can safely and efficiently capture the location of previously inaccessible points and points that were hazardous to access. Even for surface features it provides a much safer way to measure locations. For example, a technician can be on the sidewalk safely taking pictures of man holes in a busy roadway. GNSS with visual positioning can also be used in conjunction with 3D laser scanning and ground penetrating radar (GPR) to accurately locate remote-sensed underground utilities reducing the need for vacuum and hydraulic excavation.
Visual positioning not only expands the range of points that can be safely and efficiently accessed, but creates a permanent record. Additional images can be captured as changes are made creating up to date as-built documentation with real-world conditions that will reduce risks and costs later on.
This post is based on Simon Pedley's talk at the Canadian Underground Forum (CUF). You can listen to all the talks at CUF on the GeoIgnite CUF Youtube channel.
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