In most jurisdictions worldwide the low quality of underground infrastructure location data has been identified as a top cause of underground utility damage. Existing underground data, most of which is stored in network owners' utility records, is inaccurate, out of date or simply missing. In North America in a survey of 402 locate professionals better facility maps received nearly unanimous agreement as a top strategy for improving the efficiency and quality of locates. Singapore's Digital Underground project is unequivocal about the impact of low quality underground location data stating that "virtually all stakeholders are aware that much of the available information is unreliable and that this has repeatedly led to losses of time, money and opportunities."
Until now there has never been a standard for an “As-Installed” (often referred to as “As-Built”) utility data record for which location, positional accuracy, geometry, dimensions, type, function, ownership, materials, or operational status of a utility feature are among the data documented. The lack of a standard for utility as-installed record data has resulted in utility records that are of inconsistent data quality, content, and formats not conducive for data exchange.
ASCE 75-22 Standard
The purpose of this standard guideline is to specify essential elements for recording and exchanging data about the location, size, orientation, function, ownership and other attributes of newly installed, relocated, or otherwise exposed utility infrastructure. The benefits of such a standard are the exchange of consistent underground and above ground utility data among construction stakeholders, support for digital project design standards such as building information modelling (BIM), and reduced underground utility damage through sharing of more consistent and reliable infrastructure data.
The 75-22 standard should be used whenever subsurface utility infrastructure is exposed such as newly installed, relocated and daylighted utility infrastructure. Various techniques of modern reality capture including LiDAR and photogrammetric scanning and high accuracy GNSS should be used to record the location of exposed infrastructure to ensure survey accuracy.
Data exchange framework
The 75-22 standard defines a data exchange framework which includes a minimum, optional, and conditional set of elements that every utility data model may include. Utility infrastructure data stakeholders have developed different data models and management tools to handle the data. The data exchange framework identifies specific data elements to collect and manage for newly constructed or exposed utility infrastructure to facilitate a variety of functions including exchange of consistent underground and above ground utility data among construction stakeholders and reduced underground utility damage through sharing of reliable infrastructure data.
The data exchange framework is derived from standard geospatial data models and includes: feature types, geometry types and feature attributes. Every 2D geospatial data model includes at a bare minimum the spatial data types: line strings, points, and polygons. Among the data elements defining most utility data models can be found the following data types (often with a different name): segment, device, access point, support structure, containing structure, secured utility area, encasement, marker, and tracer. Associated with these feature types are attributes that define the properties of individual features.
ASCE 38-22 Quality Levels
A Utility Quality Level is assigned by a Professional to each subsurface Utility Feature. The Quality Level identifies the uncertainty of a subsurface Utility Feature’s existence and actual location to that of its documented location.
Utility Quality Level A (QLA): A value assigned to that portion of a subsurface Utility Feature that is directly exposed and measured and whose location and dimensions are tied to the Project Survey Datum. The subsurface Utility Feature shall be tied to the Project Survey Datum with an accuracy of 0.1 ft (30 mm) vertical and to 0.2 ft (60 mm) horizontal for the outside limits of the Utility Feature that is exposed.This is the default quality level for newly installed utility equipment.
Utility Quality Level B (QLB): A value assigned to a subsurface Utility Feature whose existence and horizontal position is based on Geophysical Methods combined with professional judgment and whose location is tied to the Project Survey Datum.
Utility Quality Level C (QLC): A value assigned to a Utility Segment not visible at the ground surface whose estimated position is judged through correlating Utility records or similar evidence to Utility Features, visible above ground and/or underground.
Utility Quality Level D (QLD): A value assigned to Utility Feature not visible at the ground surface whose estimated position is judged through Utility records, information from others, or from visual clues such as pavement cuts, obvious trenches, or existence of service.
Positional accuracy
Utility company records comprise the vast majority of the data we have about underground utilities. Much of this data is inaccurate, out of date or simply missing (especially in the case of abandoned infrastructure and infrastructure without a known owner). Jurisdictions around the world recognizing the critical importance of accurate data are implementing policies to ensure data quality. ASCE 75 is central to these initiatives and has already been mandated in Montana and Colorado as the "as-installed" standard for new infrastructure in state right of ways.
The ASCE 75 standard includes positional accuracy for horizontal and vertical data points.
In the context of underground infrastructure location data locators, professional surveyors and engineers, network operators, and one call centres have different responsibilities in collecting and certifying underground utility location data. For example, Colorado and Montana mandate ASCE 75 compliant as-installeds be submitted for completed public construction projects. Understanding the equipment used to gather information and the reliability of that technique in that particular project situation is the responsibility of the person putting their sign and seal on the data. Assigning quality levels compliant with ASCE 75 requires sign off by a professional engineer.
Among the attributes required by ASCE 75 are several that define data quality for different utility feature types. The horizontal and vertical datums shall be managed and exchanged at the record level using the horizontal spatial reference and vertical spatial
reference elements in the framework for data exchange;
- Horizontal spatial reference: Coordinate system, datum, associated with the X and Y coordinates
- Vertical spatial reference: Datum and geoid model for the Z coordinate
- Horizontal accuracy: Error of horizontal coordinates at the 95% confidence level
- Vertical accuracy: Accuracy level of vertical coordinates at the 95% confidence level
- Accuracy units Units: used to express horizontal and vertical positional accuracies.
During construction or when underground utility infrastructure is subsequently exposed, the horizontal and vertical coordinate observations are obtained at each distinct feature; at horizontal bends, vertical bends, and points of deflection; and along each distinct feature with sufficient interval frequency to achieve the required Positional Accuracy Level.
The Horizontal Accuracy and Vertical Accuracy requirements help determine the type of data collection equipment and data collection methodology. Different Positional Accuracy Levels are possible when using global navigation satellite system (GNSS) equipment and real-time kinematic (RTK) methods, post-processed mapping GNSS equipment, and mapping GNSS equipment without post processing. The combined effect of the characteristics of the positioning equipment, data collection procedures, and postprocessing support a specific statement of accuracy. For example, equipment and data collection and post-processing procedures that are designed to achieve a 3 ft Positional Accuracy Level cannot be used to certify an accuracy smaller than 3 ft.
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