As we are reminded by the recent underground utility hits in San Francisco, CA, Durham, NC, and Calgary, Alberta, not knowing the location underground utilities represents a serious risk for the public. There are 400,000 hits per year in the U.S. alone. Standards for reporting the reliability of the locational information about underground utilities have been in place for decades but these standards do not reflect recent advances in underground utility detection technology. In the U.S. the 2003 ASCE 38-02 ,which is been used for classifying the quality of location information about underground infrastructure, is widely seen as being out of date. In France the 2012 presidential decree defines three explicit levels of cartographic accuracy for underground structures: A - less than 40 centimeters, B - 40 centimeters to 1.5 meters, and C - greater than 1.5 meters. In Canada the CSA S250 (2011) standard also specifies absolute precision with three different quality levels for exposed utilities and one for remote sensed detection. In the UK the 2014 Publicly Available Specification (PAS) 128, developed under the auspices of the British Standards Institution (BSI) and sponsored by the Institution of Civil Engineers (ICE) and others, not only includes the A,B, C, D quality levels of the U.S. standard, but extends it with explicit precision levels B1 to B4 similar in this respect to the French decree and the Canadian S250 standard. I have just had a chance to chat with Andy Rhoades who was responsible for locating and mapping underground infrastructure at Heathrow airport for many years and is now leading the process to update PAS128. Andy told me that this process will kickoff in a couple of weeks with a target of completion roughly six weeks after that.
Just as a reminder PAS128 defines quality levels A through D based on the type of survey used to locate underground utilities.
QLD - Desktop search techniques were used to identify existing utility data by identifying known utility owners within the survey area, requesting asset information from each of them, typically as-builts, and compiling the collected information. Locational accuracy generally poorly known.
QLC - Requires site reconnaissance to identify above-ground visible physical features that are indicative of and help locate utilities within the survey area. Locational accuracy undefined.
QLB - For this quality level PAS requires that at least two remote sensing techniques must be used in detecting utilities. Electro-magnetic locating (EML) and ground penetrating radar (GPR) are required, but others may be used in addition. Most EML systems only detect metal, do not have the capability of digitally recording what was detected, and can only provide rough estimates of depth. GPR is able to provide 3D location, is not able by itself to identify the type of utility, and does not work well in certain types of soils such as clay. Post-processing is often used with GPR to improve the interpretation of GPR data by resolving weak and intermittent signals or distinguishing multiple targets.
In addition PAS128 breaks the QLB level into several sublevels with specified absolute precision depending on the results of remote sensing.
QLB4 - A utility cable or pipe is believed to exist but has not been detected. Locational accuracy undefined.
QLB3 - Only the horizontal location of the utility has been detected by one of the geophysical techniques used. Locational accuracy ±500 mm (XY), Z locational accuracy undefined.
QLB2 - Horizontal and vertical location of the utility was detected by one of the geophysical techniques used. Locational accuracy for X,Y ±250 mm or ±40% of detected depth whichever is greater, for Z ±40% of detected depth.
QLB1 - Horizontal and vertical location of the utility was detected by multiple geophysical techniques. Locational accuracy for X,Y ±150 mm or ±15% of detected depth whichever is greater, for Z ±15% of detected depth.
QLA - Involves exposing the target utilities within the survey area by excavation to confirm and record the location and other attribute data. Locational accuracy for X,Y ±50 mm, for Z ±25 mm.
In contrast with the standards in other countries PAS128 reflects the advances in underground detection technology - hardware and software - that have occurred in the last few years. That it is now being updated suggests that PAS128 will continue to stay with the underground detection technology curve.
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