Canadian standard specifies absolute precision for underground infrastructure exposed by excavation

Infrastructure in the older cities of Canada was installed more than 100 years ago when as-built drawings, if any existed, referenced surface features that may have disappeared.  Recording the accurate location of such utilities was not formally required. Today, many of the records that do exist are in formats that are incompatible between utilities and may represent schematics rather than geospatially accurate maps of underground utilities. Inaccurate location of underground infrastructure has implications for public safety and disaster management and represents a significant drag on the economy. A number of national standards have been developed including in the U.S. (ASCE 38-02), U.K (PAS 128)., France and Australia. 

In Canada the purpose of the Canadian Standards Association CSA S250 Standard from 2011 is to specify the mapping records requirements used to identify and locate underground utility infrastructure. This Standard is intended to promote the use and drive the advancement of mapping records during the planning, design, construction, and operation of underground utility infrastructure.  Impetus for the widespread adoption of S250 could be provided by a Senate Bill S-229 that is currently before the House of Commons that opens the door to a digital system (similar to the Dutch KLIC and Belgian KLIP) that does not require an operator to actually visit planned excavation sites by providing digital maps instead.

The standard defines levels of accuracy for recording the location of underground infrastructure when the infrastructure has been exposed either by potholing or during excavation.

• Record accuracy level 0 — There is no information available related to spatial accuracy. The position as shown on the drawings is the best possible estimate of the actual location.
• Record accuracy level 1 — For exposed infrastructure.  Horizontal and vertical location of underground utilities shall be measured while exposed and recorded in a real world coordinate system by a competent individual. Accuracy ± 25 mm in the x, y, and z coordinates.
• Record accuracy level 2 — For exposed infrastructure. Horizontal and vertical location of underground utilities shall be measured while exposed and recorded by a competent individual. Accuracy ± 100 mm in the x, y, and z coordinates,
• Record accuracy level 3 — Horizontal and vertical location of underground utilities shall be measured while exposed and recorded by a competent individual. Accuracy ± 300 mm in the x, y, and z coordinates.
• Record accuracy level 4 — For exposed infrastructure. Horizontal and vertical location of underground utilities shall be measured while exposed and recorded by a competent individual. The measurements shall be tied into either a relative feature or geodetic datum. Accuracy ± 1000 in the x, y, and z coordinates.
• Record accuracy level 5 — For infrastructure that has not been exposed.  Location measured and recorded by a competent individual using geophysical methods. These less precise measurements shall be tied to either a relative feature or geodetic datum. Accuracy ± 1000 mm in the x and y coordinates.  Accuracy of the z coordinate if measures is not quantified.

Furthermore the standard states that if the criteria for accuracy level 0 to 5 are not used, the owner is referred to other standards, specifications, or instructions from the utility.  The A, B, C, D quality levels of the American Society of Civil Engineers (ASCE) 38 standard for underground infrastructure is explicitly referenced for this purpose.  A rough mapping between these two standards appears to be as follows

• Record accuracy level 0 - Quality level D
• Record accuracy level 1 - Quality level A
• Record accuracy level 2 - Quality level A
• Record accuracy level 3 - Quality level A
• Record accuracy level 4 - Quality level C
• Record accuracy level 5 - Quality level B

The Canadian standard is similar to the Australian standard in specifying different levels of absolute accuracy for exposed utilities.  This represents a substantive difference between the Canadian and Australian standards and ASCE 38-02. 

The S250 standard has adopted the widespread but dated practice that the only way to accurately determine the location of underground infrastructure is by exposing and measuring location.  The S250 standard references geophysical methods including electromagnetic methods, ground penetrating radar, and sonding methods requiring insertion of a source of a magnetic field (the sonde) into a pipe or conduit and detecting it at the surface, but the S250 standard clearly doesn't reflect confidence in locations determined by these technologies. 

IDS Stream multi-channel GPR utilized by T2 Utility Engineers

Recent advances in ground penetrating radar and other remote sensing technologies and the high cost of potholing, estimated at $30,000 per pothole, suggest that standards for the reliability of the location of underground infrastructure should include estimated precision for remotely sensed infrastructure.  The U.K. PAS 128 standard, which was adopted in 2014, does this.  The ASCE 38-02 and the PAS 128 standards are being updated, possibly to reflect the development of improved underground remote sensing hardware and software.

Thanks to Mark Braiter and Laverne Hanley for providing access to the S250 standard.