Most existing pipes and cables are known only from the owners' records, which may be decades old and metres out. Utility survey standards grade every line by how it was found, from a desktop record to a line exposed and measured. The grade decides how close you may design and dig.
Every linear project crosses existing networks: water, gas, electricity, telecoms, drainage, sometimes oil and district heating. Most of them are known only from the owners' records, drawn at different times, to different standards, often from memory of where the trench went. Hitting one can kill someone, cut a town's supply or stop the job for weeks.
The UK specification PAS 128 and the US standard ASCE 38 both grade utility information this way, with sub-levels for detection accuracy. Read the edition your contract names for the exact definitions; the principle is the same: say how each line was found.
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