These are fleeting, colourful maps that depict the topography of the land beneath our feet. They tell those digging the road exactly where the pipes and cables are and, just as importantly, how deep they are. Here, we interpret the curves in the road as discovered by our readers.
The different colours used for road markings are the result of convention rather than law and reflect specific purposes of use.
For example, red is the color of electricity. Blue represents water, yellow represents gas, green represents CCTV or cable companies, and white represents telecommunications (or general information for building contractors). The numbers between the lines indicate the distance from the curb to a specific pipe or cable, or (if D is used) the depth of burial.
As the network of pipes and cables becomes more and more dense, more and more specialist ‘search companies’ are beginning to map this underground world. John Robinson is the managing director of Subscan Technology, one such mapping company. Here he deciphers an image sent in by a reader.
”It looks like this is a project that was going to have a crosswalk or something like that installed,” Robinson said. “These numbers are for the size of the PVC pipe that will be installed underground to accommodate the cables.”
”It’s a gas pipeline and its location was discovered using ground penetrating radar (GPR),” Robinson said.
”The lines marked with an R (for radar) next to them indicate the location of major roads running parallel to the road.
”The dots indicate the location of each cable or channel. The term ‘dual channel’ indicates that there are two cables or channels in one channel.”
”I think it’s probably related to multiple IT pipes,” said Robinson, who said there were reportedly 11 pipes in total and the depth at the location was 1.5 metres.
“This is an inspection cover that contains the cable systems (electrical and IT) that control the traffic lights.
“The red circles show how many cable channels are coming out of the bay, and the blue dots show how many channels are active and have cables running through them.
”Using two different detection methods will improve the accuracy and reliability of the detection process,” Robinson said.
”It is an electrical substation that converts high-voltage cables (11,000 volts or more) to lower voltages to power small businesses and homes.
”High voltage enters the right side of the device and comes out of the left side with lower voltage.
”The red cables (and paint) represent high voltage, and the green ones represent low/medium voltage. The numbers should relate to some numbering scheme used to identify electrical distribution,” Robinson said.
”Here’s a typical suburban pavement with power lines (red), British Telecom cables (white), water mains (blue) and cable TV (green),” Robinson said. “Each mark indicates the location of a service. The number next to the line indicates the depth in centimetres from the top of the service.”
”In this case, the excavation was planned and marked with paint,” Robinson said. “It appears to be for the installation of high-voltage cables.”
“The excavations are complete and the paving stones have been re-laid, but not in the original order!
“Originally [30 years ago] we represented electricity as a line with a zig-zag wave inside, which symbolized the flow of electricity [a bit like a lightning strike].
Robinson explained that it was a gas pipeline, the location of which was discovered using ground penetrating radar (GPR), hence the “R” on the ground.
”These drains have a U-shaped bend, like a toilet, and this line indicates the direction of the drain pipe.”
Post time: Feb-18-2025