How to Read Utility Marks on the Ground
Marks are a language, and most of the people digging near them were never taught to read it. It takes ten minutes.
The Colors
- Red — electric power, cables, conduit, lighting
- Yellow — gas, oil, steam, petroleum
- Orange — communication, alarm, signal lines
- Blue — potable water
- Green — sewer and drain lines
- Purple — reclaimed water, irrigation, slurry
- Pink — temporary survey markings
- White — proposed excavation
This is the APWA Uniform Color Code, and private locators use it too, so a marked site reads consistently regardless of who painted which line.
White Comes First
White is not a utility. It is the excavator saying "this is where I intend to dig", laid down before the one-call ticket is submitted so that every responding locator sweeps the right ground. A site with white lining gets tighter, more usable marks back. Several one-call programs require it for large or ambiguously described excavations.
What the Writing Means
A mark is often more than a line. Common annotations include the facility owner or locating company's initials, the line size and material, the number of conduits in a duct bank, and arrows where a line changes direction or terminates. Offset marks — a line painted a stated distance away from the actual utility, labelled with that offset — are used where the utility itself sits under something that cannot be painted.
If an annotation is ambiguous, ask. Guessing at a duct bank count is how a crew ends up in the middle of one.
A Mark Is Not a Survey Line
Every mark carries uncertainty, and the law accounts for it with the tolerance zone: a buffer on either side within which mechanical excavation is prohibited and only careful hand or vacuum digging is permitted. The width is set by state or provincial law and is not the same everywhere, so check the rule that governs your dig.
Marks also say nothing about depth. Depth is not part of the one-call deliverable, and depth from a private locate is an estimate unless a test hole was cut.
When to Stop and Re-Locate
- Marks are faded, tracked over, or partially washed away
- The site has changed — grading, stripping, or stockpiling since the marks went down
- A member responded "no facilities" for a utility you can see a meter or pedestal for
- The marks contradict the site plan, and you do not yet know which is wrong
- The job has run past the validity window the one-call program sets
Frequently asked questions
Do private locators use the same colors?
Yes. Private marks follow the same APWA code, so private and public lines sit side by side on the ground and mean the same thing. Some locating companies also add their own initials or a job reference so it is clear on a busy site which marks came from which visit.
What does an offset mark mean?
It means the painted line is deliberately placed a stated distance away from the actual utility, usually because the utility runs under something that cannot be painted or because painting on top of it was impractical. The offset distance is written next to the mark. Measure from the mark, in the direction indicated, before you dig.
How do I preserve marks on a long job?
You largely cannot, which is the real answer. Paint and flags do not survive weather, traffic, or a construction season. Photograph the marks with a scale and permanent features in frame, record offsets on a sketch, and plan re-marks into the schedule. If the information has to last, buy a mapped deliverable with coordinates rather than relying on paint.
Related guides
View all811 Call Before You Dig: What It Covers and What It Misses
Call 811 — or your provincial one-call centre in Canada — before every dig. Then understand precisely what the ticket did and did not cover, because that gap is where most private-property strikes happen.
Utility Strike Prevention: A Working Playbook
Strikes are overwhelmingly preventable, and the causes are boringly consistent. Here is the sequence that removes them.
Marks, Sketches, or a Map: Choosing a Locating Deliverable
Paint is gone in a few weeks. Whether that matters is the whole question, and it decides what you should be buying.
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