Line Marking

What Line Marking is

Line marking is the point where instrument readings become something a crew can dig to. A locator traces each line with electromagnetic equipment, ground penetrating radar, or both, then puts that traced route on the surface using the APWA Uniform Color Code so the marks are readable by anyone on site regardless of who placed them. A complete set of marks records the horizontal route, the utility type by color, and usually the facility identifier, duct or conduit count, line size, and an estimated depth written beside the route. Under ASCE 38-22, designating a line on the surface from geophysical data is Quality Level B; Quality Level A requires physically exposing the line and measuring it. Marking is normally bundled into a private utility locating visit rather than sold on its own, but it is the part of the job the excavation crew actually sees and relies on.

How it works

After tracing, the locator sprays inverted water-based marking paint along the route as a dashed line, adds wire-staff flags or stakes in turf and gravel where paint will not hold, and writes the owner or facility identifier next to the marks. The APWA Uniform Color Code assigns red to electric power lines, cables, conduit, and lighting cables; yellow to gas, oil, steam, petroleum, and other gaseous or flammable material; orange to communication, alarm, and signal lines, cables, and conduit; blue to potable water; green to sewers and drain lines; purple to reclaimed water, irrigation, and slurry lines; pink to temporary survey markings; and white to the proposed excavation. Where paint is not permitted or the line runs beneath a structure, the locator sets an offset mark and writes the offset distance and direction beside it so the crew can measure back to the true route. Depth notes, duct counts, and sizes are written in the same color as the utility they describe, which keeps a congested corridor legible. Good practice is to photograph the finished marks and, on larger sites, capture them with GPS or a total station before they are disturbed, because paint is a perishable record and a coordinate is not.

When to choose Line Marking

Line marking is what you want any time equipment will be working on ground you have no reliable map for: excavation on private property past the meter, boring and directional drilling, post and foundation holes, landscaping and irrigation work, and any dig inside a plant, campus, or fenced facility that one-call locators do not enter. White lining, marking the proposed excavation in white before the one-call ticket is filed, narrows the area the public locator has to cover and generally produces cleaner, faster public marks. On long or phased projects, budget for re-marks rather than a single visit, since marks do not last as long as construction schedules. If you need positions that survive the dig, pair marking with utility mapping so the same data lives in a drawing or GIS layer instead of only on the dirt.