Flags and painted lines on the ground tell you the horizontal direction the facility is running. They do not tell you its depth. If you are going to cross or work near an existing facility, you may want to first verify its depth by hand digging.
Yes, there are standards for installing underground facilities at a certain depth. But that depth can change over years.
In the example above, the power line was buried at a depth of three feet. Later, the area was developed and landscaped. At the berm the line is now five feet underground, but at the swale it is only one foot below the surface. Accidentally discovering this situation could literally be a heart stopping experience.
The best way to know how deep the line is buried it to expose it using the safe digging methods mentioned in another module.