A line locator is a fairly simple tool – a coil (or coils) in the receiver produces an electric charge when exposed to a moving magnetic field. That charge is communicated to the user via an audible tone and/or a visible display. The source of that moving magnetic field, known as signal, is usually a transmitter, but it doesn’t have to be. That’s it. A current generated in the receiver indicates the presence of a signal. The stronger or more proximate the signal, the greater the induced current. As the receiver is moved horizontally through the magnetic field, current increases and decreases relative to that proximity.

It’s a straightforward mechanical function that is highly reliable in most circumstances. It provides such a sense of reliability that state legislatures have seen fit to dictate 18-24 inch requirements for horizontal accuracy. Although disputes occur, locators and excavators are for the most part comfortable operating within these parameters. It’s a testament, really, to the beauty and reliability of a technology that was first deployed a human lifetime ago.

For all the focus on horizontal accuracy, locate technicians are loathe to provide depth measurements to excavators, and in spite of their stringent requirements regarding horizontal accuracy, legislatures are remarkably mum on the subject of depth. Why is that?

While horizontal accuracy is a mechanical function, modern locating instruments provide estimated depth measurements based on an electronic calculation that compares relative signal strengths from multiple coils within the receiver housing. The calculations required for electronic depth measurement depend on very clean information received from each reference point, and as any experienced field locator can tell you, they are often embarrassingly inaccurate.

Still, our excavating customer knows that the receiver can provide depth information, are understandably interested in that information, and often request it even in the absence of a legal requirement that it be provided.

This puts locating technicians and their employers in a bind. The excavator knows that with the touch of a button, depth information is available to the locator. The locator knows this information is often unreliable and that there is no statutory requirement to provide it. By doing so, they are essentially putting themselves on the line to do someone a favor. If the reading is correct, the excavator is temporarily pleased, but beyond that, there is no reward or benefit to the locator. However, if the locating tech provides a depth and it proves wrong, they are made to look foolish or incompetent, their skill and the accuracy of their equipment are called into question and, in the worst case, the excavator tries to hold them liable for a damage caused after they received inaccurate information. In short, the downside of giving out depth information vastly outweighs the upside. Why provide something for free, that we’re not required to do, that may end up making us look dumb or getting us in trouble? Would the excavator do the same for us?

A word to excavators: when a locator gives you depth information, they are doing you a big favor. They’re going out on a limb they don’t have to and they’re risking a lot. Don’t be surprised if they’re not interested in taking the risk, or if they ask you not to hold them to it. Then do them a favor back. Don’t complain if the information turns out to be unreliable.

Christopher Koch is a training consultant and President of ZoneOne Locating. He is past president of Nulca and worked on both the 2009 and 2015 revisions to the Nulca Professional Competency Standard. He can be reached by email at or on Twitter @kochauthor.



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