Utility Locate Depth Cross-Check and Distortion Screen
A locator's depth readout assumes a single isolated conductor, and when it is wrong it can read DEEPER than the truth.
Example
You enter
- Instrument depth readout (in) 52
- Offset where the signal halves (in) 71
- Left null offset from the peak (in) 34
- Right null offset from the peak (in) 52
- Signal current near the transmitter (mA) 42
- Signal current further along (mA) 19
- Current drop that counts as sharp (%) 25
You get
- Geometric depth (in) 71
- Depth difference (in) 19
- Depth disagreement (%) 36.5385
- Null asymmetry (in) 18
- Signal current along the line 54.7619
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeThe dangerous direction, because it promises cover that is not there. Put a second utility nearby, a rebar mat overhead, or a poorly grounded signal return in the picture and the field is no longer that of an isolated line, so the computed depth is wrong, and a reading deeper than the truth tells an excavator there is more cover than there is. THE 45-DEGREE CHECK IS THE DISCIPLINE THAT CATCHES IT. Move perpendicular to the line until the signal drops by half; the horizontal distance moved equals the depth, independently of the instrument's own calculation. An instrument reading 52 in against a half-signal offset of 71 in is a 37% disagreement, and at that point neither number should be trusted. Two more checks are free and travel with it. The response nulls to either side of the peak, and the two nulls should sit at equal offsets -- 34 in on one side against 52 in on the other is a field being pulled by something. And a locator that reads current should show it decreasing GRADUALLY along the line; a drop from 42 mA to 19 mA over one span means the signal has left the target and coupled onto something else, and everything located beyond that point may be a different utility entirely. That is how a crew ends up potholing confidently in the wrong place. Three checks on readings the user takes. None of this replaces exposing the utility, which is what actually establishes position and depth, and none of it is a substitute for the one-call notification and the locate marks a facility owner is responsible for. It does not select a frequency, address sonde work, passive locating, or non-conductive facilities, and it does not evaluate the accuracy of the marks themselves. The utility owner, the one-call system, and the applicable damage prevention law govern.
Formula or tablethe 45-degree method: the horizontal offset at which the signal halves equals the depth; null symmetry is the difference between the two null offsets over their mean; the current gradient is the drop between two points along the line over the near reading.
Source and editionThe 45-degree locating method and the null-symmetry and signal-current diagnostics by name. Exposing the utility is what establishes position and depth; the utility owner, the one-call system, and the applicable damage prevention law govern.
Free accessComparison of readings the technician takes in the field; no instrument's internal model is reproduced.
What governsEstimate. AHJ and licensed professional govern.
API field namesinstrument_depth_in, half_signal_offset_in, left_null_in, right_null_in, current_near_ma, current_far_ma, sharp_drop_pct, geometric_depth_in, depth_difference_in, depth_disagreement_pct, null_asymmetry_in, current_drop_pct
- Distortion can read DEEP a reading deeper than the truth is the dangerous direction: it promises cover that is not therelocating practice
- The 45-degree method is independent it does not use the instrument's own depth computation at alllocating practice
- A sharp current drop means the signal left the line and everything beyond it may be a different utilitylocating practice