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Figure 22: Tracing Line Showing High Distortion
3.Use the Guidance Arrows, Proximity Number, Signal Strength, and Tracing Line to guide the line trace. These pieces of information are generated from discrete signal characteristics to help the operator understand the quality of the locate. An undistorted signal emitted from a line is strongest directly over that line. (Note: Unlike the Signal Trace lines, the guidance arrows require that the user orient the locator so that the guidance arrows point 90 degrees to the Signal Trace line. (See Figure 23).
Note: an undistorted line will also be clear rather than blurred on the screen, and the sound accompanying the image will have no “static” in it.
Note: Confidence in the accuracy of a locate can be increased by maximizing the Proximity Signal (and/or Signal Strength), balancing the Guidance Arrows and centering the Tracing line on the screen. Confirm a locate by testing whether the Measured Depth reading is stable and reasonable. Agreement among all these indicators means the probability of an accurate locate is high.
node is placed closer and closer to the target utility. Rechecking the Measured Depth and position periodically during the excavation process can help avoid damage to a target utility and may identify additional utility signals that were not noticed prior to excavation.
When line tracing, it is important to remember that tees, curves, other conductors in the vicinity, and nearby masses of metal can add distortion to the field, requiring closer scrutiny of the data to determine the true path of the target utility.
Clarifying the situation can be done by assessing whether the distortion is due to a poor signal that needs to be improved, a local interference such as a
Circling the last location of a clear signal at a distance of about 20 feet (6.5 m) can clarify if the distortion is coming from a local turn or tee in the line, and enable the operator to again pick up the line nearby.
Always cross check the locate by ensuring that:
•The Tracing Line shows little or no distortion response (blurriness).
•The Proximity Signal and the Signal strength maximize when the Tracing Line crosses the map center.
•The Measured Depth increases appropriately as the unit is raised vertically and the Tracing Line remains aligned.
Measured Depth readings should be taken as estimates and actual depths should be independently verified by potholing or other means prior to excavating.
Maximized
Maximized
Figure 23: High Probability Locate
As always, the only way to be certain of the location of a utility is through visual confirmation by exposing the utility. The accuracy of position and depth measurement improves as the
WARNING
Care should be taken to watch for signal interference that may give inaccurate readings. The Tracing Line is only representative of the position of the buried utility if the field is UNDISTORTED. Do NOT base a locate solely on the Tracing Line
If the signal is clear, the
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