Accuracy of a locator, that is to say a locator’s ability to find the correct position of a buried line and its depth, is difficult to define and is often ignored.

Electromagnetic locators do not locate pipes and cables; they locate alternating magnetic fields.

Errors of accuracy can arise from two factors:

• The locator’s capability to measure the precise point at which a magnetic field is at a maximum (or minimum) and to correctly measure a field gradient.
• The cylindrical magnetic field around a line can be deformed or distorted so that the maximum value is no longer directly above the target line and the field gradient is not suitable for making an accurate depth measurement.

Locator’s capability to measure accurately

The obtainable degree of resolution defines one source of error from the locator. Assuming a long straight isolated target line, a visual output device that can be resolved to within 1% of full scale and vertical alignment of the antenna, a vertical aerial can resolve position to better than ±5% of depth, a single horizontal aerial to ±10% of depth and twin horizontal aerials to ±5% of depth of the line.

Manufacturing tolerance and error is another possible source of error.

The single horizontal aerials are least affected by mechanical tolerances. Vertical aerial gives twice the error of a horizontal aerial if the aerial is misaligned to the same extent. 

Magnetic field distortion

Most of the problems of accurate location of buried lines in the highway are due to situations that distort magnetic fields. While there are an almost infinite number of ways that fields may be distorted by other lines at various angles and carrying various signals, a useful analysis of accuracy can be obtained by considering just two specific situations:

• A 90° bend in the line.
• Two close parallel lines carrying equal signals or currents. Equal signal currents are, of course, an unlikely eventuality but the example serves as a useful reference for comparison. 

Distortion close to a 90° bend: a locator starts giving faulty information as it comes under the influence of the magnetic field of the perpendicular part of the target line.

Error is expressed as a percentage of the depth d of the line. Relevant distances are expressed in units of d. Measurement of two types of error are useful; maximum degree of plan location error and the length of line along which location error exceeds 10% of depth.

Vertical aerial. A null locator traces a path that is outside the actual bend. The only point where the reading is correct is exactly over the point of the right angle bend. Maximum error is at a point 0.7d from the bend and amounts to 33%. 10% error band extends 5d either side of the bend.

Horizontal aerial. The locator traces a path that cuts across the inside of the bend. Maximum error occurs at the point of the bend and is 25%. 10% error band only extends 0.5d from the bend.

Twin horizontal aerials. Similar to the single horizontal aerial locator, it traces a path to the inside of the bend with a maximum error of 16% and the band of the 10% error is only 0.33d.

Distortion due to parallel lines buried close together:

1. Similar strength signals on parallel lines following the same direction:
   
   Vertical aerial. Error of less than 10% is only achievable if lines are more than 10d apart. The error will indicate that the lines are closer together. If the lines are closer than 2d there will be a single null in the centre of the lines rather than two separate indications.
   
   Horizontal antenna. Locator indicates lines are closer together but separate indication of each line is possible down to separation of 1.2d when error will be up to 60%. Accuracy of better than 10% is possible if separation is twice depth or greater.
   
   Twin horizontal antenna. Error of 50% at a separation is greater than 1.5d.
   
   
2. If similar strength signals on parallel lines run in opposite directions the following may be expected:
   
   Vertical aerial. The locator will show two positions outside the actual position of the lines. It will still show two separate responses even if lines are almost touching and error will be 100%. Accuracy better than 10% is only possible if the two lines are nearly 10d apart.
   
   Horizontal aerial. The locator gives response outside the true positions but with maximum error of 60%. Error falls to 10% when lines are 1.7d apart. There is a sharp null response between lines
   
   Twin horizontal aerial. Similar response to the single horizontal antenna but a maximum error of 50% reducing to 10% error when separation is 1.2d.

Summary

The above data indicates several conclusions:

• The vertical aerial locator gives responses unacceptably wide from the actual position of lines when more than one line with the same signal is present in a small area.
• Twin horizontal aerial system provides the best and the most useful response.
• Comparison of responses from vertical and horizontal aerials can be used to determine if interference fields are affecting accurate location. Interference is present if the responses from the two systems do not coincide.

This comparison permits multi-aerial instruments such as Radiodetection Precision locators to check if a response is accurate and if the signal is suitable for making an accurate depth measurement.