How Locating Instrument Works – Part 5

Well it has been a little while since I put pen to paper and for this I apologise – a little bit of R and R with the family got in the way.

In part 4 we looked at the shape of the electromagnetic field that leaves the conductive target line. We said that it ALWAYS wants to leave in a perfectly round shape. Unfortunately there are many factors that can affect this and cause distortion to the shape of the field. If the field is not round we cannot accurately locate it with EM Locating.

So the next major step in our journey is to be able to identify if the field is round or not round. This is where the correct use of the equipment is most important. If you only use your locator in peak mode, you are not getting the full picture and run the risk of inaccurate locates.

To be able to tell if the electromagnetic field is round or not you need to examine the field in both peak and null modes – specifically there are 5 tests that can be done that will identify round and non-round fields.

Peak and Null Comparison

The first is a comparison of the Peak and Null positions. When a field is round the examination of the electromagnetic field in either Peak or Null modes will indicate the same result – in other words the peak location will match the null location. Only round fields will indicate this.

Electronic Depth Verification

Electronic depth is calculated in Peak mode and uses the signal strength readings from two peak antennae’s in the receiver that are placed at a fixed and known distance inside the receiver. By calculating the strength of the signal received by each antennae and factoring in the set distance between the two fixed antennae’s a depth reading is calculated.  If the magnetic field is round you will be able to raise the receiver a known distance (say 200mm) and the depth reading will also increase by the same amount. A non-round field will not have this result.


This has been around for many, many years as a way of determining the depth of the utility. However it is also useful in determining the shape of the electro-magnetic field that we are examining. Triangulation is carried out in Null Mode – the process is to firstly identify the possible location of the utility, mark the ground. Then tilt the receiver at 45o to the level of the ground and whilst still in Null Mode move the receiver away from the utility until you record another null reading – mark this point. Move the receiver back to the target line and tilt it 45o in the opposite direction – repeat the process. If the two marks are equally distant from your target line mark then the field is round.

Peak Analysis

A peak analysis can be carried out two ways:

  1. Mark the highest peak reading that you find – this is the possible location of the target. Then whilst still in peak mode move the receiver left (or right) of the target line until you have a specific peak strength reading on your receiver – say 40 (as an example) – mark this point. Then do exactly the same on the opposite side of the target line (again to a reading of 40) and mark this point. The distance from the target line to each mark – left and right should be the same, if it is the field is round.
  2. The second method measures the same distance either side of the marked target line – say 400mm (as an example), and then taking a peak signal strength reading at each of these points. If the signal strength is the same e.g. 50 on either side then the signal is round.

Null Analysis

Finally we can analysis the field with a Null Analysis. This method will indicate if the field is balanced, as you raise the receiver above the target line. In other words, find the null position at ground level, then with care vertically raise the receiver, if the field is round your reading will remain constantly in balance.

You should always test the shape of the electromagnetic field by using at least 2 of these methods. If the field is not round then we need to work on making it round.

Locating Unlimited thoroughly investigates and demonstrates each of these methods plus a whole heap more in our 5 day advanced EM locator Training and the 2 day locator awareness training. Contact me now for further details: