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The difference between a CAT4 and RD8200 | When to use a precision locator

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This is one of the questions I am asked the most. “The CAT 4+ is on our approved kit list and we just wanted to check that it is what you use for your survey?”

The answer is always the same. No.

Electromagnetic location (EML) is a key method in conducting a buried services survey. Utility survey providers are more likely to use a precision locator from the Radiodetection range or from an alternative manufacturer such as Vivax. Here we are going to look into the reasons for this and provide an insight into what the differences are between a CAT 4 and a precision locator such as the RD8200.

What is a CAT 4?

The CAT 4  is the current designation of locators in the current Radiodetection Cable Avoidance Tools range. There is a reason why we still colloquially refer to EML equipment as a CAT and Genny. The CAT has been the staple of this sector for many years.

The CAT 4 comes in a variety of models. These are:

  • CAT 4
  • CAT 4+
  • gCAT 4+
  • SuperCAT 4+

There are also two different signal generators with the locators. These are the Genny 4 for the CAT 4 models and the T1 for the SuperCat.

There are of course other manufacturers that produce cable avoidance tools. These include recognisable names such as Leica and C-Scope. However the original CAT is a Radiodetection product.

For the purposes of this we will look at the top of the range CAT 4, which is the gCAT 4+ model. The reason for this is principally we are asked if we have the ability to log use data. This is the only one in the lineup that has a data logging function. Also, the SuperCAT is straying more into the precision locator market with its wider selection of frequencies over the CAT 4 range.

The Genny 4 signal generator has a maximum output of 1 Watt and operates on two frequencies simultaneously. These are the standard 33 Mhz and 131 Mhz. The user does not have the ability to select either of the frequencies to be transmitted independently.

It is completely expected that we should be asked to record use data. This provides a trail to when the survey is undertaken should anything go wrong. This should be an essential element of quality control for any utility survey company, regardless of the equipment they use.

What is a precision locator?

One of the things that surprises me most is the lack of awareness of what a precision locator is. When I am asked if we use a CAT 4+ I am mostly confronted with blank faces when I say we use its big brother, the RD8200 precision locator.

A precision locator performs the functions of a CAT, including the data logging of its use, but is different from a standard CAT in the following key area:

  • Ability to trace all of the frequencies outputted by the TX transmitter. Some of these frequencies are industry specific.
  • Constant depth and current readout only when over the induced target, providing greater reliability of a trace.
  • Automatically rejects strong interfering signals.
  • Selection of power filters to establish if a signal is from one source or multiple.
  • Ability to connect to RTK GPS for highly accurate on the go maps using RD MAP software.
  • Fault finding mode.

The signal generator paired with the RD range is the TX. It is available with an output of 5 or 10 watts, TX5 or TX10. Much like the standard genny, the TX range should be used with a range of accessories to get the best out of the equipment.

The key differences between the TX range and the standard Genny are the increased output power and the extensive frequency range that is available for different functions and industries. 

Where the standard Genny 4 transmits two frequencies simultaneously, the Super CAT 4+ transmitter outputs five frequencies, the TX range can output on thirty four active frequencies. The TX10 model also has additional frequency options in the fault find mode.

What is the difference between the CAT 4+ and an RD8200?

The simple answer is features, power output of the TX  and ability to use a broader range of frequencies for locating a target utility.  There are other differences, such as the continuous depth readout, filters for isolating signals in congested locations and the five precision antennas within the locator. The core takeaway is that the precision locator performs the same functions as a CAT 4+ and much more besides.


The CAT 4 is definitely the tool for those undertaking excavation works who need to check underground services. It is simple to use, the 4+ adds depth measurements and the gCat provides data logging for QA and traceability.

For surveyors, the broader options, range of frequencies and functionality of a precision locator are what makes it the correct tool for the task. The job of a utility survey is to provide as accurate a picture as possible of the subsurface, taking into account the limitations of geophysical techniques. The best way to do this is to have trained professionals using state of the art equipment.

Both of the ranges of kit have their place in reducing the H&S and true cost of a utility strike. The CAT 4+ may be state of the art for cable avoidance, but the RD8200 is currently state of the art for utility location and this is why we use them.

Andrew Botterill