What is the accuracy of the PLT 400 and how can I ensure it stays accurate?

Disclaimer: for official accuracy information, please see the operations manual. Accuracy of layout and measurements is strongly determined by end-user competence and understanding of the tool.

Accuracy Specifications for the PLT 400 - Explained

The PLT 400 has millimeter-level accuracy at 100m and is ideal especially for general construction applications. Several factors need to be considered when answering accuracy questions, which this article will address. For a quick answer on accuracy, note the following points:

The PLT 400 has a 2-4 arc-second angular error, which means that at 100m, only a 1mm-2mm deviation, respectively, will be present at that distance from the tool's angular precision error.
At 100m, the PLT 400 has a distance measurement error of 2.2-3mm, depending on how the measurement is being made (see below).
Errors are tested under ISO 17123-3 and 17123-4, meaning they’re consistent and traceable.
The PLT 400 uses an automatic field calibration method to ensure the tool is consistently leveled and that the tool head unit components are properly aligned for accurate measurements.
The Hilti Construction Layout software is intuitive and flags inconsistencies in measurements for the end-user, as well as provides visual cues to guide corrections.

Relying just on the tool accuracy specifications is not enough to maintain accuracy in layout applications, however, as jobsite conditions and end-user competence also can affect how the tool performs. Therefore, this article will later discuss end-user strategies to help keep the tool accurate and mitigate potential errors. First, though, the tool-specific angular and distance measuring specifications are clarified and explained below.

Angle Measuring Precision (ISO 17123-3)

The PLT 400 can be purchased as a 2 arc-second tool or a 4 arc-second tool, depending on where you are located. An arc-second is a very small unit used to measure angles, especially in fields like surveying, astronomy, and navigation, and it makes part of a singular angular degree. To visualize: a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 arc-seconds.

In other words, regardless of which PLT 400 version is purchased, the angular precision has a very low error. Naturally, a 2 arc-second tool is more accurate, or has less angular error, than a 4 arc-second tool, but often, these errors will be very difficult to see unless extremely long measurements (300+ meters) are being made with the tool.

For instance, for construction jobsites, the typical maximum layout range of work is around 100 meters. At this distance, the maximum technical angular inaccuracy of a 2 or 4 arc-second tool is 1mm and 2mm lateral deviation, respectively. While there are other factors that affect accuracy with total stations, such a low arc-second precision error should be very encouraging.

When should I consider even lower arc-second accuracy tools?

For especially long-range layout projects, such as land surveying, road and bridge work, etc., where prisms are being measured consistently beyond 300m away (even up to nearly 1 mile or 1.6 km), perhaps a tool with an even smaller arc-angle inaccuracy should be considered. Angular accuracy errors are more of a factor at extremely long distances from the tool.

Distance Measuring Precision (ISO 17123-4)

There are three different ways the PLT 400 can make a measurement, and it depends on what the tool is looking at or tracking. Below are the precision technical details for these three measuring methods available, as it refers to distance measuring:

Standard Measurement Precision:

+/-2mm + 2ppm. This is referring to general measurements with the tool's laser to reflective targets designed for total stations, like reflective tape, foils, or even prisms. This is an EDM error, or electronic distance error, from the tool's laser. What this means is that when you are using the laser and measuring a reflective target that is designed for total stations, there is a fixed chance of a 2mm distance measurement error and a variable error of 2ppm (explained below).

As mentioned, prisms can be measured to in standard mode, but most commonly, they are "locked" onto via the prism tracking mode (also explained below), which has a different precision error. The reason for this is because without the lock-on prism tracker, users must manually aim to the center of a prism, which is typically less likely to be as accurate as simply locking onto it with the prism tracker.

Reflectorless Measurement Precision:

+/-2mm + 2ppm. This is referring to general measurements with the tool's laser (EDM) to general surfaces. Examples include natural or man-made matte (non-shiny) surfaces like concrete (most common), wood, painted non-glossy surfaces, and rough, non-metallic surfaces. This has the same distance measurement error as the standard measurement precision: there is a fixed chance of a 2mm distance measurement error and a variable error of 2ppm (explained below).

It is important to note here that the laser (EDM) is not ideal to measure highly reflective surfaces (other than reflective total station targets). In addition, caution should be taken when measuring to an area that could be affected by highly reflective material (like a dark wall that is next to very shiny objects). The reason for this is because the laser may get scattered or absorbed in this light, the total station telescope may retrieve false reflections causing measurements to fail, or, there may be larger distance measuring inaccuracies.

See this article related to working in dark or light conditions for more information.

Prism Tracking Precision:

+/-3mm. This is referring to the prism tracker in the head unit, which is a combination of the EDM and the tool's telescope devices that help find and track the center of a prism. The total station is using the camera system within the tool to find the center of a prism via infrared reflections, and then uses the EDM to ping back to the telescope the measurement distances.

This has a fixed error of 3mm when the tool is actively following or measuring to a prism. It is a combination of the standard EDM error, as well as the error associated with tracking a prism.

What does the PPM error mean for distance measurements?

To take the example of the standard measurement precision mentioned above, +/-2mm + 2ppm means that there is a fixed +/- 2mm error in distance measurement, regardless of the distance you are measuring to. In addition, there is a variable 2ppm error, or 2mm error added on to that every 1,000 meters.

So - at 100m, your additional error from the ppm would be 2mm + (2 x 0.1mm)= +/- 2.2 mm error.

How can the PLT 400 be an accurate solution, even with internal fixed errors?

Errors are almost a given, regardless what kind of layout work is performed. Nothing is perfect. The benefit of a total station is that the errors are known, the errors are consistent, and the errors are manageable.

This means that end users can completely prepare for them, and can understand their implications in their own work. Total station errors can be accounted for, while traditional layout or measuring methods have much more room for error and inconsistency.

With the PLT 400 and other Hilti layout tools, the Hilti Construction Layout software is intuitive, making the operation of total stations less intimidating and easier to understand.

What factors could affect the total station accuracy?

While the tool itself is a very accurate tool for general jobsite layout, care must be taken by the end user to keep it accurate. Important action steps to maintain accuracy are listed below:

Before Starting Work

Acclimate the instrument to the ambient temperature (especially after transport/storage) - give it time to sit in the environment you intend to work in before use so the internal temperature is acclimated to ambient conditions. See the following links for more information regarding weather impact for total stations: weather acclimation and working in wet or dusty conditions.
Let the tool complete its field calibration & self-leveling - this happens automatically with the PLT 400, and will continue to happen at regular intervals throughout the workday. Field calibrations are critical to maintaining accuracy. See this link for more information on field calibrations.
Inspect the optics/lenses - make sure the mirrors around the telescope and the main telescope lens itself is cleaned of water and debris. If you see fog inside of the lens, giving time for the tool to acclimate to the outside temperature should eventually see the fog disappear. See this link on cleaning the mirrors.
Secure the total station to not be influenced by jobsite vibrations and movements - jobsites are usually very active, and care should be taken to protect the total station from being affected by the traffic or vibrations of jobsite activity. See this article for help regarding securing a total station properly to the jobsite.
Have a versatile distribution of control points for stationing - stationing is critical for a total station to conduct layout and measuring applications accurately. See this introduction article to stationing topics, and this article related to control point best practices for more information.
Calibrate or check the prism pole - the prism pole must be level when being used for layout to stay accurate. Verify that the bubbling level provided on the pole is reading accurately, and adjust the bubbling leveling screws if needed.

During Use

Regularly conduct backsight checks - checking established control points regularly even after you have set up the tool will help you verify if your accuracy is consistent. See this article, specifically, regarding the backsight checks application. This is especially important if you have just moved the tool to a new location.
Regularly allow the tool to conduct field calibrations - as mentioned above.
Monitor weather conditions - be willing to pause layout if the weather conditions are not ideal for accuracy requirements (wind, vibrations, rain, snow, dust, etc.).
Keep prisms clean and scratch free to ensure they are being properly targeted - using a cloth to simply clean dirty prisms could help ensure the total station is finding the center accurately and making accurate measurements.
Choose the correct prism type for the layout or stationing you are conducting - sometimes the simplest of mistakes can cause measurement errors. When using HCL, make sure the targets you are measuring to are also correctly indicated within the software.
Mark your points cleanly - a total station may be able to measure and guide accurately to a point, but if the marking process is clumsy or inconsistent, the accuracy will diminish simply because of how the points are eventually marked by the end user.

After Use

Clean and inspect - Wipe down the instrument, especially of dust and water. Storing the tool in a dry case is important to prevent the humidity of the case environment to allow fog to collect inside the telescope of the tool. In addition, having dust caked on the outside of the telescope could lead to scratches. Anything inhibiting the telescope could lead to inaccuracies.
Store the tool at room temperature and in dry places - Avoid storing the tool, especially long term, in extreme temperature conditions. This protects the internal components like sensors and optics from expanding or contracting. It also helps to prevent condensation from building up in the lens, as moving the tool from a freezing (or near freezing) environment to one that is very warm, for instance, could cause internal condensation.

Remember, consistent checks of your work area need to occur to maintain an optimal work environment for digital layout. Control points might get bumped, hidden, or knocked down, dust or fog may be heavy that is affecting measurement readings, weather temperature and barometer readings may be input incorrectly in the tablet, an end user might forget to regularly perform backsight checks to ensure consistency in layout, etc.

While the tool is very accurate, diligence by end users is required.