Accuracy and precision explained: surveying with high precision GPS

When it comes to GNSS data measurements, both accuracy and precision are of great importance. Surveyors always have to consider the precision of the receivers used and the acceptable accuracy level for their projects. While the precision quoted in the technical specifications of these GNSS / GPS receivers may seem clear, the actual accuracy might not always be straightforward.

Indeed, one of the most common questions we deal with is how to set up Reach receivers to get accurate data for a project. To answer this question, let’s consider both terms concerning their everyday use.

Learning the distinction between accuracy and precision

In high precision GPS as a part of GNSS technology, the terms accuracy and precision are crucial. These terms, often used interchangeably, have distinct meanings that are pivotal in fields like GPS surveying.

Accuracy refers to how close a measurement aligns with the correct or accepted value. For example, if a surveyor measures a land boundary and their measurement aligns within millimeters of the previously established data, this measurement is considered accurate. On the other hand, precision relates to the consistency of repeated measurements. Precision does not necessarily mean that the measurements are accurate, but that they are consistently similar, indicating control over the variability of the measurement process.

Consider a practical scenario where a person arrives at a train station between 6:42 and 6:45 a.m. every morning despite the train scheduled arrival at 7:00 a.m. This demonstrates high precision due to the consistent arrival times, although the accuracy is not aligned with the train’s schedule.

This distinction is even more significant when applied to high precision GPS systems used in various surveying applications, where both accurate and precise measurements are crucial for project success.

Precision in Real-Time Kinematic

Now that we have figured out what precision and accuracy mean in everyday life, let’s delve into their application within Reach receivers using the Real-Time Kinematic system. This system significantly enhances the precision of RTK GPS measurements by correcting the satellite signals with information from a base station, achieving precision down to the centimeter level. Such precision is crucial for high-detail applications like construction site mapping, precision agriculture, or geological surveying.In RTK, the solution statuses observed in ReachView 3—Single, Float, and Fix—indicate the level of precision:

• Single: The receiver calculates its position solely based on satellite signals without any corrections from the base, usually achieving meter-level precision. This initial status demonstrates the basic capability of GNSS / GPS technology without enhancements.
  
• Float: At this stage, corrections are received from the base station, but not all ambiguities are resolved. This status typically provides sub-meter precision and is suitable for applications where centimeter-level precision is not necessary, but improved accuracy is beneficial.
  
• Fix: The pinnacle of RTK system capability, where all ambiguities are resolved using corrections from the base station, achieving centimeter-level precision. This status means that the rover has achieved the highest precision possible with the RTK GPS system, ensuring that all measurements are consistent and precisely aligned with the base’s position
  

This progression underscores how RTK systems enhance the precision of measurements relative to the base station’s position, ensuring consistent and precise data collection. With each step from Single to Fix, the high precision GPS capabilities of the system are further leveraged to meet the stringent requirements of modern surveying and precision-dependent projects.

Accuracy in RTK systems

When discussing the accuracy of RTK systems, it’s essential to understand the critical role played by the base station’s setup. The base station’s accuracy is pivotal, as it transmits corrections to the rover, enhancing the rover’s positional accuracy within the GPS framework. This improvement is crucial whether you are using single-band or multi-band receivers.

Types of accuracy in RTK systems

• Relative accuracy: This type of accuracy is paramount in scenarios where the spatial relationship between points is more crucial than their absolute geographical locations. For instance, in GPS surveying for a new subdivision, the relative positioning of each plot to one another is vital. It’s similar to contour farming where the alignment of parallel rows is essential, and their exact Earth coordinates are secondary. This kind of accuracy, often utilized within a defined project area, relies on setting up the base station relative to a local site rather than a global coordinate system. When setting up for relative accuracy, the “Average SINGLE coordinates entry method” in ReachView 3 is typically employed to record the base station’s location relative to the survey area.

• Absolute accuracy: On the other hand, absolute accuracy is crucial for projects where precise geolocation is necessary. This includes legal boundary definitions and extensive geographical data sharing that require alignment with a global coordinate system. Achieving absolute accuracy involves setting the base station over a point with known coordinates or using high-accuracy correction services. This accuracy is particularly vital in construction projects where exact property boundaries must be established for legal documentation and planning purposes.

Setting up for optimal accuracy

To ensure optimal accuracy in your RTK systems, several methods can be used to establish the base station’s accuracy:

• Known coordinate points: Using pre-established survey marks that are tied to global coordinate systems.
• Averaging position: Calculating the average position over a significant duration to minimize errors.
• Correction services: Utilizing services that provide corrections based on highly accurate coordinate systems.

These methods enhance the reliability of data obtained from GNSS and RTK systems, ensuring that both relative and absolute accuracies are maintained according to project requirements. Whether you’re laying out a new subdivision with precise plot relationships or defining legal boundaries in a construction project, understanding and applying the correct type of accuracy is fundamental.Remember, the choice between relative and absolute accuracy should align with the specific needs of your project. By setting up your base station correctly and choosing the right coordinates entry method in ReachView 3, you can achieve the necessary precision for successful project outcomes in surveying.

Accuracy & precision with Reach receivers

Summing up, the accuracy level of your RTK system depends on the method you use to set up your Reach base, while the precision level is closely tied to the rover’s solution status in GNSS / GPS applications.

To get the FIX status needed for centimeter precision, do the following:

• Provide both units with a clear sky view.
• Make sure there are no electronics nearby.
• Check that the rover is receiving corrections from the base.
• Keep the distance between the units within the specified limit.

To achieve the required accuracy, every time you start a survey project, determine your goals first: whether you need to tie the obtained coordinates to the real geographic position or not. Depending on that, place the base and choose the coordinates entry method in ReachView 3—Average FIX or Manual, for projects requiring relative and absolute accuracy, respectively. Well, you know the rest!

You might also be asking about the accuracy level when using CORS and NTRIP networks instead of the ‘traditional’ base stations. Usually, you will get absolute accuracy if the distance between the reference station and your rover doesn’t exceed 10-15 km for single-band receivers and 60 km for multi-band receivers. In this case, everything you need is to configure your Reach for RTK mode with NTRIP.

Remember that your experience may vary in different conditions. Always follow appropriate survey practices! For applications requiring absolute accuracy, manually inputting the precise coordinates of the base station or using a fixed point with known coordinates ensures that the positional data is both accurate and precise.

Frequently asked questions

What is a good example of accuracy and precision?

Accuracy and precision can be illustrated by a surveyor using a GNSS device to measure land boundaries where measurements are both consistent (precise) and align closely with the true boundaries’ ground coordinates (accurate).

How do you determine accuracy?

When checking the accuracy, specialists usually compare the measurements against a set of verified and known coordinates. The process can also involve using tools calibrated in line with universally accepted standards.

Why is precision important?

If you want to reach the consistency, quality and reliability of measurement results precision is crucial. This is especially critical in surveying field where precise measurements are crucial for accuracy and repeatability.

Do you need advice about your survey project?

Navigating the complexities of high precision GPS and ensuring that both accuracy and precision meet the demands of your project are critical. By understanding and applying these concepts effectively, you can significantly enhance the reliability and success of your work.

If you’re unsure about the type of data required for your project or have questions about setting up your equipment, Emlid offers robust support options. Our community forum is a great place to start, where experienced users share insights and solutions. Alternatively, our support team is always ready to assist at info@emlid.com.