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Simple Intro to Accuracy and Precision

Accuracy and Precision

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. Even if everything is more or less obvious with the precision quoted in the technical specifications of the receivers, it isn’t always straightforward with accuracy.

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.

 

General Idea of Terms

Although often used interchangeably, the terms have different meanings. Accuracy reflects how close a measurement is to a known or accepted value, while precision reflects how reproducible measurements are, even if they are far from the accepted value. To make it clear, we need a real-life example.

Imagine that every weekday morning, you take a train to your office that arrives according to the schedule. Your train arrives at 7:00 a.m., and we can note here that it’s definitely a known value. If you don’t want to miss it, you regularly arrive at the station between 6:42 a.m. and 6:45 a.m. Here there is only a 3-minute window—your arrival time has very little scatter, which means you are very precise.

But are you accurate? Actually, it is not like that. Remember when we noted that the train arrival time is a known value, and your arrival time is 15-18 minutes away from it. Therefore, you are not accurate, but the good thing is that you definitely won’t miss your train.

Looking ahead, we can say that you are indeed accurate relative to your personal time management system, which allows for a cup of hot coffee before the arrival of your train. We will return to this later.

 

Precision in Real-Time Kinematic

Now that we have figured out what precision and accuracy mean in everyday life let’s turn to their use with reference to Reach receivers and the Real-Time Kinematic technique. Speaking of precision, remember the solution statuses you observe in ReachView 3: Single, Float, and Fix.

We should begin with Single—it means that the receiver calculates its solution relying only on the satellite signals; it does not receive any data from the base yet. In this case, its precision is quite low, usually a few meters.

To improve the situation, we need corrections from the base station. We add the base, and here the Float status comes. Float means that the rover receives corrections from the base. However, it can’t resolve all ambiguities at that moment, and its precision is usually at the submeter level. It may happen when the rover needs more time to calculate the solution due to various factors.

When the rover finally resolves all ambiguities using the base corrections and confirms the solution quality using the internal verification algorithms, we come to the Fix status. It means that the rover achieved the solution with centimeter precision.

With this in mind, we’ve come to the point—we can see that the rover is positioned relative to the base, and its position is always precise in relation to the base’s position. It means that all the rover’s measurements are consistent with the base’s position.

 

Accuracy in RTK technique

And what about accuracy? The actual accuracy, the one related to CRS, will always be set by the accuracy of the base position, which is determined by the base setup method. Depending on the method, you can obtain relative or absolute accuracy. Let’s consider the examples.

Relative accuracy can be seen as the relation of points within one project. It’s like in contour farming when we need to make parallel rows in the area, and it doesn’t really matter where this area is in the world. In this case, it’s enough to determine your base’s position coordinates without reference to the coordinate system. Remember that ‘coffee moment’ in the real-life example above?

But let’s get back to surveying. When your project requires relative accuracy, all you need is to physically mark the point on the ground, place the base over it, and get its coordinates using the Average SINGLE coordinates entry method in ReachView 3. Don’t forget to put down the obtained coordinates for further use within your project.

Projects like the one with parallel rows have very high relative accuracy but poor absolute accuracy. And here it is, absolute accuracy is the degree to which the calculated position of your project corresponds to its actual position in the real world.

For example, you have a construction project, the boundaries of which should be determined for further recording in the land registry. In this case, you should place your base over a point with the coordinates tied to the coordinate system.

How can you get such a point? First, you should find a permanent survey mark or determine base coordinates yourself. When it’s done, place the base over that point and enter its coordinates in the Manual coordinates entry mode. You will obtain absolute accuracy. Of course, everything depends on how accurately the point’s coordinates were determined.

 

Accuracy & Precision with Reach Receivers

Summing up, the accuracy level depends on the method you use to set up your Reach base, while the precision level is related to the rover’s solution status.

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!

 
Accuracy of Your Project
 

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!

 

Do you need advice about your survey project?

Deciding what level of accuracy and precision you need is a fairly tricky topic. If you still have doubts about the type of data required for your project, feel free to ask us questions. Check our dedicated community forum—here our experienced users may help with a suitable solution for you. Or get in touch with our Support team, contacting us at info@emlid.com. Emlid application engineers will assist you with different types of technical questions.