Setting up your GNSS base during a survey is perhaps the most important part of the entire job. Accurate GNSS surveys in RTK will help you obtain the most accurate results, and if the setup is not right, you might end up with bad ones.
Your coordinates are only as accurate as your base’s placement, and any shifts in the base will only affect the rover’s measurements. So to help you get everything in order and avoid any costly mistakes, and come up with perfect and accurate results, we are going to go one step at a time and explain the entire process, which will help you learn to set up your GNSS base to achieve centimeter-level accuracy.
Coordinate System Setup
Selecting the appropriate coordinate system enables you to represent the position of objects on Earth accurately. But it mainly depends on the coordinate system your base’s coordinates in. At your disposal, you actually have a lot of CS options to choose from, and picking the correct one will affect the accuracy of your results.
To help you determine the most appropriate CS, you may consult the official project documentation, where the parameters should be specified in the technical assignment. However, if there aren’t any specified, you can always check some public resources, like the EPSG registry for example, and find the best system for your region.
If you are using an app, the things to configure are the coordinate system, along with the datum and its projection, and the vertical datum, which provides an ellipsoidal height.
Choosing Proper Positioning
After properly setting up your coordinate system comes the part where you have to determine the proper positioning. In RTK, the base acts as your reference point for the rover. This basically means that it does not matter if you are measuring with centimeter precision, the rover coordinates will only be as accurate as the base placement. Any changes in the absolute base position will affect the rover measurements. Therefore, there are two ways that you can consider when setting the position of your base - Relative and absolute.
Relative
If you are planning to understand how measured objects relate to one another, calculating the distance without having their exact positions, it is more than enough for you to have relative positioning. In such a situation, you can average the position of your base and get meter-level accuracy.
This approach works in standalone mode. So when you average a position from the same location, the coordinates will vary by several meters. Relative accuracy between points remains consistent, but absolute accuracy is limited.
You also need to know that this method is not commonly used in professional surveying, especially for those that require precise georeferencing.
Absolute
Absolute positioning helps you get centimeter-accurate results that are consistent with both the rover and the coordinate system. It requires setting up your base to known and accurate coordinates.
Entering the true position of your base station, you can pin your RTK setup to the correct location on the ground. That way, all measurements taken by the rover are georeferenced precisely within the selected coordinate system and can thus deliver centimeter-level accuracy.
Using an app, you can do that by setting it manually or using an average FIX method. Both ways actually help your data align with other geospatial datasets. Making them perfect for professional surveying, construction layout, GIS mapping, and any type of tasks that require high-precision positioning.
Setting Up Base Coordinates Using an App
Depending on the field conditions and the available reference data, you have several ways available to you to set up your GNSS base station over a known point. Sometimes using an app makes things much easier, but it also helps to get the desired accuracy you need for the type of survey you are doing. So here are some base setup methods that you can try and do to achieve that.
Setting up The Base Over a Known Point
If you have a known point, the best way for you to start is to place your base directly over it. You can then manually enter the coordinates. Since it ties your project to a predefined coordinate system, this method enables high levels of accuracy.
With the help of an app, you can enter the coordinates of a known point or even set up a permanent survey mark. You also have the option of loading them from a previous project.
However, if you are in a situation where the known point is obstructed, you can go for the base shift method, which allows you to adjust the base’s coordinates. Set your base near a point and apply a linear shift based on the linear point with the help of the app.
Using Average FIX for Base Coordinates
If you have no known points available but you still have internet access, use real-time corrections with a NTRIP service and average your position in FIX. In such a case, your base will be treated as a GNSS rover, which then allows it to receive NTRIP corrections. It will allow you to gather accurate base coordinates for a short period of time.
Getting Base Coordinates Using Static Data
If you happen to be working in a desolate area where there are no known points and no internet connection, you can use Static processing as a reliable way to set up your base. But you are going to need a nearby CORS station. You can log raw data on your base, download CORS logs, and simply post-process them later on. You only need to enter the accurate coordinates after you are done processing it.
FAQ
For a bit more insight, check out the answers below.
What is a GNSS base station and why is its setup so important?
A GNSS base station serves as a fixed reference point for your rover during RTK (Real-Time Kinematic) surveys. Its placement and coordinate accuracy directly influence the precision of all rover measurements. Any errors or shifts in the base station will be reflected in your survey results, making proper setup crucial for achieving centimeter-level accuracy.
Which coordinate system should I use for my GNSS base?
The choice of coordinate system depends on your project requirements and location. Refer to your project’s technical documentation for specified parameters. If none are provided, consult public resources like the EPSG registry to select a system appropriate for your region. Always configure the coordinate system, datum, projection, and vertical datum (for ellipsoidal height) in your survey app.
How do I set up my GNSS base over a known point?
Place your base station directly over a surveyed point with known coordinates. Enter these coordinates manually into your app, or load them from previous projects or permanent survey marks. If the known point is obstructed, use the base shift method: set up near the point and apply a linear correction using your app.
How do I verify that my GNSS base is set up correctly?
After setup, perform a test measurement with your rover on a known point or check for consistency with previous surveys. Ensure that your app displays FIX status and that all coordinate system parameters match your project specifications.
What are the risks of incorrect GNSS base setup?
Improper setup can lead to systematic errors in all rover measurements, resulting in inaccurate or unusable survey data. Always double-check coordinate system settings, base placement, and input methods to avoid costly mistakes.
