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Eight common mistakes in GNSS surveying and how to fix them
Modern GNSS surveying equipment goes a long way in helping you achieve high-accuracy data collection, but the human factor still plays a role. Some common surveyor mistakes can significantly compromise GNSS accuracy. This article will walk you through eight GNSS surveying mistakes that can easily trip you up. We will outline the problem and describe practical solutions.
Multipath error
Challenge
Multipath errors occur when the satellite’s signals are reflected and refracted off a surface before reaching the receiver. These surfaces can be buildings, cars, water, or many reflective surfaces, including the ground. When the signal arrives at the receiver from a ‘bent’ signal, the distance measured becomes skewed, and thus the GNSS accuracy of the measurement is compromised. These multipath errors are more likely to occur in urban areas than rural areas.
How to deal with multipath?
The best way to overcome these errors is to avoid placing your base near significant metal structures and elevating your receiver as high as possible off the ground. It is also helpful to use a good antenna with a built-in ground plane that can block low-angle reflected signals from the ground. The Emlid Reach receivers come with this type of antenna.
Level bubble out of plumb
Challenge
When the level bubble comes out of plumb, it can significantly impact the accuracy of your GNSS surveying measurements. There are many reasons for this, including vibrational shocks and the warming and cooling of the environment, which causes the metal components to expand and contract. It is recommended that periodic in-office checks be done.
How to calibrate the level bubble?
If you’re in the field and want to check it, you can find a vertical structure like a building’s wall to see if your level bubble is out of plumb.
- Remove the receiver from the pole and place it two inches from the structure, making sure it is parallel to the structure.
- Then rotate the pole 180 degrees and check the bubble; if it is not centered, it is out of plumb.
While it’s best practice to calibrate the pole, there is a workaround that allows you to still make accurate measurements with the bubble out of plumb. Just follow this procedure:
- Measure for 60 seconds, then rotate the pole 180 degrees and take another 60-second measurement.
- Then, average these two shots to remove the error. There are many different ways to check and calibrate the bubble.
YouTube has many reference materials to help you here. Also, with the Emlid Reach RS3, you can enable tilt compensation, which allows you to collect accurate measurements even with a tilted pole.
Too long of a baseline
Challenge
The baseline is the distance and direction between the base station and the rover. As this distance increases, the accuracy of the measured readings will degrade. This happens because the base station and rover are located far enough away to experience different atmospheric and ionospheric conditions.
Environmental conditions can distort GNSS signals. If both receivers are affected similarly, the errors tend to cancel out. This assumption is built into positioning algorithms, allowing us to effectively exclude these common errors from the calculations.
However, if the signal distortions differ between the receivers, these errors cannot cancel out, leading to positioning inaccuracies. While centimeter-level accuracy is achievable even at distances of up to 60 km in RTK and 100 km in PPK, for optimal results, keeping the baseline as short as possible is recommended. The RTK error model—7 mm + 1 ppm—illustrates how accuracy decreases with distance.
How to avoid a long baseline?
One way to overcome this challenge is not to rely on CORS but to set up a local base station on your site. The Emlid Reach receivers can act as a rover and a reliable base station.
Not performing localization
Challenge
When you perform a survey and compare it to a previously gathered dataset, and do not know the parameters in which it was measured, your measured data will not accurately align with the known data. This happens commonly on a mining site job or a subdivision’s construction site.
How to survey a site with an unknown coordinate system?
When you need your data to line up with a previously surveyed site, you want to do a localization called a site calibration or local transformation. To do a proper localization, you will need (ideally) more than four known points to measure in your known coordinate system. Read our article for step-by-step guidance on localization with the Emlid Flow app.
Unit mismatch
Challenge
Projected coordinate systems use linear measurement units, typically meters or feet. A common mistake is selecting a coordinate system that uses meters when your project requires feet. This mismatch can prevent your data from aligning correctly with previous survey work.
How to know which linear unit to choose?
Using a site like epsg.io, you can investigate further by querying for your desired projection and looking into the metadata to see which linear unit is used.
Emlid Flow offers a dedicated setting that lets you choose the appropriate unit of measurement for your project:
We recommend double-checking the selected units, even if the coordinate system name includes “ftUS.” The good news is that if you choose the wrong unit and only realize it in the field, you can still adjust it using the same setting in Emlid Flow.
No documentation of the setup
Challenge
When using ad hoc markers instead of nails or targets, it is important to document exactly where you measured. If, for example, you are measuring at the intersection of parking stripes in the bottom left corner, another survey professional may easily misinterpret this point.
How to document a setup?
The best practice is to include this specific location information in the dataset’s metadata. One way to speed up this documentation is to take photos with your phone. You can then reference those photos when inputting the documentation back at the office.
Miscalculated height of the device
Challenge
When entering the receiver’s height, it is easy to measure the wrong span, as different manufacturers require measurements from other places on the instrument. Check with your receiver manufacturer to see precisely what span they want you to measure. The antenna height is usually different from the receiver’s height. And different poles have different scales.
What height do I need to enter when surveying with Emlid Reach?
With Emlid products, for example, you only need to input the distance from the base point to the bottom of the receiver. The distance from the bottom of the receiver to the antenna height is calculated within the software.
Learn more about how to set the antenna height for the RTK in our guide.
Keeping ellipsoids and geoids straight
Challenge
If you’re unfamiliar with elevation reference or vertical datums, you might make mistakes that affect the accuracy of your measurements.
GNSS receivers measure height above the ellipsoid, a smooth mathematical model of Earth’s shape, like a slightly squished basketball. However, this doesn’t reflect real-world terrain or gravity variations.
How to deal with ellipsoid and orthometric heights?
We need orthometric height for practical use—the elevation seen on road signs at mountain passes. Since GNSS gives height over the ellipsoid, we assign a geoid to get orthometric height. It represents sea level and accounts for terrain variations. Choose the correct geoid for the project to get accurate measurements. In the Emlid Flow app, you can simply select the appropriate geoid from the list.
In the U.S., surveyors often use NAVD88 with GEOID12B to obtain precise elevation data. Different regions worldwide have different vertical datums and associated geoids that are most appropriate for getting the best vertical elevation measurements.
How do you avoid these mistakes for more reliable GNSS surveys?
While this list doesn’t cover every potential pitfall, it highlights some of the most common mistakes that can reduce the accuracy of your GNSS surveys. Awareness of these issues will help you collect high-precision data, avoiding multipath errors to baseline length and unit mismatches.
The best way to ensure accuracy? Use reliable hardware, follow best practices, and double-check your setup before collecting data. With Emlid Reach receivers and the right workflow, you can minimize errors and achieve centimeter-accurate results every time.
If you have any questions, contact our support team or visit our community forum.