Modern GNSS equipment is there to help us produce the most accurate and best results possible in surveying. They are there for us to provide the most accurate results, no matter the situation or conditions in the field. But there is so much that modern GNSS surveying equipment can take us, as we still need to account for the human factor and the possible errors that may be produced.
A GNSS error in equipment failure is possible, but it does not happen as often as human errors do. For that reason, you have to know how to properly treat the equipment you are using, calibrate it correctly, and basically follow the best practices for each one of them in order to avoid mistakes in GNSS surveying.
To help you avoid these, sometimes costly, errors in GPS in surveying, we thought it would be a great idea to go over the most common ones individually and provide you with solutions or how to avoid them in general.
The Origin of GNSS is GPS
To give you a bit of a rundown of the basics. The origin of the GPS, or more precisely, the Global Positioning System, is that it was developed by the US Department of Defense for the purpose of being a worldwide navigation resource for both military and civilian use. It was released in 1978 and was originally based on a constellation of 24 satellites orbiting Earth, acting as reference points.
Today, GPS is joined by various other satellite constellations which including the likes of GLONASS, Galileo, BeiDou as some of the more popular ones. These additional satellites helped to greatly expand the range of networks, and together they are referred to as GNSS (Global Navigation Satellite Systems).
The way GNSS receivers work is that they listen to a series of coded messages that are transmitted by each satellite. A receiver located on the ground can therefore calculate how long it takes for a signal to get from a satellite to an antenna. To calculate the distance, the time it takes the signal to travel from the antenna to the satellite is multiplied by the receiver by the speed of light.
A GNSS receiver requires information from at least four satellites in order to triangulate its location anywhere on the planet’s surface. It is able to make calculations with multiple satellites at the same time.
Common GNSS Challenges and Errors
Mistakes in GNSS surveying calculation can happen if the view between the ground and space is obstructed. This can happen even with multiple satellites being present. The most common errors in GNSS occur when signals from satellites bounce off objects on their way to the receiver and not in a straight line. This is referred to as “multipath” and can be caused by some very common objects that can act as obstructions, such as buildings, large structures, and even trees.
While it is possible to maneuver around multipath challenges, there are certain errors in GNSS positioning that surveyors don’t have control of. There are atmospheric errors, receiver malfunction, or even errors that originate directly from the satellite.
It all depends on how your receiver deals with the errors. Luckily, with the help of modern technology, common surveying mistakes rarely happen. These have the capacity to work accurately in urban canyons, rural areas obstructed by canopy, and even close to tall buildings. Of course, this was not the case some 10 years ago when GNSS surveying technology was much more modest.
Unfortunately, whether it was 10 years ago, today, or even some time in the future, the human element is the one causing the most errors in GNSS surveys. To avoid mistakes in GNSS surveying, people have to be prepared and understand how things work in order to avoid any GNSS mishaps that could happen during surveys. Here are a few situations and solutions that you might be facing in the field.
Multipath Errors
Multipath errors happen when a satellite’s signal is reflected off a certain surface before being able to reach the receiver. The surfaces in question include cars, buildings, or even water. In fact, the interference can be made with any type of reflective surface. The bent signal that a receiver picks up is often skewed, compromising the accuracy of the measurement. Multipath errors in surveying usually happen in urban areas and are less likely to occur in rural ones.
To avoid multipath problems, it would be best for you to place your base away from any significant metal structures. Also, it would be wise to try and elevate your receiver high off the ground as much as possible. You can also use a good GNSS antenna with a built-in ground plane. It can help to block low-angle reflected signals from the ground.
Level Bubble Out of Plumb
If your level bubble comes out of plumb, it can hamper the results of your GNSS surveying measurements. There are actually many reasons for this to happen, including vibration, or both cooling and heating from the environment. Either way, it can lead to mistakes in surveying that you would want to avoid.
One of the ways to prevent this from happening is regular maintenance. Periodic checks can prevent mistakes in GNSS surveying equipment from happening. You also need to know how to calibrate the level bubble in the field. The best way to do it is to find a vertical structure to determine if the level bubble is out of plumb. Remove the receiver and place the GNSS pole two inches from the structure. Rotate the pole 180 degrees and then check the bubble. If it is out of plumb, you are going to need to recalibrate it.
Too Long of a Baseline
The baseline represents the distance between the base station and the rover. The greater the distance, the more inaccurate it becomes. This happens when the base and the rover are too far away from each other, so they start to experience different atmospheric and ionospheric conditions.
One way to deal with this problem and avoid mistakes in GNSS surveying data is not relying on CORS but setting up a local base station on your site.
Not Performing Localization
During a survey, when comparing your data to a previously collected dataset, but without knowing the parameters it was measured, you risk your measured data not accurately aligning with the known data. This is more common while working on a subdivision construction site or on a mining job site.
Therefore, when you need your data to align with a previously surveyed site, you need to perform localization, like calibration or local transformation. Ideally, you would need four known points measured in your known coordinate system for proper localization. It is the only way to avoid mistakes in GNSS surveying accuracy.
Unit Mismatch
One of the common GNSS surveying mistakes is dealing with unit mismatch. Projected coordinate systems use linear measurement units, which are usually displayed in either meters or feet. Surveyors often make a mistake when they select a coordinate system that uses meters, while the project requires the use of feet. The mismatch will mean that your data is not going to align correctly with your previous survey work.
The most obvious way to avoid this is double-checking everything, especially the selected unit,s even if the coordinate system name includes “ftUS.” Luckily, there is a way for you to easily change the units even if you are in the filed after just realizing that a mistake has been made.
No Documentation of the Setup
If you decide to use ad hoc markers instead of targets or nails, you should document the exact location where you measured. There might come a situation where another survey professional comes along and misinterprets the point.
To avoid this from happening, you should provide the information of the specific location in the dataset’s metadata. To speed things up, you can take photos with your phone and reference those photos when inputting the documentation back in the office.
Miscalculated Height of the Device
Sometimes when entering the receiver’s height, surveyors might easily measure the wrong span. This is because different manufacturers might need measurements from other places on the instrument. For that reason, you should check the manufacturer of the receiver to determine the span they want you to measure. Different poles usually have different scales, while the antenna height will most definitely differ from the receiver height.
Keeping Ellipsoids and Geoids Straight
People who are unfamiliar with elevation reference or vertical datums often make mistakes in GNSS surveying. Top GNSS receivers measure the height above the ellipsoid, but this does not reflect the terrain in the real world, nor does it include gravity variations.
Therefore, for practical use, we need orthometric height, which is the elevation seen on road signs at mountain passes. As GNSS gives height over the ellipsoid, you need to assign a geoid to get orthometric height. This is because it also includes sea level and accounts for terrain variations. So, you basically have to pick the correct geoid for your project to get precise measurements.
How to Avoid Mistakes for More Reliable GNSS Surveys?
While we covered almost all of the main challenges of GNSS surveying, there are still some that could arise and give you problems in the field. The best way to deal with them is to come prepared with awareness of potential issues is key to solving them and collecting precise data for your project.
Also, make sure to use reliable software, follow best practices, double-check your setup before surveying, and take care of your surveying equipment properly. By doing all of this, you can minimize errors and avoid making mistakes in GNSS surveying.
FAQ
To understand the basic mistakes in GNSS surveying better, check out these added FAQs below.
What are the most common mistakes in GNSS surveying?
The most frequent mistakes include multipath errors (signal reflections from surfaces), equipment not being level, using too long of a baseline between base and rover, not performing localization, unit mismatches between coordinate systems, lack of documentation, miscalculated device height, and confusion between ellipsoids and geoids for elevation data.
What is a multipath error in GNSS surveying and how can it be avoided?
Multipath errors occur when satellite signals reflect off surfaces like buildings, cars, or water before reaching the receiver, causing inaccurate measurements. To minimize multipath, place your base station away from metal structures, elevate the receiver, and use a GNSS antenna with a ground plane to block low-angle reflected signals.
How can I minimize human errors and ensure reliable GNSS survey results?
Prepare thoroughly by understanding potential errors, using reliable equipment and software, following best practices, double-checking your setup, and maintaining your equipment. Regular calibration and awareness of field conditions will help you collect precise and reliable data.
What are the weaknesses of GNSS?
GNSS has several weaknesses, including vulnerability to signal blockage or reflection (multipath errors) from buildings or trees, reduced accuracy in poor atmospheric conditions, weak signals indoors or under dense cover, and susceptibility to human errors in setup and calibration. GNSS signals can also be affected by intentional interference (jamming or spoofing) and require a clear line of sight to multiple satellites for reliable results.
