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Common Challenges in GPS Surveying

In this article, we are going to look at what the limitations of GPS surveying are, and how you can tackle those issues in the field. Over the next several weeks, we are going use our blog platform to do some deep dives on GPS technology and other surveying topics. Learn about how GNSS has changed surveying, what limitations you should be aware of, and some tips on how to overcome those limitations! 

Introduction

Urban Canopy: Challenges Near Buildings

Urban Canopy: Tips and Tricks

Challenges of Tree Canopy

Foliage Tips and Tricks

General Limitations of Surveying

Conclusion

FAQs

Introduction

GPS surveying has revolutionized the way we measure and map the world around us. Whether it’s for construction, land development, or scientific research, GPS surveying provides precise and reliable positioning data at speeds once thought impossible. Entire industries have been revolutionized, with surveying crews shrinking in size, and the efficiency of individual surveyors being doubled it not tripled.  

However, despite its advanced capabilities, GPS surveying is not without its challenges. This blog post explores some common issues faced by surveying professionals, especially when working near buildings and trees, and delves into the general limitations of GPS technology. Understanding these challenges and knowing how to mitigate them is crucial for achieving accurate results. Here at Bench-Mark we are dedicated to providing high-quality surveying equipment that addresses these challenges and ensures optimal performance in various environments.

Urban Canopy: Challenges Near Buildings

Surveying in urban environments presents a unique set of challenges. The presence of buildings makes surveying and the deployment of GNSS technology very difficult. Buildings can cause significant issues for GPS surveying, primarily through signal obstruction and multipath errors. Unfortunately, not all buildings are the same shape, or made of the same material. This means that the challenges experienced in these environments are never the same or entirely predictable.  

Signal Obstruction and Multipath Errors

The most obvious challenge that buildings present is that they can block GPS signals, preventing them from reaching the receiver. This obstruction can lead to loss of satellite lock, resulting in incomplete or inaccurate data. 

Multipath errors occur when GPS signals reflect off surfaces like buildings before reaching the receiver. These reflected signals travel a longer path, causing errors in the calculated position. The material of the building will have a large effect on this. For instance, buildings that have exteriors that are largely glass or metal will cause a greater number of reflections.  

Examples of Interference

Imagine a surveyor working in a dense cityscape. Tall buildings can create “urban canyons” where GPS signals are either blocked or reflected multiple times. This can cause the receiver to record incorrect positions, sometimes by several meters, making it challenging to achieve the precision required for professional surveying tasks. See the image below for an idea of what is happening to the satellite signals as they encounter these buildings. 

Example of signals in an urban canopy. The receiver has a difficult time distinguishing which signals to filter out, and which to include when the signals are reflected. On top of that critical information is lost as signals are obstructed by buildings.  

Urban Canopy: Tips and Tricks

Before beginning any survey in an urban environment, the above limitations must be considered. However, you are not completely out of luck. There are several strategies that can be used to mitigate and overcome these effects.

Positioning

Whenever possible, place the receiver in open areas, away from tall buildings. When positioning in open areas is not possible, aim to find locations with minimal obstructions directly overhead. For users in the Northern Hemisphere, users will want to limit the obstructions in the Southern portion of the sky. The further North you are, the more helpful this will become. You can learn more about satellite orbits here: Satellite Characteristics: Orbits and Swaths (NRCAN).

Timing

Although significantly less important with modern 7th generation RTK receivers, the time of day can have an effect on RTK performance. It is best practice to conduct surveys during times when satellite visibility is highest. This timing maximizes the number of visible satellites and improves positioning accuracy. There are several free applications that can help you do this, personally I prefer GNSS Viewer on iOS. 

Example of a live GNSS viewer satellite scope. This includes all satellites in the area.

Antenna Selection 

Use high-quality, multi-frequency antennas that can minimize the effects of multipath errors. Receivers that are capable of receiving GPS, Glonass, Galileo, and Beidou satellites will give the best performance. However, you will also want to consider what signals the receiver can receive. Those that can receive the latest signals will give significantly better performance. For a better idea of what the difference between even 6th and 7th generation technology will mean in these conditions check out the below video.

RTK and Positioning Engine

As with most technology, RTK technology has gotten better and better over time. The leap from 4th to 7th generation RTK technology is enormous. Environments where 3rd generation receivers, units like the Topcon Hiper II and Hemisphere S320 would not be able to obtain a position, newer units like the 7th generation Hemisphere S631 can easily obtain a fixed, and repeatable position. 

However, not all manufacturers have created equally capable receivers. The individual RTK engines, or the algorithms that solve the RTK equations are different between manufacturers. Between 7th generation receivers, like the Hemisphere S631, Trimble R12 and Leica GS18 are all going to perform different in these conditions. These algorithms are designed to mitigate the effects of multipath and signal obstruction and still give you the best performance. See the below videos for an idea of how the different receivers perform.

Importance of RTK GPS in Urban Environments

Real-Time Kinematic (RTK) GPS is particularly useful in urban settings. RTK GPS uses a base station to provide real-time corrections to the GPS data, significantly improving accuracy. By incorporating RTK GPS, surveyors can reduce the impact of multipath errors and signal obstruction, achieving centimeter-level precision even in challenging urban environments. With the right receiver, surveyors can begin to leave their total stations in the truck, and work faster then ever. 

Challenges of Tree Canopy

Surveying in forested or tree-dense areas can be equally challenging due to the natural obstruction of GPS signals by foliage. Challenges are very similar to those in urban environments, with the added caveat that the entire sky may be partially obscured. 

Signal Attenuation and Blockage by Foliage

Trees and other vegetation can block or weaken GPS signals, a phenomenon known as signal attenuation. The denser the foliage, the more significant the attenuation, leading to potential loss of satellite lock and reduced accuracy. 

With the introduction of the latest Block III GPS and Beidou-3 satellites, the latest signals are designed to help mitigate these effects. The latest L5 type signals are specifically designed to better penetrate canopy and are more powerful to reduce signal loss. 

Impact of Seasonal Changes

Seasonal changes can also affect GPS accuracy in wooded areas. Where it was possible to obtain a fix in the winter, may be impossible in the summer. As foliage coverage increases over summer months, the ability to reliably obtain fixed solutions is going to become more difficult. With the latest RTK technology, this may mean in the winter a fixed position can be obtained in a couple of seconds, but in the same location during the summer months, time to obtain a fix could increase to several minutes.   

A surveyor in heavy tree cover. Older generations of receivers would have struggled in this type of terrain, necessitating the need for a total station. 

Foliage Tips and Tricks

Foliage can cause a lot of issues when attempting to survey. Fortunately, there are several ways to give yourself the best odds of achieving a fixed result. 

Clearing Lines of Sight

When possible, clear small areas to improve satellite visibility. This can involve trimming branches or removing small trees that obstruct the line of sight to the satellites. Anything to reduce the number of obstructions between the sky and the receiver will improve results. 

Using Elevated Antennas

Place antennas on poles or tripods to reduce the impact of ground-level obstructions. As you increase the elevation of the receiver, you will receive two benefits. The first is that you may be able to clear some of the lower lying obstructions. Additionally, you may be able to pick up some of the receivers that are lower on the horizon, adding to the data your receiver can use to obtain a fixed solution.

Base Station LocationThe most important factor in your setup that will give you the best possible performance is the location of your base. By ensuring that your base is in a location that is completely open to the sky, with little to no obstructions, you will get the best performance at the rover. In an RTK solution, the rover can only use the information the base is providing. Providing the best possible information to the rover is the best way to get the best performance.

General Limitations of GPS Surveying

Beyond specific challenges near buildings and trees, GPS surveying has inherent limitations that can affect its accuracy and reliability.

Factors Affecting GPS Accuracy

Several factors can impact the accuracy of GPS measurements, including:

Atmospheric Conditions: Variations in the ionosphere and troposphere can delay GPS signals, causing errors. RTK engines attempt to correct for these conditions by modeling ionosphere. 

Satellite Geometry: The relative positions of satellites (known as satellite geometry) can influence the precision of GPS data. Poor geometry can lead to higher positional errors.

Overview of GPS Signal Errors

GPS signals are subject to various errors, including:

Ionospheric and Tropospheric Delays: Changes in the Earth’s atmosphere can slow down GPS signals, leading to inaccuracies.

Clock Errors: Minor discrepancies in the timing of signals between the satellite and receiver can result in positioning errors.

For more information on these factors, check out our article: How Does A GNSS Receiver Work?

Conclusion

GPS surveying is a powerful tool for achieving precise positioning, but it comes with its share of challenges. Working near buildings and trees can introduce significant errors due to signal obstruction and attenuation. Additionally, inherent limitations of GPS technology, such as atmospheric delays and satellite geometry, can impact accuracy. By understanding these challenges and utilizing advanced techniques like RTK GPS and RTK GNSS, surveyors can improve the reliability and precision of their measurements. Investing in high-quality surveying equipment that addresses these issues is crucial for success in the field. Explore our range of surveying equipment designed to help you overcome these challenges and achieve accurate results every time.

Frequently Asked Questions (FAQs)

What is the difference between RTK GPS and Using Your Phone?

RTK (Real-Time Kinematic) GPS provides real-time corrections to GPS data, significantly improving accuracy to centimeter-level precision. Traditional GPS, without RTK, relies on single-point positioning, which can result in errors ranging from a few meters to several meters. RTK GPS is particularly useful in environments with significant signal obstruction or multi-path errors, such as urban areas or wooded regions.

How does RTK GNSS improve surveying accuracy in challenging environments?

RTK GNSS (Global Navigation Satellite System) uses corrections from multiple satellite constellations (such as GPS, GLONASS, Galileo, and BeiDou) to enhance accuracy and reliability. This multi-constellation approach helps surveyors achieve more precise positioning even in challenging environments where signal blockage and attenuation are common, such as near buildings or under dense foliage.

What steps can I take to minimize GPS errors when surveying near buildings?

– To minimize GPS errors near buildings, consider the following steps:
– Place the receiver in open areas, when possible, to avoid signal blockage.
– Conduct surveys during times when satellite visibility is highest, such as early morning or late afternoon.
– Use high-quality, multi-frequency antennas to reduce multipath errors.
– Implement advanced correction techniques like RTK GPS to improve accuracy.

Can GPS surveying be effective in heavily wooded areas?

Yes, GPS surveying can be effective in heavily wooded areas, but it requires specific strategies to mitigate signal attenuation and blockage by foliage. Using elevated antennas, conducting surveys during seasons with less foliage, and employing RTK GNSS technology can significantly improve accuracy in wooded environments. Additionally, clearing small areas to enhance satellite visibility and using multi-path resistant equipment can further reduce errors.

Bench Mark Equipment & Supplies is your team to trust with all your surveying equipment. We have been providing high-quality surveying equipment to land surveyors, engineers, construction, airborne and resource professionals since 2002. This helps establish ourselves as the go-to team in Calgary, Canada, and the USA. Plus, we provide a wide selection of equipment, including global navigation satellite systems, RTK GPS equipment, GNSS receivers, and more. We strive to provide the highest level of customer care and service for everyone. To speak to one of our team today, call us at +1 (888) 286-3204 or email us at [email protected]

About the Author

Nolan
Nolan has been working in the surveying field since 2017, starting as a part-time student at Bench-Mark while attending the University of Calgary. He now works in technical support and sales helping customers find the right product for them.

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