Choosing the Right Positioning Method: RTK, PPK, or GCP?

As a professional surveyor, you have stumbled upon terms such as RTK, PPK, and GCP and know exactly what they are. But if you are new to the field of land surveying, you are very likely confused about what they mean. The thing is, when you are using a drone to create a map, there are different kinds of options available to you to achieve different levels of positional accuracy. Positional accuracy refers to how precisely a mapped location matches its true position on the Earth’s surface.

In surveying and especially drone mapping, the accuracy and the results that you are creating will largely depend on the positioning technology you are using. Standard GNSS is enough to provide meter-level accuracy (±1-3m), and drones often come with standard GNSS receivers built into them. 

But with RTK, PPK, and GCP also available, inexperienced surveyors are left asking themselves which one is better to use. This is what we will be answering today as we help you to understand how to choose the right positioning method. 

RTK (Real-Time Kinematics)

Real-Time Kinematics, or RTK as some of us prefer to call it, is a correction technique that helps to improve the accuracy of GPS positioning systems. You will commonly see it used in drone mapping and various other operations. The way to use it is through a GNSS base station that helps to establish a known reference point.

In that situation, the base station will send correction data in real time to mobile GPS receivers or rovers. This way, error correction is achieved through centimeter-level positional accuracy, with it being at around ±1 to 2 cm. Correction data is usually provided via a UHF signal from satellite, or the internet when using NTRIP. Unfortunately, it does come with some limitations as it always requires a stable RTK correction signal, and if the connection is lost during a survey, accuracy may revert back to to standard GNSS levels.

PPK (Post-Processed Kinematics)

Post-Processed Kinematics, PPK for short, is a correction technique that you will usually experience being used on the payload side. This is where the raw satellite data is logged while the drone is flying and processed after the mission to compute precise positions. PPK enables centimeter-level accuracy by decoupling correction from real-time constraints. 

PPK is also able to perform in areas where RTK cannot be used or is unavailable. For that reason, it is ideal for both mapping and surveying, but also for several other applications that require high precision. 

Its accuracy is measured at ±1 to 2 cm, which is the same as RTK, with the difference that this method is more reliable. It is even able to handle GNSS errors much better. The downside is that it can’t be done in real time and is a process that requires more time after the flight.

GCPs (Ground Control Points)

Ground Control Points, or GCP, is something that is very much used in photogrammetry besides surveying. They present physical markers that are placed in various locations within the mapping area. The precise coordinates are measured via very accurate GNSS equipment and a base station are later imputed into a photogrammetry software that can then improve the geometric accuracy of the map or a 3D model.

Its accuracy is measured at ±1 to 3 cm, but it can actually depend on the GCP survey and the number of points that the surveyor uses. The worst part when using this method is that it is time-consuming. The more GCPs you have, the better the accuracy, but it also means added work for each one of them. 

When to Use RTK, PPK, or GCPs?

It is more than logical for you to doubt certain techniques and try to figure out which one will provide you with the results you need. For instance, RTK is a correction technique that is useful for inspection work as it can improve the accuracy in real time. Compared to PPK, it is a correction technique that is applied to the data itself. This means that as long as you are able to collect GPS data and are able to collect GNSS observations from a fixed base station, you will also be able to post-process your GPS data to increase accuracy. 

GCPs are mostly beneficial for small to mid-sized surveys, where the deployment can be done fast. They provide ground-truth data to anchor aerial imagery during photogrammetric processing. Unfortunately, its practicality dips when deployed for large-scale surveys. Placing and measuring more points can at one point become expensive and time-consuming, which can be particularly rough when working on difficult terrain. 

Generally speaking, if you are planning on doing some localized projects, GCP is the method that you would want to choose. All three of them, RTK, PPK, and GCP, offer advantages that can be effective depending on the hardware that you possess. More so, many surveys believe that combining methods, like pairing RTK and GCP, is what produces the best results. 

The key element of picking correctly is understanding each method and knowing how to properly use the given technique. That way, you can optimize your data quality and workflow effectively for the most accurate results. 

Pros and Cons of Each Method

FAQs

Check out some of the most asked questions about the use of RTK, PPK, and GCPs.

What is the main difference between RTK, PPK, and GCPs?

RTK (Real-Time Kinematics) and PPK (Post-Processed Kinematics) are both GNSS correction techniques that achieve centimeter-level accuracy. The key difference is when the correction is applied. RTK corrections are sent in real-time from a base station to a drone, while PPK involves collecting raw data during the flight and processing it after the mission. GCPs (Ground Control Points) are physical markers placed on the ground with known coordinates, used to anchor and improve the geometric accuracy of a map or 3D model during photogrammetric processing.

Which method provides the highest accuracy?

All three methods, RTK, PPK, and GCPs, can achieve centimeter-level accuracy, typically in the range of ±1 to 3 cm. The final accuracy depends on several factors, including the quality of the equipment, signal stability, the number and placement of GCPs, and the specific workflow. In many cases, combining methods, such as using RTK with a few GCPs, can produce the most reliable and accurate results.

When should I choose RTK over PPK?

Choose RTK when you require real-time accuracy and have a clear, stable connection between the base station and the drone. This is particularly useful for inspection work or projects where immediate, precise positioning is critical. If your project involves large areas with potential for signal obstructions or if a stable base station connection is not feasible, PPK is often the more reliable choice as it is not dependent on a real-time signal.

Can I use a combination of these methods?

Yes, and in many professional surveying applications, combining methods is considered a best practice. For example, a surveyor might use an RTK drone for the primary data collection and then place a few GCPs to serve as checkpoints. This combination leverages the efficiency of RTK while using the GCPs to independently verify and potentially improve the overall accuracy and quality of the final deliverable.