When you're preparing for a drone survey, you hear this a lot these days: "With an RTK drone, you don't need GCPs anymore." It's a tempting claim. Anyone who knows the field understands just how cumbersome it is to install, survey, and maintain Ground Control Points (GCPs) one by one across wide, rugged sites. But can RTK alone really replace GCPs entirely?
Before we get into the main discussion, let's first clarify the three core concepts used for position correction. The methods of 'position correction'—aligning the coordinates of drone imagery with the actual terrain—fall broadly into the following.
A rough comparison of accuracy looks like this. Judging by the numbers alone, RTK appears the most accurate. And under ideal conditions with a perfect signal—that is, when RTK holds a 'Fix' state—that is indeed true. What deserves attention here, however, is the premise of 'ideal conditions.'
Let's first pin down why RTK is drawing so much attention. Whether you look at time, cost, or safety, the benefits RTK offers are unmistakable. The problem reveals itself when you take one step further from here.
RTK's high accuracy presupposes perfect 'signal conditions.' But signal conditions in the field can never be the same every time. Because of this, operating with RTK alone produces the following blind spots.
Meissa, too, actively uses RTK technology in the field and fully supports the related data processing. But as the party responsible for field data, rather than simply choosing between 'RTK or GCP,' we recommend a combination that gains efficiency with RTK and verifies those results with a small number of GCPs. The benefits of adopting both correction methods together are as follows.
The important thing here is that this does not mean installing GCPs densely across the entire site as before. Combining efficiency-driven RTK operation with a minimal number of verification reference points—this is closer to the practical answer that captures both accuracy and reliability at once.
Accuracy is a concept that includes 'verification.' It is hard to prove with a single number. Unverified accuracy is, strictly speaking, closer to an expectation than to accuracy. The reason Meissa recommends using RTK and GCP together is simple: to propose only data that can truly be trusted in the field—data the person in charge can rely on to make decisions. Because what works in words and what works in the field are different.
If you're wondering which capture and correction method best fits your current site conditions, please feel free to reach out anytime.
Q. How is the PPK method mentioned in the introduction used in the field? How does it differ from RTK?
A. Because PPK (post-processing correction) does not rely on a real-time communication network, it becomes an excellent alternative to RTK on mountainous terrain or coastal sites with severe signal shadowing. Even if real-time communication is cut off, precision can be improved by correcting the data after capture. However, PPK too is merely a technique for precisely capturing the drone's flight trajectory—it is not an independent reference for 'verifying' whether the output matches the actual ground surface. So you should choose whichever of RTK or PPK is advantageous depending on the site's communication conditions, but the basic principle of combining a small number of GCPs to verify the final data applies just the same.
Q. If we set up our own base station on site to stabilize the signal, wouldn't perfect surveying be possible even without GCPs?
A. Installing your own base station greatly improves communication latency and dropout issues compared with network RTK. However, it is difficult to completely block the 'multipath error' phenomenon that occurs when radio waves reflect off cut slopes or tall steel structures. Even if the device shows a 'Fix' reception state, there is a possibility of slight deviation from the actual terrain coordinates, so a minimal number of verification GCPs are still needed to prove the integrity of the data.
Q. What exactly does the 'small number of GCPs' Meissa recommends mean in terms of layout?
A. It does not mean you have to cover the entire site densely in a 200–300 m grid pattern as in the past. Since RTK (or PPK) does an excellent job of forming the skeleton of the overall terrain data, it is enough to place just 3–4 perimeter points surrounding the site boundary and 1–2 checkpoints in the central area with large terrain changes or at key work zones. It's an efficient setup that obtains reliable data while cutting the labor of installing reference points by more than 80% compared with before.
Q. We plan to fly a drone regularly to calculate earthwork volume for progress-payment inspection. Do we have to install and survey GCPs anew each time?
A. No. You only need to install a small number of GCPs well, just once, as permanent points (or markers that can be preserved long-term) on firm ground at the site perimeter or on structures that won't be damaged during construction. If you align the data against these fixed GCPs every capture run, the time-series data always accumulates on the same axis even when the weather or communication conditions change. This lets you minimize errors and secure a highly consistent, accurate basis for earthwork-volume calculation to submit to the client or supervisory team. If an installed GCP is lost or needs to be re-installed and set up, please contact Meissa and we'll help with the setup.
