In the world of drone mapping, users often encounter discrepancies between drone photo coordinates and RTK coordinates. These differences can be confusing, especially for beginners venturing into drone photogrammetry, drone surveying, or mapping with GCP and RTK. This article provides a comprehensive overview of the causes of discrepancies between drone photo coordinates and RTK, and offers solutions to align data accurately.
What Are Drone Photo Coordinates?
Drone photo coordinates are positional data directly recorded from the GPS embedded in the drone during image capture. Typically, this data includes:
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Latitude (Northing)
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Longitude (Easting)
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Altitude (Height)
However, it is important to note that drone photo coordinate accuracy without RTK typically ranges from 3–10 meters. This explains why drone photo coordinates often deviate compared to RTK coordinates.
What Is RTK Coordinate?
RTK (Real Time Kinematic) coordinates are a high-precision positioning system that can achieve centimeter-level accuracy. RTK is commonly used in:
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Topographic surveying
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GCP (Ground Control Point) measurement
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High-precision mapping
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Construction and infrastructure projects
Due to its high accuracy, RTK coordinates are often used as the primary reference in professional drone mapping.
Why Do Drone Photo Coordinates and RTK Differ?
Here are some main reasons for differences between drone photo coordinates and RTK coordinates:
1. Lower GPS Accuracy on the Drone
The drone’s built-in GPS without RTK only offers a few meters of accuracy. In contrast, RTK provides centimeter-level accuracy. This difference most frequently causes mismatched data.
2. Elevation (Altitude) Discrepancies
Many cases involve drone photo altitude at 100 meters, while GCP RTK altitude at 500 meters. This is not a data error but a difference in height reference between:
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Sea level (MSL)
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Drone’s relative height above ground
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Geoid vs Ellipsoid
3. Different Coordinate Systems
If drone photos use WGS84 but GCP RTK uses UTM, positions will inevitably differ. This error is common among beginners.
4. No GCP Used During Processing
Without Ground Control Point (GCP), software like Agisoft Metashape relies solely on drone GPS. Consequently, the model lacks precision.
5. Alignment Process Errors
Jika align photo drone tidak diikuti dengan optimize camera menggunakan GCP, maka data drone dan RTK tidak akan menyatu.
How to Resolve Coordinate Discrepancies Between Drone Photos and RTK
To ensure drone photos and RTK coordinates can align, the following recommended solutions apply:
1. Standardize Coordinate Systems (CRS)
Ensure all data uses the same CRS, for example:
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EPSG 32749 (UTM Zone 49S)
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WGS84 if required
2. Use GCP from RTK
Incorporate GCP obtained from RTK measurements into photogrammetry software such as Agisoft Metashape.
3. Perform Align Photos
Use the Align Photos feature so that drone images form an initial model.
4. Optimize Camera
Navigate to Optimize Camera Alignment and enable:
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Fit f
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Fit k1 k2 k3
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Fit cx cy
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Fit p1 p2
5. Ignore Drone Altitude If Needed
If the drone altitude deviates significantly, we can:
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Reduce camera Z accuracy
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Rely on GCP as the primary reference
Conclusion
Drone photo coordinates differing from RTK coordinates is not a mistake, but rather a difference in accuracy levels and measurement methods. With the use of GCP, a unified coordinate system, and optimize camera, drone photo data and RTK can be merged into a single accurate model.
For those working in drone surveying, drone mapping, or photogrammetry, understanding the differences between drone coordinates and RTK is a crucial step toward producing precise and professional data.
With the right methods, drone photos and RTK coordinates can definitely align and merge.
