Background
The metadata of an image captured by a drone and its sensor can tell you a lot about the Real-Time Kinematic (RTK) corrections received by the drone in order to improve the overall accuracy of the drone data. This information is stored in the metadata of the image and online tools can be used to check individual images.
This article will explain some of the metadata entries so that you can troubleshoot flights that may have encountered RTK issues.
For DJI equipment, if the flight has been flown with an RTK enabled drone then you will typically have the following entries in the metadata:
rtk_flag
rtk_std_lon
rtk_std_lat
rtk_std_hgt
For the newer Mavic 3 Enterprise family (M3E/M3T/M3M) the metadata also records the following (if an NTRIP provider is used):
rtk_diff_age
ntrip_mount_point
ntrip_port
ntrip_host
Here is an example of the metadata from an M3E drone image:
How does this relate to the accuracy of the flight data captured?
The RTK information in the metadata of a drone image plays a crucial role in determining both the absolute and relative accuracy of the drone's geospatial data. Here’s how they relate:
Absolute Accuracy
Absolute accuracy refers to how closely the drone's positioning aligns with real-world coordinates. RTK data directly influences this by:
RTK Corrections: When RTK corrections are applied (indicated by flags like 34 or 50), the positioning is typically accurate to within a few metres to centimetres. This enhances the absolute accuracy of the GPS coordinates associated with the image.
Standard Deviation Metrics: Values like
rtk_std_lonandrtk_std_latprovide insights into the precision of these measurements. Lower standard deviation values indicate higher confidence in the absolute position.
Relative Accuracy
Relative accuracy refers to the precision of positioning in relation to other points or images taken during the same flight. RTK information affects this by:
Consistency of Corrections: RTK solutions ensure that all points captured during a flight maintain consistent accuracy levels. This is essential for tasks like creating detailed 3D models or maps, where relative positioning is crucial.
Fixed vs. Float Solutions: A fixed RTK solution (e.g., indicated by an
rtk_flagvalue of 50) provides higher relative accuracy compared to a float solution (rtk_flagof 34 or 16). Using fixed solutions helps maintain reliable relative positioning among multiple images.
Further information on absolute and relative accuracy can be read here.
What is rtk_flag ?
The RTK flag in drone image metadata indicates whether the RTK correction data was used to enhance the accuracy of the GPS coordinates associated with the image. This flag helps in determining the precision of the geotagging information for the images captured by the drone. Here are the typical values for DJI drones and their meanings:
0 - No Positioning: Failed to position. No GPS correction provided to the drone. Accuracy is very low in the region of tens of metres.
16 - Single-point Positioning Mode: Low accuracy, meter-level
accuracy.
34~49 - RTK Float Solution: Floating point solution positioning, therefore
decimetre-level to meter-level accuracy, the accuracy will usually depend on the base station age (see below).
50 - RTK Fixed: High accuracy, fixed solution positioning (centimetre-
level accuracy).
What are the rtk_std entries?
These entries refer to the standard deviation of the longitude, latitude and height/altitude coordinate in the RTK positioning system. This value gives an indication of the precision of the RTK-corrected measurement.
Here's a breakdown of what each part means:
rtk: A GPS correction technology that improves the accuracy of position data.
Standard Deviation (std): A statistical measure that quantifies the amount of variation or dispersion from the mean. In this context, it indicates the variability or uncertainty in the RTK measurement.
lon/lat/hgt: The geographical coordinate that specifies the east-west / north-south and height position of a point on the Earth's surface.
For the Mavic 3 Drone Family.
What is the RTK base station age (rtk_diff_age) ?
The age of an RTK base station typically refers to the time elapsed since the last correction message was received from the base station. This "age of differential" value is an important parameter in RTK systems because it affects the accuracy and reliability of the position corrections being used by the rover receiver or drone.
RTK systems work by using correction data from a base station to improve the positional accuracy of the rover. The base station can be a physical base station set out on site by the surveyor/pilot or a virtual reference station (VRS) accessed through NTRIP. The base station sends out correction messages at regular intervals. If the drone does not receive these corrections in a timely manner, the "age of differential" increases, indicating that the correction data is becoming stale, which can degrade the accuracy of the positioning solution.
In summary, the "RTK base station age" is essentially the latency or the delay in the correction data being used by the drone. For optimal performance, this age should be as low as possible, ideally less than a few seconds. The base station age, or rtk_diff_age can be viewed in the metadata of the image.
In the image at the start of this article, the rtk_diff_age shows a low latency with an age of 0.4 seconds. This results in a high absolute accuracy and an rtk_flag value of 50.
Below is an example where the corrections are >80 seconds and this has resulted in the rtk_flag being 34.
When there is no base station correction, we can see an rtk_flag of 16 or no rtk_flag entry in the metadata, and rtk_std values will be >1m. As there was no base station connected to the drone in this example, then the rtk_diff_age is 0.
What are the ntrip entries?
The ntrip entries relate to the connection used between the drone and the ntrip caster. These being the host url, connection port and the mount point from which the corrections were provided from. This information is specific to each NTRIP provider and more information can be obtained from your correction service provider.
