Real-time kinematic (RTK) technology has been used in terrestrial surveying for years to improve the accuracy of Global Navigation Satellite System data and provide high relative accuracy. However, in order to achieve centimeter precision in absolute accuracy when flying with RTK, greater care is needed to achieve this precision.
What is Relative and Absolute Accuracy?
Relative Accuracy - This is the accuracy when comparing two points in the same dataset to each other, and then in the real world. For example if you were to measure the carriage way width of a road from one kerb to another in the model and then carryout the same measurement in the real world, then the difference in measurements taken in the real world and in the digital model would be low (to with a couple of cm).
Absolute Accuracy - This is when the digital model coordinates are compared to the real world coordinates of the same feature relative to a specific Coordinate Reference System (CRS / EPSG).
It is important to remember that whilst RTK provides high relative accuracy, it does not automatically guarantee high absolute accuracy.
RTK – Relative Accuracy.
Using the Phantom 4 Pro RTK drone and D-RTK 2 base station as an example, the base station is setup prior to the drone being flown. The base station will provide real time corrections to the drone as it is flying. Therefore the drone should never loose connection with the base station, otherwise the high accuracy corrections will be lost.
However, depending on how the base station has been setup will determine if you have both high absolute and relative accuracy, or only high relative accuracy.
If the base station is setup without manually entering a coordinate for the base station (or using an NTRIP provider) then this is known as Single Point acquisition and the operator will not obtain high absolute accuracy.
RTK - High Absolute Accuracy
When the base station is being setup you have the option of manually entering the coordinate of the base station. As the DJI D-RTK 2 base station is not suitable to calculate a known point accurately, then the known point that the base station is to be set up on needs to be established using survey grade equipment.
There is also the option to utilise an NTRIP provider in order to obtain RTK corrections without the need of a base station using services such as Trimble VRS Now.
Using either the manually entered coordinate or NTRIP options will ensure that you have both high relative and absolute accuracy in the processed data.
If you're proposing to use the D-RTK 2 base station, without establishing a known point or use NTRIP, then you will not gain high absolute accuracy, and therefore is not suitable for operations which require high absolute accuracy.