Targeted Operations
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      Mapping and spraying accuracy - PIX4Dfields

      This article explains the concepts related to accuracy and RTK corrections to obtain the best application results.

      IN THIS ARTICLE

      Introduction
      What is accuracy?
      What is Real-Time Kinematic (RTK)?
      Types of RTK corrections
      Importance of NTRIP

      Introduction

      In PIX4Dfields, when exporting Targeted Operations maps to be used by spraying drones or farm machinery, a message pops up:

      GPS RTK NTRIP usage reminder

      Please ensure you use the same GPS RTK NTRIP network for the mapping drone and the spraying drone, sprayer, or tractor. Otherwise, large location errors are possible. The same applies if no RTK network is used. It is the user’s responsibility to monitor autonomous machinery at all times and to be able to manually intervene to prevent collisions.

      It is important to read this article carefully to avoid any damage to your application machinery.

      What is accuracy? 

      When processing drone imagery, the accuracy of the outputs can be distinguished into:
      • Relative accuracy of a photogrammetric project is defined by comparing individual features on a map / reconstructed model / orthomosaic with other features on the same model. For example, the relative accuracy of the model is high if two points have the same distance between them in the model and reality.
      • Absolute accuracy of a photogrammetric project is defined by the difference between the location of features on a map / reconstructed model / orthomosaic and their true position in a certain reference frame. For example, if the position of a tree in the reconstructed model is close to its actual position on the Earth, then the absolute accuracy is high. Using RTK can greatly improve absolute accuracy.
        For more information:  What is the relative and absolute accuracy of drone mapping?

      What is Real-Time Kinematic (RTK)?

      RTK helps improve absolute accuracy. RTK is a satellite navigation technique used to enhance the precision of position data derived from GNSS (Global Navigation Satellite System) receivers. Standard GNSS systems, such as GPS, can typically achieve accuracy within several meters. However, RTK reduces this to centimeter-level accuracy by correcting the satellite data in real time. RTK can significantly enhance drone operations' accuracy, ensuring that mapping drones and application machinery are synchronized for optimal results.

      Types of RTK corrections

      Modern drones available in the market, such as the Mavic 3 Multispectral, are now equipped with an RTK antenna for improved accuracy and precision. To leverage this technology, two types of corrections are available: 

      • RTK base station: The base station is placed at a known, fixed location, and the error in the satellite signal is calculated. This error is transmitted to the rover (in this case, the mapping drone, spraying drone, or tractor) via radio. The rover applies these corrections to its own GNSS data, allowing it to determine its position with much greater accuracy.

      When using a base station, it is recommended to:

        • Place it in an open area at the most elevated point.
        • There should be no obstruction between the base station and the drone during the flight.
        • Consider marking a permanent location and installing the base station in the same spot. For example, when flying a mapping drone and later a spraying drone, the base station should be placed in the exact location.
        • Consider capturing the location coordinates and inputting them manually in the future.
        • Do not move the base station while the drone is flying.
      • Network-based correction: NTRIP (Networked Transport of RTCM via Internet Protocol) is a protocol for streaming corrections over the Internet to achieve cm-level accuracy.
        An NTRIP Service provides access to the NTRIP network to receive RTK corrections. This way, a rover (mapping/spraying drones or agricultural machines) and an Internet connection are enough to receive RTK corrections.

        To gain access to the NTRIP network, register with a local NTRIP provider to create an account. This provides a port, IP address, username, and password to access the network. Once the registration is completed, these credentials can connect the drone to the NTRIP network and receive corrections. The cost and coverage area depends on the specific service provider.

      Important:

      When entering the RTK device settings:

      • Check the NTRIP provider's website to determine which mount point is more suitable.
      • Try to find the nearest reference station to the work area. In most cases, the closest reference station is detected automatically.
      • Contact the NTRIP service provider for information on the coordinate system in which the provider broadcasts its corrections. 
      • When performing both mapping and spraying using NTRIP, the same network should be selected.

      If there is no internet connection or NTRIP service is unavailable, using the RTK base station is recommended.

      The importance of a consistent NTRIP connection

      For the highest level of accuracy, it is critical that both the mapping drone and any application machinery (such as agricultural equipment) are connected to the same NTRIP source.

      When the drone and the machinery receive correction data from the same NTRIP connection, they operate within the same reference frame. This ensures that the positional data collected by the drone matches the exact coordinates that the application machinery will use.

      If different NTRIP connections are used, even slight variations in the correction data could lead to discrepancies between the drone's mapping and the machinery's application. In precision agriculture, this could result in incorrect applications or drone crashes.