Comparing Absolute and Relative Accuracy in Drone Surveying

drone-accuracy

Choosing between High Absolute Accuracy and Relative Accuracy is crucial in drone surveying. Explore the advantages of each approach to assist in selecting the most appropriate level of accuracy for your needs, and learn about the advantages of utilizing RTK or PPK techniques.

The increasing demand for precise geospatial data underscores the significance of choosing between High Absolute and Relative Accuracy in drone surveying.

Discover the disparities between Absolute and Relative Accuracy and their implications for your surveys.

Learn how RTK and PPK methodologies can enhance accuracy.

In the swiftly evolving realm of drone surveying, accuracy stands as a pivotal factor, dictating the outcome of projects. Two commonly employed terms, Relative Accuracy and Absolute Accuracy, delineate different facets of data accuracy derived from drones. But what do they entail?

Relative Accuracy: Relative Accuracy gauges the consistency of measurements within the dataset concerning each other. It focuses on the uniformity of measurements taken across various points or instances within the surveyed area.

Absolute Accuracy: Absolute Accuracy assesses the fidelity of drone-derived measurements to real-world coordinates or ground truth. It measures how faithfully the collected data represents the precise positions and features on the Earth’s surface.

High absolute accuracy ensures meticulous alignment with ground truth, instilling confidence and dependability in tasks like mapping and infrastructure planning. Conversely, high relative accuracy fosters uniformity within datasets, facilitating robust spatial analyses and informed decision-making across different sections of the surveyed area. Both forms of accuracy play indispensable roles in maximizing the utility and efficacy of data sourced from drones across diverse fields.

accuracy diagram

DJI drones harness onboard sensors such as accelerometers, gyroscopes, and magnetometers to uphold stability and precision during flight, contributing to dependable relative accuracy. However, measurements extracted from your dataset may be accurate but might not align with a coordinate reference system, potentially leading to offsets, particularly on the z-axis.

To achieve high relative and absolute accuracy, you can adopt either RTK or PPK workflows. RTK corrections can be transmitted to the drone in real-time, whereas PPK corrections are applied post-flight in the office, leveraging data from a local base station.

Accuracy Vs Precision In Drone Surveying:

Before delving into Relative vs Absolute Accuracy, let’s distinguish between accuracy and precision, as these concepts offer two perspectives on errors encountered by surveyors:

Accuracy: Accuracy pertains to the proximity of measured values to true or known values. In drone surveying, accuracy gauges the correctness of geospatial data in relation to real-world coordinates.

Precision: Precision denotes the consistency and reproducibility of measurements under identical conditions. In drone surveying, precision evaluates the reliability and consistency of measurements within the dataset.

accuracy diagram

Relative Accuracy emphasizes precision over accuracy, underscoring the uniformity of measurements within the dataset. Absolute Accuracy prioritizes both accuracy and precision, ensuring that collected data closely align with true or known coordinates in the real world.

Understanding Relative Accuracy:

The required level of accuracy hinges on the nature and purpose of the survey. Relative accuracy proves optimal for conducting general inspections and comparative analyses within a specific dataset. Applications such as monitoring vegetation changes, tracking construction progress, or assessing the aftermath of natural disasters find relative accuracy well-suited. This level of accuracy provides relevant data for specific land sections, without the necessity for precise global geospatial coordinates.

Benefits of Relative Accuracy:

Rapid Deployment: Relative accuracy allows swift deployment of drones without extensive ground control and surveying, ideal for time-sensitive scenarios.

Flexible Data Collection: Relative accuracy offers adaptability in data collection without set ground points, facilitating agile operations, such as rescue missions.

Sufficient Intra-Dataset Comparisons: Reliable surface-level data supports comparisons and analyses within the same dataset.

Localized Mapping: Missions in small areas like rural settings often don’t require highly precise global geographical data points.

Understanding Absolute Accuracy:

High absolute accuracy becomes indispensable when precise coordinates and comprehensive data are needed for long-term comparisons and specific applications such as georeferencing, precision architecture, or topographical mapping.

High Absolute Accuracy: Use cases include tracking inspection and survey data over time, demonstrating visual changes in areas/buildings, and following exact coordinates for every mission.

Benefits of High Absolute Accuracy:

Interoperability and Integration: Seamless integration of drone survey data with other geospatial datasets facilitates comprehensive mapping and analyses across large areas.

Legal Compliance: Adherence to strict standards and regulations, particularly in industries like construction, agriculture, and environmental monitoring.

Survey Control and Ground Truthing: Crucial for validating and refining survey results, especially in large-scale projects.

Long-Term Comparisons: Ensures consistent measurements for monitoring changes in landscapes, infrastructure, or environmental conditions over extended periods.

Precision Agriculture: Vital for crop monitoring, yield prediction, and resource application based on accurate spatial information.

RTK and PPK:

RTK and PPK methodologies refine geospatial data accuracy in drone surveying. RTK utilizes real-time differential correction to enhance GPS positioning data during flight, ensuring centimeter-level precision. PPK corrects GPS errors post-flight, offering flexibility and comparable accuracy through post-processing.

Tips for achieving your desired accuracy

Use this table to decide the best action to achieve your ideal accuracy type. 

Level of accuracy required  How to achieve it 
High Absolute AccuracyConnect to your own DJI D-RTK2 base station via (LORA) radio and ensure the base station is installed over a known point.Next, add measured GCPs into the software to align the model with these more accurate points.Or use EMLID NTRIP Caster – EMLID RS3 and broadcast corrections to the drone over the internet and position it over a known point.
Relative Accuracy  Draw the mapping boundary and launch the mission.The data processed using drone mapping software – DJI Terra etc.
High Relative Accuracy and Low Absolute AccuracySet up a base station anywhere that is best for you.
High relative and absolute accuracy.Set the base station to known points; this could be marked with a GNSS Receiver, such as an Emlid RX or EMLID RS3.heliguy™ always recommends Emlid as it is three times more accurate than drone data.GCPs must be marked with a device that is at least 3 times more accurate than the geospatial data being tested and traversed with a total station as a part of the control network.

Summary:

The selection of accuracy level is influenced by the mission’s purpose, frequency, time constraints, and budget. High absolute accuracy proves indispensable for applications necessitating precise spatial positioning, legal compliance, or recurrent inspections. Understanding the unique advantages of each accuracy metric enables drone surveyors to tailor their approach to meet project-specific demands.