With the advancement of drone technology, drone mapping, with its efficiency, flexibility, and cost-effectiveness, has brought unprecedented changes to the geographic surveying and mapping industry.
In surveying and mapping, accuracy and reliability are crucial, and these are determined by absolute and relative position data. Here, we will explain absolute and relative position in detail and explore their role in drone mapping.
What is absolute position?
Absolute position refers to the precise coordinates of an object in a global or fixed reference coordinate system, typically based on the Earth's geographic reference system. It is independent of the local environment and is determined by an external global positioning system.
Key Features:
- Coordinate System: This primarily uses a global coordinate system, directly interfacing with GPS/GNSS systems. Position is expressed using longitude, latitude, and altitude. For example, the absolute position of a point might be: 39.9042° North, 116.4074° East, 50 meters above sea level. This means that data points collected by drones are unique.
- Acquisition Method: Relies on satellite navigation systems such as GPS, GLONASS, or the BeiDou Satellite Navigation System. Drones calculate their real-time position on Earth by receiving satellite signals.
- Accuracy Requirements: In drone mapping, absolute position accuracy typically must reach centimeter-level accuracy to ensure that the surveyed data is aligned with the global map.
Advantages and Disadvantages of Using Absolute Position in Drone Mapping
In drone mapping, absolute position is used to record the geographic coordinates of waypoints, geotag images, and georeference the final product.
In these cases, mapping drones often overlay orthophotos with existing maps or GIS data for tasks requiring high coordinate accuracy, such as engineering surveying, land title confirmation, and mining surveying.
Advantages: Strong global consistency, easy integration with other GIS data, and support for large-scale regional mapping.
Limitations: Susceptible to multipath effects or signal loss in weak signal areas (such as urban canyons, indoor areas, or forests), resulting in positioning errors.
What is Relative Position?
Relative position describes the position of an object relative to a local reference point or coordinate system. It does not directly rely on a global system but instead calculates based on a local reference, making it commonly used in scenarios requiring internal consistency.
Key Features:
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Coordinate System: Utilizing a local coordinate system, data points are expressed relative to the drone's takeoff point, the flight path, or other reference points. For example, relative position might be expressed as: 100 meters east, 200 meters north, and 50 meters up, relative to the takeoff point.
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Acquisition Method: Calculated primarily using the drone's built-in IMU+GPS, VO, or SLAM technology. Relative position is based on the accumulated displacement data of an initial reference point.
- Accuracy Requirements: High accuracy over short distances, but drift accumulates over time and distance. For example, after a 10-minute flight, relative position error may reach meter levels.
Advantages and Disadvantages of Relative Position in UAV Mapping
In UAV mapping, relative position is often used for flight path planning, obstacle avoidance, and local feature matching. For example, when constructing 3D point clouds, relative position helps calculate the pose relationship between cameras and enables stitching of multiple images.
Advantages: Insensitive to satellite signals, suitable for environments with poor GNSS signal quality; high real-time computation performance, facilitating real-time attitude control.
Limitations: Lacks a global reference, resulting in drift over time, and inability to directly align with external maps.
The Difference Between Absolute and Relative Position
Absolute and relative position are used in complementary ways in UAV mapping. The differences between the two are primarily reflected in the following aspects:
| Aspect | Absolute Position | Relative Position |
| System | Globally Fixed Coordinate System | Local Reference Point or Coordinate System |
| Positioning Foundation | Satellite Signals (GPS/RTK) | Sensor Data (IMU, VO, SLAM) |
| Accuracy and Stability | High accuracy but susceptible to environmental interference | Short-term high accuracy, prone to accumulated drift |
| Application Scenarios | Large-Scale Map Generation, Georeferencing | Real-Time Navigation, Local Modeling |
| Error Sources | Signal Multipath, Satellite Geometric Distribution | Sensor Noise, Integrated Drift |
| Data Fusion | Direct Integration with GIS | Requires Absolute Position Calibration |
| Accuracy Level | Meter to Centimeter (Depending on the Device) | High relative accuracy, but no global reference |
The Relationship Between Absolute and Relative Position
In real-world surveying and mapping, the two are often used in combination. For example, a drone initializes a reference point with an absolute position at takeoff. Relative position is then used for real-time tracking during flight. RTK or post-processing software then converts the relative data into absolute coordinates for high-precision output. This fusion, called "absolute-relative hybrid positioning," effectively reduces drift errors.
For example, in a typical drone mapping workflow:
1. Initialization: Use GPS to obtain an absolute position as a reference.
2. In-flight acquisition: The IMU calculates relative displacement and records image sequences.
3. Post-processing: Correct the relative position to the absolute system using ground control points or RTK data to generate an orthophoto or DSM (digital surface model).
Optimal Surveying Drones
UAV surveying significantly improves geographic data collection, processing, and application capabilities in scenarios such as agricultural surveying, indoor or underground surveying, and urban planning.
With RTK, GPS, PPK technology, and visual assistance, surveying drones can fully address the challenges of surveying precision and accuracy, as well as those posed by complex environments. The Autelpilot store offers Autel mapping drones: Autel EVO II PRO RTK V3, Autel EVO II Dual 640T RTK V3, Autel EVO MAX 4T RTK, and Autel EVO MAX 4N RTK.
Autel RTK modules achieve centimeter-level accuracy and support RTK, PPK, and network RTK services. Combined with the powerful performance of Autel commercial drones, they can be used for surveying and mapping in a wide range of scenarios.
Autel RTK drones can generate 2D/3D models, and when used with Autel Mapper, they ensure high accuracy and reliability in modeled data.
Summary
In drone mapping, absolute position provides a global perspective, ensuring data consistency and shareability, while relative position enhances flexibility and real-time performance. The right combination of these two is key to achieving high-precision mapping.
Drone mapping is driving continuous innovation in surveying, monitoring, and modeling. Order an Autel RTK mapping drone to empower your mapping career.
