Introduction
Inertial Measurement Units (IMUs) have established themselves as an unavoidable instrument in the contemporary surveying and geospatial data capture processes. Such sensors are generally accelerometers and gyroscopes in combination, used to calculate the position, orientation and velocity of an object without external references in terms of specific force, angular rate, and occasionally magnetic field.
In surveying, IMUs can be used to offer vital data when there is poor or no GPS signal example in urban canyon, forest canopy, within tunnels and indoor settings. Combining IMU measurements and other positioning methods, surveyors get the accuracy and reliability in the most difficult situations they have never had before.
Understanding IMU Technology
Core Components
IMUs usually include three accelerometers and three gyroscopes orthogonally set up to quantify linear acceleration in three-space and three-axes angular velocity. More advanced units can be provided with magnetometers to keep track of the heading.
MEMS Revolution
The Micro-Electro-Mechanical Systems (MEMS) technology has shrunk the IMUs, thus making them cheap and easy to acquire. MEMS-based IMUs now become as small as mounted systems in drones and achieve professional level accuracies.
Sensor Fusion
IMUs hardly operate independently. In combination with GPS, LiDAR and camera data, using advanced algorithms (such as Kalman filtering), IMU data is used to form very accurate and highly reliable positioning systems (in particular when there is a GPS outage).
Dead Reckoning
Without external signals, IMUs can be used in dead reckoning navigation, in which the position is recalculated continuously through prior-determined positions and a calculated speed over time elapsed.
Key Insight: IMU counterbalances loss of position provided by the GPS by giving constant position readings in case of outages. In the case of the GPS signal loss, the IMU assists on correcting the cumulative errors with sensor fusion algorithms and keep the survey-grade precision within a demanding environment.
IMU Surveying Process
Initialization & Alignment
The IMU system must be initialized with its known position and orientation information which is usually that of the GPS when present. It is by this process of aligning that this forms the reference frame that all other measurements are based.
Continuous Data Collection
The IMU measures acceleration and roll/pitch/yaw rates very fast (usually 100-1000Hz) as the survey platform (drone, vehicle, or backpack system) is in motion.
Sensor Fusion
Kalman filtering algorithms are used to combine IMU data with the positions of the GPS. The GPS gives absolute position references and the IMU gives smooth, high frequency between GPS fixes.
Error Correction
The bias, scale factors (systematic errors) are constantly approximated and eliminated. Extra sensors (magnetometers, wheel odometers) give more data to fight the drift.
Surveying Applications of IMU Technology
Mobile Mapping Systems
On-board systems integrate IMU with LiDAR and cameras to provide high-accuracy surveys of roads, highways, and urban areas at a rate at which only centimeters of any area can be observed with accuracies even in the GPS-problematic regions.
Aerial Surveying (UAV)
Lightweight IMUs are used on drones to stabilize cameras and sensors, which can perform precise aerial mapping, photogrammetry and 3D modelling in the absence of ground control points in most applications.
Underground & Indoor Mapping
IMU-based systems do work in GPS-denied locations where surveyors are able to survey mines, tunnels, caves, and large indoor facilities with the SLAM (Simultaneous Localization and Mapping) technology.
Hydrographic Surveying
Through IMUs, sonar hardware on survey boats stabilizes and counters the off-shore wave action to produce accurate maps of seafloors, riverbeds and sub-sea structures.
IMU vs. Traditional Surveying
| Feature | Traditional Surveying | IMU-Enhanced Surveying |
|---|---|---|
| GPS-denied environments | Limited capability | Excellent performance |
| Data collection speed | Slow (point-by-point) | Very fast (continuous) |
| Equipment portability | Moderate | High (drone/backpack systems) |
| Operating costs | Higher (labor-intensive) | Lower (automated collection) |
| Position drift over time | Minimal | Requires periodic correction |
Future of Surveying
IMU technology has revolutionized the current surveying procedures by making it possible to collect data very quickly in areas that were thought to be unreachable. With the MEMS technology still developing and sensor fusion algorithms improving, it can be assumed that even more accuracy to affordable prices will be achieved with IMU-based systems in the future.
IMU integration with other new technology such as AI, 5G connectivity and edge computing offers the potential to further transform geospatial data collection by enabling high precision surveying to become faster, more cost effective and available to more application areas than ever.