Not all LiDAR is equal – Part 2: The role of the IMU
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This week Anditi’s chief software architect and LiDAR dataset diagnostics guru Patrick Poissant and Head of Business Operations and quality queen Catherine Pepper combine forces to focus on the role of the Inertial Measurement Unit (IMU) in LiDAR capture and what factors you should consider to ensure the data reporting from the IMU is as accurate as possible.
This is part 2 of the 4 part blog series on common pitfalls to avoid in the capture of LiDAR with a discussion on things to consider when planning your capture program. See ‘Part 1: Planning for success’ for ways to plan your project to ensure you’re setting yourself up to succeed.
Have a high quality Inertial Measurement Unit
The IMU on an aircraft is the primary device for determining the altitude of the aircraft as the LiDAR sensor is taking measurements. The IMU records measurements in three dimensions as roll, pitch, and yaw, and this data is used, along with the GPS, to determine the exact location of each laser pulse as it exits the sensor. The accuracy and precision of the IMU is a key determinant in the quality of the output from your LiDAR capture program. Starting with a poor quality IMU will reduce the quality of the resultant LiDAR point cloud.
Constant motion collection improves IMU precision
The less lateral and vertical movement the LiDAR collecting instrument has while collecting data, the better the overall precision of collection. This is due to the fact that the IMU’s precision gets better and better as constant motion is accomplished. In the same way, it can be destroyed pretty quickly if it changes direction quickly, e.g. due to turbulence. Achieving constant motion collection is a combination of careful planning of the capture program to avoid factors that increase the likelihood of lateral movement (e.g. high winds), and ensuring that there is sufficient coverage of the area of interest to ensure the aircraft isn’t changing direction while over an area where a highly accurate dataset is required.
Why flying at stall speed isn’t a good idea
Flying at stall speed may sound like a great way to increase the point density. In theory, flying slower should result in a higher number of LiDAR returns per square metre. However, apart from the safety challenges of flying at or near stall speed, in practice this actually increases the risk of perturbing the accuracy of the point cloud due to the increased likelihood of pitch and roll of the aircraft. Particularly if the collection is made with a low quality IMU, this approach can generate more problems than benefits.
What to do if the quality of your LiDAR dataset isn’t up to scratch
If your LiDAR dataset comes back at a quality that makes it difficult to achieve the kind of outcomes you were hoping for, there are ways to review the data, identify any problems, and determine whether the dataset can be fixed through post processing or whether another capture round is required. If you find yourself in this predicament, our dataset diagnostics and repair tools can help. Drop us a line or give us a call on 1300 326 170.