Jacob Amacker explains the value of boresight calibration for oil & gas applications
Using lidar in oil and gas engineering is nothing new, however, sophisticated and compact solutions for its use are. There are many successful use cases and applications being used in the field today. In many of these cases, an inertial navigation system (INS) is used alongside a lidar device to accurately measure roll, pitch and heading. The result being an accurate georeferenced 3D point cloud.
The importance of accurate boresight calibration
In this situation, where an INS is being used alongside a lidar, it is crucial that there are no imprecisions in the angles measured between the tracking device (INS) and the surveying device (lidar). Where an imprecision occurs, it can result in the point cloud being distorted, displaced or even unusable. This is known as boresight misalignment. The diagram on the right demonstrates this problem.
Small errors in heading, pitch and roll measurements are often unimportant when considering your own position, but when surveying objects that are 10-200+ metres away these small imprecisions can be magnified considerably.
Ideally, these angles must be measured to an accuracy of 0.1°. This is extremely difficult with physical measuring devices and CAD modelling is expensive, inflexible and time-consuming.
Solving the problem with boresight calibration
Another way of addressing the problem is to use a data-driven approach to calibrating the position of the INS in relation to the lidar device. This data-driven approach to correcting the misalignment is the boresight calibration.
Within the OxTS Georeferencer software, a boresight alignment tool is included. The tool automatically aligns the INS and lidar reference frames using a proprietary algorithm. The algorithm analyses returns from two planar targets to find the precise alignment of the INS and lidar.
This method reduces user workload and saves time and has also been designed to be portable and easy to use in real-world applications.
Boresight calibration set up
The procedure to set-up the calibration is very simple and can be reproduced for whichever application is required, as long as an OxTS INS with OxTS Georeferencer is used in conjunction with the compatible third-party lidar device.
Firstly, two retro-reflective targets are set-up. These can be made simply using a flat wooden board and some cheap retroreflective tape. Once they are in place the INS and lidar device should be configured to begin surveying.
Once ready, the vehicle (UAV, car, plane or any other mapping vehicle) should be manoeuvred in such a way that the lidar device sees the targets from a range of angles and distances. This procedure should take no more than five minutes and will only need to be done once if the set-up isn’t altered. As soon as this exercise is complete the survey can begin.
At the point of data processing OxTS Georeferencer will guide the user through the calibration process and the software does the hard work for the user.
Benefits of boresight calibration
This calibration technique can be added on to any survey-run and will reduce the amount of time and effort taken to set-up for a survey, whilst simultaneously allowing users to produce more accurate 3D point clouds.
Jacob Amacker is product engineer at Oxford Technical Services (OXTS)
