Boresight calibration
Boresight calibration for your mobile mapping system: GEORIZON determines the angular offset between IMU and sensor automatically and delivers corrected data within 24 hours.
A mobile mapping system derives position and orientation from a coupled GNSS and INS solution, and measures its surroundings with laser scanners and cameras. Between the reference frame of the IMU and that of each sensor sit two systematic offsets. The lever arm is the translation between the two origins: it is fixed and can usually be measured physically. The boresight angles are the residual angles in roll, pitch and heading that remain because no mounting bracket is ever perfectly square.
The effect of such a residual angle scales linearly with measurement range. An offset of 0.01 degrees displaces a point by less than a millimetre at five metres, but by almost nine millimetres at fifty. In practice the boresight therefore decides whether façades, road markings and cables stay sharp at range or smear into a double edge.
Because the error is angular rather than a simple shift, it does not disappear by fitting the finished point cloud to ground level or to control points afterwards. It has to be resolved in the same frame in which the trajectory was computed.
GEORIZON estimates the boresight angles from the overlap already present in your own acquisition. The algorithm looks for correspondences between planes and linear features captured from several driving directions or several passes, and minimises the remaining distance between those observations in a least squares solution. Roll, pitch and heading are solved together, so the corrections cannot quietly compensate for one another.
That the estimate rests on your own project is the essential difference from a calibration determined once on a test track. A system that has been rebuilt, transported or exposed to a serious temperature swing almost never sits exactly as it did during factory calibration. That drift is picked up per project.
The same solution carries over to the cameras. Panoramas and point cloud are tied to the same IMU frame, so the colouring of the points and the projection of image features onto the cloud genuinely coincide.
Before any correspondences are searched for, GEORIZON automatically masks the parts of the imagery and the point cloud that belong to the survey system itself: bonnet, roof, mast and the reflections on them. That structure travels with the vehicle, so it is perfectly stable in every frame while telling you nothing about the world.
Without a mask it is exactly that apparently stable structure which dominates the fit and pulls the estimate towards an angle that does not describe the real mounting. Masking is configured per acquisition system and needs no manual outlining.
The same principle applies on the laser side: an occlusion mask keeps out points caused by the vehicle or by the scan pattern itself. What remains are observations whose geometry actually means something.
The boresight angles and lever arm that were applied can be inspected afterwards as readable values, together with the residual offset between the strips. You can check the result on a cross section instead of taking it on trust, and carry the values over into your own acquisition software.
