How to understand the global coordinates?

[guiyuliu]

hello, I'm not familiar with coordinates calibration.
I see two parameters, sensor2ego, ego2global, I want to know what is the origin of global coordinates？
I see the tranlation between global to lidar is about 400，500,... is it measured in meters?
how to calibrate between ego and global ?

[holger-motional]

Yes, they are meters on one of the 4 maps. You can see them rendered at https://www.nuscenes.org/nuscenes#data-collection .
Ego depends on the rotation and translation. I suggest you follow the code in https://github.com/nutonomy/nuscenes-devkit/blob/master/python-sdk/nuscenes/nuscenes.py to try to understand better what we are doing.

[guiyuliu]

Yes, they are meters on one of the 4 maps. You can see them rendered at https://www.nuscenes.org/nuscenes#data-collection .
Ego depends on the rotation and translation. I suggest you follow the code in https://github.com/nutonomy/nuscenes-devkit/blob/master/python-sdk/nuscenes/nuscenes.py to try to understand better what we are doing.

I know ego2global means the translation and rotation between ego and global , if ego means car itself, i want to know what is global ?

[holger-motional]

Ego vehicle: The self-driving car, more exactly the center of the rear axle projected to the ground.
Global: See #56 (comment)

[rockywind]

Hi,
What’s the global coordinates definition?
What's the X, Y and Z's orientation of global coordinates?

[whyekit-motional]

@rockywind pls see #687 (comment)

[northeastsquare]

@whyekit-motional @holger-motional the question "how to calibrate between ego and global ?", have not answered, thank you

[whyekit-motional]

@northeastsquare an example of the transformation from ego frame to global frame can be found here:

nuscenes-devkit/python-sdk/nuscenes/nuscenes.py

Lines 887 to 890 in 9b165b1

	# Second step: transform from ego to the global frame.
	poserecord = self.nusc.get('ego_pose', pointsensor['ego_pose_token'])
	pc.rotate(Quaternion(poserecord['rotation']).rotation_matrix)
	pc.translate(np.array(poserecord['translation']))

[northeastsquare]

@whyekit-motional Thank you for reply! I know use rotation and translation matrix to do it. But how to calibrate between ego and global? I mean how do you got the rotation and translation matrix? If use gps and IMU, but gps location error is in several meters. How do you got the rotation and translation matrix with location error in centimetres?

[whyekit-motional]

@northeastsquare this discussion might be useful for you: #481