When we consturct the 3D coordinates of the checkerboard corners, does the unit of length matter?
I have read multiple articles and example codes for camera calibration with OpenCV. To represent the 3D coordinates on a checkerboard, some use centimeter as a unit, some use meter as a unit, some simply consider the length of one checkerboard box as 1 without any unit of length involved.
So how is the length of unit determined when we calibrate a camera?
If you are doing intrinsic calibration (focal length, lens distortion), the scale doesn't matter at all because your single camera can't see it anyway and the pose of the board itself is of no interest (the intrinsics are).
The scale does matter if you are doing extrinsic calibration (multiple cameras), pose estimation (of the board), or any kind of Structure from Motion.
You can choose your unit of length freely. It will be the base unit for everything, i.e. determines how to interpret any values.
*Scale meaning both values (1, 1000, 25.4) and units (m, mm, inches, parsecs).