Reprojection Error

Reprojection error is the difference between observed feature points and the reprojected feature points (from 3D to 2D) in the image plane.

So basically, assuming you have 3 measurements

• $X$ the point in 3D
• $x$ is the observed point in the image coordinate system.
• $\hat{x}$ the point measured in image coordinate system
• $P$ is the Calibration Matrix

$\hat{x}=PX$ You have your measurement, so $x-\hat{x}$ is the projection error

Introduced well through SLAM Textbook.