# 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.
- $x$ the point measured in image coordinate system
- $P$ is the Calibration Matrix

$x=PX$ You have your measurement, so $x−x$ is the projection error

Introduced well through SLAM Textbook.