# Rectification forward and backward error

From GRASS-Wiki

**Question:** What are the forward and backward RMS errors in i.points, i.rectify, etc.?

**Answer:** i.points, i.rectify, etc. calculate transformation equations from ground control points (GCPs). RMS (root mean square) errors are then determined by transforming each GCP and calculating the root mean square from the deviations of the transformed coordinates from the GCP coordinates. Since you can transform coordinates from source to target (forward) and from target to source (backward), you can also calculate two RMS errors. The forward RMS error is probably of more interest and ideally smaller than the target resolution (sub-pixel transformation accuracy).