The basic concepts: zero measurement, reference points, monitoring points, epochs and reliability.
Zero measurement and epochs
Deformation monitoring compares the position of points over time. The first measurement — the zero measurement or reference epoch — forms the basis. Each later measurement (epoch) is compared to it to compute displacements. The more careful the zero measurement, the more reliable every derived movement.
Reference vs monitoring points
Reference points lie outside the zone of influence and are assumed stable; they form the fixed frame. Monitoring (object) points sit on the structure to be watched. By relating monitoring points to stable reference points, systematic errors are eliminated.
Significance and noise
Every measurement contains noise. A measured difference is only a real movement if it is significantly larger than the measurement uncertainty. Statistical testing distinguishes genuine deformation from measurement noise — essential to avoid false alarms.