The difference between accuracy and precision, and why uncertainty must always be stated.
Two distinct concepts
Accuracy indicates how close a measurement is to the true value. Precision indicates how well repeated measurements agree. A system can be very precise yet inaccurate due to a systematic error.
Measurement uncertainty
Every measured value should be accompanied by an uncertainty, often expressed as a standard deviation. In monitoring this is crucial: only when a displacement exceeds twice the standard deviation do we generally speak of a significant movement.
Sources of error
Errors arise from the instrument, the setup, the atmosphere and the observer. Good monitoring minimises systematic errors (calibration, reference points) and averages out random errors (repetition, redundancy).