Error is the difference between the measured value and the ‘true value’ (NPL, 1999).  Errors can come from the measuring device itself, including bias, changes due to wear, instrument drift, electrical noise and device resolution.  Other errors can be introduced by difficulties in performing the measurement and by operator skill.  To avoid sampling error, sufficiently dense measurements in space and time should take place to make sure that full variability is captured e.g. diurnal cycles, variations across a site.

Errors can be random or systematic (NPL, 1999).  With random errors, each measurement gives a different result, so the more measurements (of the same thing) the better the estimate and the more certain the measurement becomes.  Systematic errors arise from a bias, e.g., a stretched tape measure, and more measurements do not produce a better estimate of the ‘true value’.