If two points were sampled a vanishingly small distance apart they would give different values. This is known as the nugget of the semivariogram . A non-zero nugget
effect can be attributed to measurement errors or spatial sources of variation at distances smaller than the sampling interval. Measurement error occurs because of the error inherent in measuring devices. Natural phenomena can vary spatially over a range of scales. Variation at micro scales smaller than the sampling distances will appear as part of the nugget value.
Note: The nugget can be interpreted as the variation among repeated measurements at the same point.