the nugget effect is the value of the variogram when lag distance is equal to zero, and according to litterature, it's dependant on measurement error, i don't if any one can help me to calirify this point.
thank you Louadj yacine
Thank you very much indeed vasu for this valuable paper, it will help me for sure .
Best regards yacine
B
In general the nugget indicates a variability at the origin, it means that for shortest lag distance (up to zero) you could find a variability. You can calculate the nugget graphically by constructing a line that pass through the first two points of the variogram up to intersect the Y axis.
A large nugget effect relative to total variability can be caused by sampling that is too sparse with respect to spatial variability, or because of measurement error. In either case it is small-scale variability.
Generally you fit a nugget effect by eye from the variogram. The first few points--two or more--are the most instructive. However, you can also use your experience as a guide in fitting the nugget effect.
If you are doing variography as a prelude to kriging or simulation the nugget effect affects the degree of smoothing in the results.
You can't "calculate" the nugget effect, you can only infer it. theoretically the value of the variogram is zero for zero lag distance but sometimes the apparent shape of the variogram graph suggests a jump discontinuity at the origin. The nugget is the magnitude of the jump. Measurement error could contribute to it but it may also be a consequence of no data locations close enough together. As pointed out by Michael it affects the smoothness of the interpolated surface since the kriging estimator is exact.
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