Definitions
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A function of the spatial dependence of variance; a graph of this function
Etymologies
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Examples

The variogram is the key function in geostatistics as it will be used to fit a model of the spatial/temporal correlation of the observed phenomenon.
More ISO2000 Weather Stations from Jones and Hansen « Climate Audit

Before I had this thing running, the measure of autocorrelation I was using in my accent model was to crank out a semivariogram for all the point pairs, add up the average valuedistance for all the pairs on the “near” half of the variogram, add up the average for the “far” half, and compare the ratio of the “near” and “far” averages.

By contrast the variogram is well defined for the much wider class of socalled intrinsic processes, its classical estimator is unbiased when the process is only mean stationary and an alternative but natural estimator has only a small bias even when the process is neither mean nor variance stationary.

For observations at locations the empirical variogram is defined as Cressie 1993:
More ISO2000 Weather Stations from Jones and Hansen « Climate Audit

When a variogram is used to describe the correlation of different variables it is called crossvariogram.
More ISO2000 Weather Stations from Jones and Hansen « Climate Audit

The sampling variogram, unlike the semivariogram and the variogram, shows where a significant degree of spatial dependence in the sample space or sampling unit dissipates into randomness when the variance terms of a temporally or insitu ordered set are plotted against the variance of the set and the lower limits of its 99% and 95% confidence ranges.
More ISO2000 Weather Stations from Jones and Hansen « Climate Audit

This explains why only a limited number of variogram models are used like the linear, the spherical, the gaussian and the exponential models, to name only those that are the most frequently used.
More ISO2000 Weather Stations from Jones and Hansen « Climate Audit

Note that the experimental variogram is an empirical estimate of the covariance of a Gaussian process.
More ISO2000 Weather Stations from Jones and Hansen « Climate Audit

A plot of the semivariance versus h is the variogram.
More ISO2000 Weather Stations from Jones and Hansen « Climate Audit

One is thus making a distinction between the experimental variogram that is a visualisation of a possible spatial/temporal correlation and the variogram model that is further used to define the weights of the kriging function.
More ISO2000 Weather Stations from Jones and Hansen « Climate Audit
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