This example illustrates the Variogram-cloud Arcview-XGobi link. The variogram-cloud link is used when exploring the spatial dependence in a data set and when looking for spatial outliers. In this option, the points displayed in XGobi represent all possible pairs of sampling locations. For each pair of locations, XGobi plots the square-root of the absolute difference between attribute values at the locations versus the euclidean distance between the locations. In data sets exhibiting strong spatial dependence, the variance in the attribute differences will increase with increasing distance between locations. Locations that are near to one another, but with large attribute differences, might indicate a spatial outlier, even though the values at both locations may appear to be reasonable when examining the data set non-spatially.
In this example, a variogram cloud has been plotted for precipitation values at precipitation sampling sites. This plot shows that a group of points seems to lie above the main group of points.
When several of these "outlying" points are brushed in red, red lines are used to connect the pairs of sampling locations that correspond to each point brushed in the variogram cloud. This indicates that all of the brushed points have a single sampling location in common.
By zooming in on the site in question (below), and displaying elevation contours, it becomes obvious that the site in question sits on top of a mountain, which causes it to have higher values than other sites in the area.
