Dear list,

I am doing block kriging using the gstat package in order to estimate

the mean and variance of a variable over an irregular polygon. I do this

by passing to the krige function a SpatialPolyons object under the

newdata argument. The krige block variance is calculated (as I

understand) using the point-to-point, point-to-block and within-block

variograms.

My question is about how this calculation is done in gstat. It seems that

gstat's krige function deals with an irregular polygon by discretising

it using the spsample function to select a number of regular nodes in

the polygon (~500 by default). But according to Goovaerts (1999,

Geostatistical Tools for Deriving Block-Averaged Values of Environmental

Attributes, p.91), this approach is difficult because "the variance of the

global estimator cannot be derived as a mere combination of the kriging

variances at each discretizing point". But clearly gstat is producing a

krige variance estimate. So how it is done?

At the end, what I want is to be sure that the variance estimate is

valid. :)

Many thanks for the clarification,

Julian

--

Julian Mariano Burgos, PhD

Hafrannsóknastofnun, rannsókna- og ráðgjafarstofnun hafs og vatna/

Marine and Freshwater Research Institute

Botnsjávarsviðs / Demersal Division

Skúlagata 4, 121 Reykjavík, Iceland

Sími/Telephone : +354-5752037

Bréfsími/Telefax: +354-5752001

Netfang/Email:

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