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corSpatial and zero distance

2 messages · valerio.bartolino at uniroma1.it, Edzer Pebesma

Original
valerio.bartolino at uniroma1.it
# 
Dear list,
it is probably a very simple question with an obvious answer, that unfortunately I cannot find by myself.

Why I cannot fit a spatial correlation structure model if I've some observations in the same location? Shouldn't the nugget account exactly for this small-scale variability and measurement errors?

I'm using one of the correlation models in the generic function 'lme' (package:nlme)

Thank you

Valerio
Edzer Pebesma
# 
lme fits using (restricted) maximum likelihood, and that requires a
covariance/correlation matrix that is non-singular, to be inverted each
iteration.

Under spatial correlation models, even the pure nugget model,
observations with identical location are perfectly correlated (as in:
the correlation of X with itself), and so result in duplicate rows/cols,
making the covariance/correlation matrix singular.
On 10/31/2010 11:21 AM, valerio.bartolino at uniroma1.it wrote: