[R-sig-Geo] How to perform a variogram model in gstat?

[Carolina Lang]

Dear Julia, 

About your question, the variogram fit depend on what you want!!!, 
if you just want to fit a model to describe structures you can choose many options like eyefit, Index GOF (Goodness of fit), AIC (Akaike) or RSS to decide which parameters, models and values select, If you want an interpolate data with kriging then you would use a cross validation to compares values.

Regards, 

Carolina Lang A. 
Investigador 

----- Mensaje original -----
De: "Julia Chacon Labella" <juliachacon at gmail.com>
Para: r-sig-geo at r-project.org
Enviados: Martes, 25 de Septiembre 2012 4:57:08 GMT -04:00 Georgetown
Asunto: [R-sig-Geo] How to perform a variogram model in gstat?

Please excuse these basic questions, but I am a bit lost in geostatistics.

I am trying to make a kriging of the soil properties of a plot. I am
already using gstat with R and I don't have any problems with the scripts
and so on...but, I don't really know how to choose the nugget, psill and
range for the variogram model. Moreover, I am not sure about how to include
the anisotropy in the model, I have already include a linar model in some
cases, to remove any trend.

I am following the steps recomended in the gstat help, but I am a bit
befuddled.

Let me show you my scripts (the psill, range and nugget are choosen just to
make the test).

coordinates(corg)= c("x", "y")

v = variogram(SOC~1, corg)

SSErr.a <- numeric(4)
names(SSErr.a) <- c("Exponencial","Esférico","
Gausiano","Lineal")
v.fit = fit.variogram(v, vgm(psill=0.18, model="Exp", range=5, nugget=0.1))
SSErr.a[1] = attr(v.fit, "SSErr")
v.fit = fit.variogram(v, vgm(psill=0.18, model="Sph", range=5, nugget=0.1))
SSErr.a[2] = attr(v.fit, "SSErr")
v.fit = fit.variogram(v, vgm(psill=0.18, model="Gau", range=5, nugget=0.1))
SSErr.a[3] = attr(v.fit, "SSErr")
v.fit = fit.variogram(v, vgm(psill=0.18, model="Lin", range=5, nugget=0.1))
SSErr.a[4] = attr(v.fit, "SSErr")
SSErr.a

v = variogram(SOC~x+y, corg)

SSErr.b <- numeric(4)
names(SSErr.b) <- c("Exponencial","Esférico","Gausiano","Lineal")
v.fit = fit.variogram(v, vgm(psill=0.18, model="Exp", range=5, nugget=0.1))
SSErr.b[1] = attr(v.fit, "SSErr")
v.fit = fit.variogram(v, vgm(psill=0.18, model="Sph", range=5, nugget=0.1))
SSErr.b[2] = attr(v.fit, "SSErr")
v.fit = fit.variogram(v, vgm(psill=0.18, model="Gau", range=5, nugget=0.1))
SSErr.b[3] = attr(v.fit, "SSErr")
v.fit = fit.variogram(v, vgm(psill=0.18, model="Lin", range=5, nugget=0.1))
SSErr.b[4] = attr(v.fit, "SSErr")
SSErr.b

Thanks a lot,
Cheers.

-- 

 Julia Chacon Labella [PhD candidate]

Área de Biodiversidad y Conservación

 Escuela Superior de Ciencias Experimentales y Tecnología

 Universidad Rey Juan Carlos

 c/ Tulipán, s/n.

 E-28933 Móstoles (Madrid)

 España


-- 
Julia

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