R: error with autofitVariogram (automap package)

Question

Using autofitVariogram() function from automap package I have generate following error:

Error in vgm_list[[which.min(SSerr_list)]] : attempt to select less than one element in get1index

Example code:

model <- as.formula(Value ~ Elevation)
data <- matrix(c(11.07,42.75,5,62.5,
                 8.73,45.62,234,75,
                 12.62,44.03,12,75,
                 10.87,45.38,67,75,
                 8.79,42.53,64,75),
               nrow = 5, byrow = TRUE)
data <- as.data.frame(data)
names(data) <- c('Lon', 'Lat', 'Elevation', 'Value')
library('sp')
coordinates(data) = ~Lon+Lat
library('automap')
autofitVariogram(model, data)

What causes this error? Do interpolated values cause some kind of 'singularity'?

Thx!

Answer 1

This error is caused by the fact that gstat cannot generate an experimental variogram given this number of observations:

library(gstat)
library(sp)

data <- matrix(c(11.07,42.75,5,62.5,
                 8.73,45.62,234,75,
                 12.62,44.03,12,75,
                 10.87,45.38,67,75,
                 8.79,42.53,64,75),
               nrow = 5, byrow = TRUE)
data <- as.data.frame(data)
names(data) <- c('Lon', 'Lat', 'Elevation', 'Value')
coordinates(data) = ~Lon+Lat
variogram(Value ~ Elevation, data)
## NULL

When given insufficient observations, gstat::variogram returns NULL. This in turn causes autofitVariogram to fail.

The solution is to simply have more data if you want to use kriging. A rule of thumb is that you need about 30 observations to generate a meaningful variogram to fit a variogram model to.

Answer 2

Recently, I also come across this problem. I find out the reason is that there are some Inf values in my data, and if I delete them, the package works well. Hope this could help you.