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!
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.