Create variogram in R's gstat package

Question

Suppose I have rainfall data taken at four weather stations over the span of 2004-2016. I fed the data into a database for retrieval in R. My goal is to take the data for every single day from that period, and krige using those values, repeatedly.

So right now my data looks like this, each row corresponds to one of the points, and the columns in order are: lat, long, and rainfall_data.

I followed this tutorial: https://rpubs.com/nabilabd/118172, to help me get started. So here's my code so far:

day_1 <- dbGetQuery(con, "SELECT lat, long, rainfall_data FROM schema.sample")
coordinates(day_1) <- ~lat+long
day_1.vgm <- variogram(rainfall_data~1, day_1)...

My problem starts at the last piece of code, every time I run that, all I get is a null (empty) result (as seen in RStudio). I can't even make it to the next step which is:

day_1.fit <- fit.variogram(day_1.vgm, model=vgm(1, "Sph", 900, 1))

Because when I do, it throws an error which reads:

Error in fit.variogram(day1.vgm, model = vgm(1, "Sph", 900, 1)) : object should be of class gstatVariogram or variogramCloud

I know that the dataset is SUPER LACKING having only 4 points, and I know that makes for some really poor results, but its the one I got so I'm sticking with it. But irregardless of dataset size, this should work, unless I'm missing something.

If I'm average at Java, then R is a completely alien language to me (although not impossible to learn) and statistics is far from my list of skills (I'm an IT guy not a statistician).

Am I doing something wrong, can anyone give me directions? Please help. Thanks.

EDIT: The data looks like this:

lat    long    rainfall_data
7.16   124.21    0.25
8.6    123.35    1
8.43   124.28    125.6
8.15   125.08    4.3

Answer 1

I would say it's unwise to try to fit a variogram to just 4 points. However, if you really want to do that, you can do something like this :-

The error you are getting is because the day_1.vgm object is NULL so you need to look at the documentation of variogram. There are parameters namely width and cutoff which you need to change. For example, try the following

day_1.vgm <- variogram(rainfall_data~1, day_1, width = 0.02, cutoff = 1.5)

If you look at the plot of this variogram, it will look something like this:-

Now you are trying to fit a variogram to these points. So, you can use the fit.variogram command as you have used. However, be careful to look at the parameters. Let's fit a simple spherical model as a start.

day_1.fit <- fit.variogram(day_1.vgm, model=vgm("Sph", psill = 8000, range = 1))

You will likely get a warning here about a singular fit, which is probably because of very few points.

The fitted variogram will look something like this:

You can change the parameters of the fit appropriately.