Dear all,
I'm having trouble interpolating a number of gridded datasets that I have. I'm
quite new to R so any help/advice that can be offered would be much appreciated!
Firstly I'll describe my dataset. The data is a grid of the planet at 1 degree
spatial resolution, with each grid cell containing a value describing a
particular variable (e.g. population density) for coordinate pairs. The
dimensions of the grid are 360col x 180row. The x-coordinates of the values
within the grid are longitudes (from -179.5(W) to 179.5(E)) and the
y-coordinates of the grid values are latitudes (from 89.5(N) to -89.5(S)).
What I want to do is convert this 1 degree dataset to a finer resolution of 0.5
degrees, thereby quadrupling the number of cells within the frame (i.e. convert
to 720 x 360 grid). I wish to have longitudinal values ranging from -179.75 to
179.75 and latitudinal values from 89.75 to -89.75. I selected the
'interp.surface' function in the 'fields' package to do this as it provides
bilinear interpolation from one grid to another. I have written the following
code to try and carry out my interpolation...
> library(fields)
> x.mat <- sprintf("%.2f", seq(from = -179.5, to = 179.5, length=360)) #
> longitude grid values
> y.mat <- sprintf("%.2f", seq(from = 89.5, to = -89.5, length=180)) #
> latitude grid values
> z.mat <- read.table("BF_200501.txt", colClasses="numeric") # dataset
> z.mat <- data.matrix(z.mat)
> obj <- list(x=x.mat, y=y.mat, z=z.mat)
#Setting up new grid
> a.mat <- sprintf("%.2f", seq(from = -179.75, to = 179.75, length = 720)) #
> longitude grid values
> b.mat <- sprintf("%.2f", seq(from = 89.75, to = -89.75, length = 360)) #
> latitude grid values
> loc <- make.surface.grid(list(x=a.mat, y=b.mat))
#Interpolation
> new <- interp.surface(obj, loc)
However I get the following error message:
Error in x.new - min(x) : non-numeric argument to binary operator
Can anyone explain what this error message means and why I am getting it? Or
are there other more effective ways to perform the interpolation on this sort
of dataset? Like I said I'm new to R so any advice will be gratefully received.
Many thanks,
Andy
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