Convert XYZ to stars curvilinear grid and plot

Question

I have data in XYZ format (long, lat, elevation) that does not fit on a rectangular grid, but I would like to plot it as grid cells of different sizes.

To illustrate what I mean, the last two plots in this stars vignette illustrate exactly what I would like to do: https://r-spatial.github.io/stars/articles/stars4.html

In that vignette, they are able to plot the data as points, but then they are able to plot the data as cells of different sizes, which essentially fills in the gaps.

But I am having a hard time figuring out how to convert the data into a stars object and plot. An example of one of my files can be downloaded here: https://www.dropbox.com/s/u230o28109hhxee/elevation_8.0.dat?dl=1

Here is what I have so far:

library(stars)
library(sf)

f <- 'elevation_8.0.dat'

temp <- read.table(f)
colnames(temp) <- c('x','y','alt')
sfObj <- st_as_sf(temp, coords=1:2, crs=st_crs(4326))

plot(sfObj, cex=0.15)

dat <- st_as_stars(sfObj)

dat
> dat
stars object with 1 dimensions and 1 attribute
attribute(s):
      alt          
 Min.   :-4895.79  
 1st Qu.:-1813.55  
 Median : 1191.53  
 Mean   :   48.76  
 3rd Qu.: 1829.30  
 Max.   : 3702.25  
dimension(s):
         from    to offset delta                       refsys point
geometry    1 43990     NA    NA +proj=longlat +datum=WGS8...  TRUE
                                                        values
geometry POINT (-128.867 42.1795),...,POINT (-102.176 28.3599)


plot(dat, as_points = TRUE)

Error in .image_scale(values, colors, breaks = breaks, key.pos = key.pos,  : 
  object 'mar' not found

Answer 1

After reading the data into temp, construct matrix of X and Y coordinates and data:

> x = matrix(temp$x,nrow=265, ncol=166)
> y = matrix(temp$y,nrow=265, ncol=166)
> am = matrix(temp$alt, nrow=265, ncol=166)

Now construct an unreferenced stars object from the data matrix:

> s = st_as_stars(am)

This has coordinate dimensions called X1 and Y1:

> s
stars object with 2 dimensions and 1 attribute
attribute(s):
      A1           
 Min.   :-4895.79  
 1st Qu.:-1813.55  
 Median : 1191.53  
 Mean   :   48.76  
 3rd Qu.: 1829.30  
 Max.   : 3702.25  
dimension(s):
   from  to offset delta refsys point values    
X1    1 265      0     1     NA FALSE   NULL [x]
X2    1 166      0     1     NA FALSE   NULL [y]

Replace those with the x and y matrices:

> s = st_as_stars(s, curvilinear=list(X1=x,X2=y))

to get....

> s
stars object with 2 dimensions and 1 attribute
...
                            values    
X1 [265x166] -128.867,...,-102.157 [x]
X2   [265x166] 25.3431,...,44.8624 [y]
curvilinear grid

a curvilinear grid! Plot:

> plot(s, as_points=FALSE, axes=TRUE, breaks="equal", border=NA)

I'm no stars expert (first time I've tried it) so there may be a smoother way. This all relies on the order of your data in your file being in the right order to go into the matrix correctly (and I had to infer the dimension of your data by factoring 43990 rows into two integers...). If the data was down the curvilinear grid or right to left then you might have to flip or transpose the matrices. Maybe.