xisoband

The goal of xisoband is to make isoband a bit more generally useable.

Example

This example shows how to get the output of isoband as a list of lists of matrices, and to simply plot it.

We avoid

• creating redundant rectilinear vectors of $x and $y
• requirement to specify levels (we auto them)
• having to convert from iso type to sf to finish the job

library(xisoband)
e <- whatarelief::elevation()
#> [1] "/vsicurl/https://public.services.aad.gov.au/datasets/science/GEBCO_2021_GEOTIFF/GEBCO_2021.tif"
plot(iso <- xisoband::xisobands(e, extent = c(-180, 180, -90, 90)))
#> [1] 256 512
maps::map(add = TRUE, col = "firebrick", fill = FALSE)

ximage::ximage(e, extent = c(-180, 180, -90, 90))
plot(iso, add = TRUE)

We are using the hypertidy raster reader elevation() to get GEBCO 2021 elevation in a matrix, and ximage to plot it. The ‘hypertidy way’ is to specify extent directly when it is required, not a mix of xmin,xmax or xlim,ylim, or the degenerate rectlinear model used by image().

The orientation is raster-order, which is used by the GDAL packages and rasterImage(). There is a plot method for isoband,iso objects.

We otherwise would get an unuseable list of grouped x,y,id lists from isoband::isobands(), or a list of sfg geometries (matrices in a list with a class for sf) with iso_to_sfg() that further processes the isobands() output .

The plot method uses graphics::lines() currently, but could use the vectorized grid package plotting. It seems funny that isoband currently returns an output that requires further clean up, and converts to sf.

If extent is not provided we treat the matrix as living in 0:ncol,0:nrow space.

We might contribute changes to isoband to clean this up, but first I needed to be able to explore what’s already there.

A rough set of goals:

• add vectorized grid package plotting
• separate the raw isobands() output from the cleaned up version (in the same format)
• separate the conversion to sf
• put the geometry clean up deeper in the code, why do we want the raw isobands() output?, for example
• record the grid attirbutes (dimension and extent) on the output
• record the auto generated hi/lo levels

A simpler example, this shows the simplicity of not requiring even extent as input, we simply assume the data lives in grid space, and we have a convenient way to choose just one level (or none, but choosing here gives a good result).

m <- diag(5)
(x <- xisobands(m, lo = .5))
#> [1] 5 5
#> $`0.5:1`
#> $`0.5:1`$x
#>  [1] 0.5 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 4.5 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0
#> [20] 0.5 2.5 2.5 2.5 2.5 3.5 3.5 3.5 3.5 1.5 1.5 1.5 1.5
#> 
#> $`0.5:1`$y
#>  [1] 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.5 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
#> [20] 4.5 2.5 2.5 2.5 2.5 1.5 1.5 1.5 1.5 3.5 3.5 3.5 3.5
#> 
#> $`0.5:1`$id
#>  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4
#> 
#> 
#> attr(,"class")
#> [1] "isobands" "iso"
ximage::ximage(m)
plot(x, add = TRUE)

Code of Conduct

Please note that the xisoband project is released with a Contributor Code of Conduct. By contributing to this project, you agree to abide by its terms.