You measured something in some places, and you want a picture of it everywhere. guerrilla is a tour of the ways to do that, with the arithmetic left visible.
library(guerrilla)
xy <- cbind(quakes$long, quakes$lat)
depth <- -quakes$depth
op <- par(mfrow = c(1, 2), mar = c(2, 2, 2, 1))
plot(grid_barycentric(xy, depth), main = "grid_barycentric")
#> 2 duplicated coordinates collapsed with mean()
plot(grid_tps(xy, depth), main = "grid_tps")
par(op)Every method takes the same three things -- coordinates, values, and a grid to fill -- and gives back a grid. The grid is a list of four elements and nothing else:
str(unclass(grid_spec(xy)))
#> List of 4
#> $ dimension: int [1:2] 60 50
#> $ extent : num [1:4] 165.7 188.1 -38.6 -10.7
#> $ crs : NULL
#> $ values : NULLgrid_bin(), grid_voronoi(), grid_barycentric(), grid_idw(),
grid_tps(), grid_kriging(), grid_gam(), grid_smooth(), and
grid_gdal() for GDAL's own gridder. grid_tps(), grid_kriging() and
grid_gam() also take statistic = "se", which is usually the more
interesting picture:
op <- par(mfrow = c(1, 2), mar = c(2, 2, 2, 1))
plot(grid_tps(xy, depth), main = "what it thinks")
plot(grid_tps(xy, depth, statistic = "se"), main = "where it is guessing")
par(op)The goal is an easy introduction to interpolating irregular data, and to help find a simple answer rather than to ensure you do the most rigorous modelling possible. That part is your job.
So where a method is normally a call into compiled code, this package
also writes the arithmetic out in R next to it, and checks that the two
agree. bary_weights() is barycentric coordinates in six lines;
grid_barycentric(engine = "R") runs the whole interpolation that way
and matches the C to floating point. On this package's own data the
linear interpolation agrees, to floating point or exactly, across four
independent implementations: Qhull's tsearch(), the R engine here,
interp::interp(), and GDAL's gdal_grid.
The tools should not get in the way of exploring, and not everything spatial is geo-spatial. Nothing in the interpolation looks at a coordinate reference system.
# install.packages("remotes")
remotes::install_github("hypertidy/guerrilla")geometry, vaster, wk and geos are required. Everything else is
suggested and needed only by the method that uses it.
vignette("interpolating")-- the tour, all the methods on one dataset.vignette("grids")-- what a grid is, and how to hand one to another package.vignette("triangulation")-- barycentric weights, Delaunay and Voronoi.vignette("projection")-- what changes when the coordinates change, and how it all lines up with GDAL.
https://statnmap.com/2018-10-28-play-with-spatial-tools-on-3d-cells-images/
https://docs.qgis.org/2.8/en/docs/gentle_gis_introduction/spatial_analysis_interpolation.html
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