Contours

using Makie, GeoMakie, CairoMakie

lons = -180:180
lats = -90:90

-90:90

Create some field of values across lons and lats.

This grid can be of any density, but note that the time it takes to plot scales with the grid size!

field = [exp(cosd(l)) + 3(y/90) for l in lons, y in lats]

361×181 Matrix{Float64}:
 -2.63212  -2.59879  -2.56545  …  3.26788  3.30121  3.33455  3.36788
 -2.63206  -2.59873  -2.5654      3.26794  3.30127  3.3346   3.36794
 -2.6319   -2.59856  -2.56523     3.2681   3.30144  3.33477  3.3681
 -2.63162  -2.59828  -2.56495     3.26838  3.30172  3.33505  3.36838
 -2.63122  -2.59789  -2.56456     3.26878  3.30211  3.33544  3.36878
 -2.63072  -2.59738  -2.56405  …  3.26928  3.30262  3.33595  3.36928
 -2.6301   -2.59677  -2.56343     3.2699   3.30323  3.33657  3.3699
 -2.62937  -2.59603  -2.5627      3.27063  3.30397  3.3373   3.37063
 -2.62852  -2.59519  -2.56186     3.27148  3.30481  3.33814  3.37148
 -2.62756  -2.59423  -2.5609      3.27244  3.30577  3.3391   3.37244
  ⋮                            ⋱                             ⋮
 -2.62852  -2.59519  -2.56186     3.27148  3.30481  3.33814  3.37148
 -2.62937  -2.59603  -2.5627      3.27063  3.30397  3.3373   3.37063
 -2.6301   -2.59677  -2.56343     3.2699   3.30323  3.33657  3.3699
 -2.63072  -2.59738  -2.56405  …  3.26928  3.30262  3.33595  3.36928
 -2.63122  -2.59789  -2.56456     3.26878  3.30211  3.33544  3.36878
 -2.63162  -2.59828  -2.56495     3.26838  3.30172  3.33505  3.36838
 -2.6319   -2.59856  -2.56523     3.2681   3.30144  3.33477  3.3681
 -2.63206  -2.59873  -2.5654      3.26794  3.30127  3.3346   3.36794
 -2.63212  -2.59879  -2.56545  …  3.26788  3.30121  3.33455  3.36788

Makie provides two main recipes for contours = contour for lines, and contourf for fills. In this example, we'll see examples of both.

fig = Figure(size = (1000, 750), Contour = (; labelsize = 14, labelfont = :bold), Text = (; strokecolor = :gray, strokewidth = .3))
ax1 = GeoAxis(fig[1,1]; title = "Plain contour")
contour!(ax1, lons, lats, field)
fig

Makie also offers filled contours via the contourf recipe:

ax2 = GeoAxis(fig[1,2]; title = "Filled contour")
contourf!(ax2, lons, lats, field)
fig

The contour recipe also offers labels, which we can activate via keyword:

ax3 = GeoAxis(fig[2,1]; title = "Contour with labels")
contour!(ax3, lons, lats, field; labels = true)
fig

Finally, we can get a filled contour plot with labels by connecting the two:

ax4 = GeoAxis(fig[2,2]; title = "Filled contour with labels")
cfp = contourf!(ax4, lons, lats, field)
clp = contour!(
    ax4, lons, lats, field;
    color = :black, labels = true,
    levels = cfp._computed_levels
)
fig

In order to control the levels, we need only set the levels for the first filled contour plot:

cfp.levels[] = -2:5
fig

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