Add more tests for trend

test/detrend.jl

@@ -10,4 +10,32 @@
   D = Tables.matrix(values(d))
   R = Tables.matrix(values(r))
   @test isapprox(D, R, atol=1e-6)
+
+  rng = MersenneTwister(42)
+  # constant trend
+  d = georef((z=rand(rng, 100),), CartesianGrid(100))
+  z̄ = GeoStatsTransforms.trend(d, :z).z
+  @test all(abs.(diff(z̄)) .< 0.01)
+
+  # linear trend
+  μ = range(0, stop=1, length=100)
+  ϵ = 0.1rand(rng, 100)
+  d = georef((z=μ + ϵ,), CartesianGrid(100))
+  z̄ = GeoStatsTransforms.trend(d, :z).z
+  @test all([abs(z̄[i] - μ[i]) < 0.1 for i in 1:length(z̄)])
+
+  # quadratic trend
+  r = range(-1, stop=1, length=100)
+  μ = [x^2 + y^2 for x in r, y in r]
+  ϵ = 0.1rand(rng, 100, 100)
+  d = georef((z=μ + ϵ,))
+  d̄ = GeoStatsTransforms.trend(d, :z, degree=2)
+  z̄ = reshape(d̄.z, 100, 100)
+  @test all([abs(z̄[i] - μ[i]) < 0.1 for i in 1:length(z̄)])
+
+  d = georef((x=rand(rng, 10), y=rand(rng, 10)), rand(rng, 2, 10))
+  𝒯 = d |> GeoStatsTransforms.trend |> values
+  s = Tables.schema(𝒯)
+  @test s.names == (:x, :y)
+  @test s.types == (Float64, Float64)
 end