Add tests for theta_map function in test_transformations.py

Includes:
- Validation against synthetic model using finite differences and FFT
- Input error handling for 1D, 3D, and NaN-containing grids
- Integration of theta_map import for test discovery

Author: ErosKerouak

harmonica/tests/test_transformations.py

@@ -26,6 +26,7 @@
     gaussian_lowpass,
     reduction_to_pole,
     tilt_angle,
+    theta_map,
     total_gradient_amplitude,
     total_horizontal_gradient,
     upward_continuation,
@@ -737,6 +738,70 @@ def test_invalid_grid_with_nans(self, sample_potential):
         with pytest.raises(ValueError, match="Found nan"):
             tilt_angle(sample_potential)
 
+class TestThetaMap:
+    """
+    Test theta_map function
+    """
+
+    def test_against_synthetic(
+        self, sample_potential, sample_g_n, sample_g_e, sample_g_z
+    ):
+        """
+        Test theta_map function against the synthetic model
+        """
+        pad_width = {
+            "easting": sample_potential.easting.size // 3,
+            "northing": sample_potential.northing.size // 3,
+        }
+        potential_padded = xrft.pad(
+            sample_potential.drop_vars("upward"),
+            pad_width=pad_width,
+        )
+        theta_grid = theta_map(potential_padded)
+        theta_grid = xrft.unpad(theta_grid, pad_width)
+
+        trim = 6
+        theta_grid = theta_grid[trim:-trim, trim:-trim]
+        g_e = sample_g_e[trim:-trim, trim:-trim] * 1e-5  # SI units
+        g_n = sample_g_n[trim:-trim, trim:-trim] * 1e-5
+        g_z = sample_g_z[trim:-trim, trim:-trim] * 1e-5
+        g_thdr = np.sqrt(g_e**2 + g_n**2)
+        g_total = np.sqrt(g_thdr**2 + g_z**2)
+        g_theta = np.arctan2(g_thdr, g_total)
+        rms = root_mean_square_error(theta_grid, g_theta)
+        assert rms / np.abs(g_theta).max() < 0.1
+
+    def test_invalid_grid_single_dimension(self):
+        """
+        Check if theta_map raises error on grid with single dimension
+        """
+        x = np.linspace(0, 10, 11)
+        y = x**2
+        grid = xr.DataArray(y, coords={"x": x}, dims=("x",))
+        with pytest.raises(ValueError, match="Invalid grid with 1 dimensions."):
+            theta_map(grid)
+
+    def test_invalid_grid_three_dimensions(self):
+        """
+        Check if theta_map raises error on grid with three dimensions
+        """
+        x = np.linspace(0, 10, 11)
+        y = np.linspace(-4, 4, 9)
+        z = np.linspace(20, 30, 5)
+        xx, yy, zz = np.meshgrid(x, y, z)
+        data = xx + yy + zz
+        grid = xr.DataArray(data, coords={"x": x, "y": y, "z": z}, dims=("y", "x", "z"))
+        with pytest.raises(ValueError, match="Invalid grid with 3 dimensions."):
+            theta_map(grid)
+
+    def test_invalid_grid_with_nans(self, sample_potential):
+        """
+        Check if theta_map raises error if grid contains nans
+        """
+        sample_potential.values[0, 0] = np.nan
+        with pytest.raises(ValueError, match="Found nan"):
+            theta_map(sample_potential)
+
 
 class Testfilter:
     """