Merge branch 'geodetic' into 'master'

Add reference to ISO [skip ci].

See merge request !14

pymagglobal/core.py

@@ -550,7 +550,7 @@ def field(z_at, splines, cov_splines=None, field_type='nez', check=True,
     field = np.einsum('ij, ijk -> jk',
                       coeffs,
                       spatial).T
-
+    # The "algorithm" for transformation can be found in ISO 16695.
     if out_gd:
         angs = z_at[0] - colats
         rot_mats = _rot_mat(angs)