diff --git a/setup.cfg b/setup.cfg
index 9464f8b8..ad32ebc6 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -54,6 +54,7 @@ docs = sphinx >= 1.4
     k3d >= 2.8.0
     mayavi >= 4.7.2
     wand
+fci = eudist
 
 [build_sphinx]
 project = $metadata.name
diff --git a/xbout/geometries.py b/xbout/geometries.py
index fa8a67b6..97003906 100644
--- a/xbout/geometries.py
+++ b/xbout/geometries.py
@@ -324,7 +324,7 @@ def _add_vars_from_grid(ds, grid, variables, *, optional_variables=None):
             # https://github.com/pydata/xarray/issues/4415
             # https://github.com/pydata/xarray/issues/4393
             # This way adds as a 'Variable' instead of as a 'DataArray'
-            ds[v] = (grid[v].dims, grid[v].values)
+            ds[v] = (grid[v].dims, grid[v].values, grid[v].attrs)
 
             _add_attrs_to_var(ds, v)
 
@@ -340,7 +340,7 @@ def _add_vars_from_grid(ds, grid, variables, *, optional_variables=None):
                     # Dataset, see https://github.com/pydata/xarray/issues/4415
                     # https://github.com/pydata/xarray/issues/4393
                     # This way adds as a 'Variable' instead of as a 'DataArray'
-                    ds[v] = (grid[v].dims, grid[v].values)
+                    ds[v] = (grid[v].dims, grid[v].values, grid[v].attrs)
 
                     _add_attrs_to_var(ds, v)
 
@@ -495,7 +495,16 @@ def add_s_alpha_geometry_coords(ds, *, coordinates=None, grid=None):
 @register_geometry("fci")
 def add_fci_geometry_coords(ds, *, coordinates=None, grid=None):
     assert coordinates is None, "Not implemented"
-    ds = _add_vars_from_grid(ds, grid, ["R", "Z"])
+
+    keys = sum(
+        [
+            [f"{pre}R", f"{pre}Z", f"{pre}xt_prime", f"{pre}zt_prime"]
+            for pre in ["forward_", "backward_"]
+        ],
+        [],
+    )
+
+    ds = _add_vars_from_grid(ds, grid, ["R", "Z"], optional_variables=keys + ["B"])
     ds = ds.set_coords(("R", "Z"))
     ds = _create_single_region(ds, periodic_y=True)
     return ds
diff --git a/xbout/tracing.py b/xbout/tracing.py
new file mode 100644
index 00000000..c0a526bb
--- /dev/null
+++ b/xbout/tracing.py
@@ -0,0 +1,182 @@
+import numpy as np
+import eudist
+
+from warnings import warn
+
+
+class OutOfDomainError(ValueError):
+    pass
+
+
+def _rz_to_ab(rz, mesh):
+    """
+    This functions finds the position (R-z) in the mesh and
+    returns the index and the relative coordinates [0,1] in that cell.
+    It uses a newton iteration.
+    """
+    ij = mesh.find_cell(rz)
+    if ij < 0:
+        raise OutOfDomainError()
+    _, nz = mesh.shape
+    i, j = ij // nz, ij % nz
+    ABCD = mesh.grid[i : i + 2, j : j + 2]
+    A = ABCD[0, 0]
+    a = ABCD[0, 1] - A
+    b = ABCD[1, 0] - A
+    c = ABCD[1, 1] - A - a - b
+    rz0 = rz - A
+
+    def fun(albe):
+        "The forwards function"
+        al, be = albe
+        return rz0 - a * al - b * be - c * al * be
+
+    def J(albe):
+        "The jacobian"
+        al, be = albe
+        return np.array([-a - c * be, -b - c * al])
+
+    tol = 1e-13
+    albe = np.ones(2) / 2
+    while True:
+        res = np.sum(fun(albe) ** 2)
+        albe = albe - np.linalg.inv(J(albe).T) @ fun(albe)
+        if res < tol:
+            return albe, ij
+
+
+def _ab_to_rz(ab, ij, mesh):
+    """
+    Calculate the position in cartesian R-z coordinates
+    given the relative position in the grid cell (ab) and
+    the grid indices ij.
+    """
+    _, nz = mesh.shape
+    i, j = ij // nz, ij % nz
+    A = mesh.grid[i, j]
+    a = mesh.grid[i, j + 1] - A
+    b = mesh.grid[i + 1, j] - A
+    c = mesh.grid[i + 1, j + 1] - A - a - b
+    al, be = ab
+    return A + al * a + be * b + al * be * c
+
+
+def _setup_mesh(x, y):
+    """
+    Setup the mesh and store some additional info.
+
+    The fci-mesh is assumed to periodic in z - but eudist does not
+    handle this so we need to copy the first corners around.
+    """
+
+    def make_periodic(d):
+        "The grid should be periodic in z"
+        return np.concatenate((d, d[:, :1]), axis=1)
+
+    assert x.dims == y.dims
+    assert x.dims == ("x", "z")
+    x = make_periodic(x.data)
+    y = make_periodic(y.data)
+    return _MyMesh(x, y)
+
+
+class _MyMesh(eudist.PolyMesh):
+    """
+    Like the PolyMesh but with extra data
+    """
+
+    def __init__(self, x, y):
+        super().__init__()
+        self.r = x
+        self.z = y
+        self.grid = np.array([x, y]).transpose(1, 2, 0)
+        self.shape = tuple([x - 1 for x in x.shape])
+
+
+class Tracer:
+    """
+    Use an EMC3-like tracing. This relies on the grid containing a
+    tracing to the next slice. The current position in RZ coordinates
+    is converted to the relative position in the grid, and then the
+    reverse is done for the end of the "flux tube" defined by corners
+    of the cells.
+    """
+
+    def __init__(self, ds, direction="forward"):
+        """
+        ds: xr.Dataset
+            a dataset with the needed FCI data from zoidberg.
+
+        direction: str
+            "forward" or "backward"
+        """
+        meshes = []
+        for yi in range(len(ds.y)):
+            dsi = ds.isel(y=yi)
+            meshes.append(
+                [
+                    _setup_mesh(dsi[f"{pre}R"], dsi[f"{pre}Z"])
+                    for pre in ["", f"{direction}_"]
+                ]
+                + [yi]
+            )
+        self.meshes = meshes
+
+    def poincare(self, rz, yind=0, num=100, early_exit="warn"):
+        """
+        rz : array-like with 2 values
+            The RZ position where to start tracing
+        yind : int
+            The y-index of the slice where to start tracing.
+        num : int
+            Number of rounds to trace for
+        early_exit : str
+            How to handle if we leave the domain before doing `num` rounds.
+            The possible values are:
+            "ignore" : do nothing
+            "warn": raise a warning
+            "plot" : try to plot the grid and where we leave
+            "raise" : Raise the exception for handling by the caller
+
+        Returns
+        -------
+        np.array
+        An array of shape (num, 2) or less then num if the tracing leaves the domain.
+        It contains the r-z coordinates at the y-index where tracing started.
+        """
+        rz = np.array(rz)
+        assert rz.shape == (2,)
+        thismeshes = self.meshes[yind:] + self.meshes[:yind]
+        out = np.empty((num, 2))
+        out[0] = rz
+        last = None
+        for i in range(1, num):
+            for d, meshes in enumerate(thismeshes):
+                try:
+                    abij = _rz_to_ab(rz, meshes[0])
+                except AssertionError as e:
+                    if early_exit == "warn":
+                        warn(f"early exit in iteration {i} because `{e}`")
+                    elif early_exit == "plot":
+                        m = meshes[0]
+                        import matplotlib.pyplot as plt
+
+                        plt.plot(m.r, m.z)
+                        if last:
+                            plt.plot(last[1].r.T, last[1].z.T)
+
+                        plt.plot(*rz, "o")
+                        plt.show()
+                    elif early_exit == "raise":
+                        raise
+                    else:
+                        assert early_exit == "ignore", (
+                            "early_exit needs to be one of "
+                            + '["warn", "plot", "raise", ignore"] '
+                            + f"but got `{early_exit}`"
+                        )
+                    return out[:i]
+                rz = _ab_to_rz(*abij, meshes[1])
+                last = meshes
+            out[i] = rz
+        return out