Add note on projecting a grid to geographic coords

doc/user_guide/transformations.rst

@@ -7,7 +7,8 @@ Harmonica offers some functions to apply FFT-based (Fast Fourier Transform)
 transformations to regular grids of gravity and magnetic fields located at
 a constant height.
 
-In order to apply these grid transformations, we first need a regular grid.
+In order to apply these grid transformations, we first need a **regular grid in
+Cartesians coordinates**.
 Let's download a magnetic anomaly grid over the Lightning Creek Sill Complex,
 Australia, readily available in :mod:`ensaio`.
 We can load the data file using :mod:`xarray`:
@@ -33,6 +34,13 @@ And plot it:
  plt.colorbar(tmp, label="nT")
  plt.show()
 
+.. seealso::
+
+ In case we have a regular grid defined in geographic coordinates (longitude,
+ latitude) we can project them to Cartesian coordinates using the
+ :func:`verde.project_grid` function and a map projection like the ones
+ available in :mod:`pyproj`.
+
 Since all the grid transformations we are going to apply are based on FFT
 methods, we usually want to pad them in order their increase the accuracy.
 We can easily do it through the :func:`xrft.pad` function.