  Angle resolved photoemission spectroscopy (ARPES) has been commonly applied
to evaluate the shape of Fermi surfaces by employing simple criteria for the
determination of the Fermi vector k_F parallel to the surface such as maximum
photoemission intensity at the Fermi level or discontinuity in the momentum
distribution function. Here we show that these criteria may lead to large
uncertainties in particular for narrow band systems. We develop a reliable
method for the determination of Fermi vectors employing high resolution ARPES
at different temperatures.
  The relevance and accuracy of the method is discussed on data of the quasi
two dimensional system TiTe_2.
