  We describe the Zonal Atmospheric Stellar Parameters Estimator (ZASPE), a new
algorithm, and its associated code, for determining precise stellar atmospheric
parameters and their uncertainties from high resolution echelle spectra of
FGK-type stars. ZASPE estimates stellar atmospheric parameters by comparing the
observed spectrum against a grid of synthetic spectra only in the most
sensitive spectral zones to changes in the atmospheric parameters. Realistic
uncertainties in the parameters are computed from the data itself, by taking
into account the systematic mismatches between the observed spectrum and the
best-fit synthetic one. The covariances between the parameters are also
estimated in the process. ZASPE can in principle use any pre-calculated grid of
synthetic spectra. We tested the performance of two existing libraries (Coehelo
et al. 2005, Husser et al. 2013) and we concluded that neither is suitable for
computing precise atmospheric parameters. We describe a process to synthesise a
new library of synthetic spectra that was found to generate consistent results
when compared with parameters obtained with different methods (interferometry,
asteroseismology, equivalent widths).
