  Spectra for 2D stars in the 1.5D approximation are created from synthetic
spectra of 1D non-local thermodynamic equilibrium (NLTE) spherical model
atmospheres produced by the PHOENIX code. The 1.5D stars have the spatially
averaged Rayleigh-Jeans flux of a K3-4 III star, while varying the temperature
difference between the two 1D component models ($\Delta T_{\mathrm{1.5D}}$),
and the relative surface area covered. Synthetic observable quantities from the
1.5D stars are fitted with quantities from NLTE and local thermodynamic
equilibrium (LTE) 1D models to assess the errors in inferred $T_{\mathrm{eff}}$
values from assuming horizontal homogeneity and LTE. Five different quantities
are fit to determine the $T_{\mathrm{eff}}$ of the 1.5D stars: UBVRI
photometric colors, absolute surface flux SEDs, relative SEDs, continuum
normalized spectra, and TiO band profiles. In all cases except the TiO band
profiles, the inferred $T_{\mathrm{eff}}$ value increases with increasing
$\Delta T_{\mathrm{1.5D}}$. In all cases, the inferred $T_{\mathrm{eff}}$ value
from fitting 1D LTE quantities is higher than from fitting 1D NLTE quantities
and is approximately constant as a function of $\Delta T_{\mathrm{1.5D}}$
within each case. The difference between LTE and NLTE for the TiO bands is
caused indirectly by the NLTE temperature structure of the upper atmosphere, as
the bands are computed in LTE. We conclude that the difference between
$T_{\mathrm{eff}}$ values derived from NLTE and LTE modelling is relatively
insensitive to the degree of the horizontal inhomogeneity of the star being
modeled, and largely depends on the observable quantity being fit.
