This is the first in a series of two papers that address the problem of the physical nature of luminosity classification in the late A-, F-, and early G-type stars. In this paper, we present precise spectral classifications of 372 stars on the MK system. For those stars in the set with Stroemgren uvby{beta} photometry, we derive reddenings and present a calibration of MK temperature types in terms of the intrinsic Stroemgren (b-y)_0_ index. We also examine the relationship between the luminosity class and the Stroemgren c_1_ index, which measures the Balmer jump. The second paper will address the derivation of the physical parameters of these stars, and the relationships between these physical parameters and the luminosity class. Stars classified in this paper include one new {lambda} Bootis star and 10 of the F- and G-type dwarfs with recently discovered planets.