1 Introduction

The photographic data presented in this paper have been obtained in the context of the re-evaluation program of the Basel RGU three-color photometric high-latitude survey of the Galaxy. The motivation and purpose of this program are derived from the major fact that a suitable RGU standard system has become available only recently through the advancements in synthetic photometry, and that consequently, the survey plates can be exploited in a more homogeneous and consistent manner, employing more comprehensive calibrations and more adequate methods of analysis than before.

In particular, results based on preliminary data (e.g., Becker 1980; Buser & Kaeser 1985; Fenkart 1989a-d) were among the first to provide quantitative evidence of a highly flattened intermediate population of faint stars, which is now known as the thick disk component of the Galaxy. It thus became clear that the unparalleled statistical weight and multiplex nature of the Basel survey data immediately called for a definitive evaluation, aimed at a more precise determination of some of the key structural properties of this newly identified population component.

Therefore, the Basel Palomar-Schmidt plates were used for the preparation of a new catalog of homogeneous RGU data for a significant sample of high-latitude fields (Buser et al. 1998b), whence the density and luminosity distributions and, most importantly, also the larger-scale chemical structure of the Galactic thick disk were to be derived.

While the ensemble of the full survey data are being analysed by detailed modelling (cf. Buser & Rong 1995; Buser et al. 1998a), the data in each individual field will be used to discuss the most important aspects of both the particular field and its analysis in terms of the classical three-color method originally developped by Becker (1962, 1965). In the present paper on the Basel field near M101 we shall show how, in the absence of proper motion data, model calculations of the space densities should be used to remedy the notorious failure of the classical, purely photometric method in identifying and properly accounting for the presence of turnoff and subgiant stars among the field populations.