  For an understanding of Galactic stellar populations in the SDSS filter
system well defined stellar samples are needed. The nearby stars provide a
complete stellar sample representative for the thin disc population. We compare
the filter transformations of different authors applied to the main sequence
stars from F to K dwarfs to SDSS filter system and discuss the properties of
the main sequence. The location of the mean main sequence in colour-magnitude
diagrams is very sensitive to systematic differences in the filter
transformation. A comparison with fiducial sequences of star clusters observed
in g',r',i' show good agreement. Theoretical isochrones from Padua and from
Dartmouth have still some problems especially in (r-i)-colour.
