A reference beam generator (1c) for guiding a marking machine, which can be moved in relation to the reference beam generator (1c), for producing ground markings has a support element with at least three adjustable legs (2a, 2b) as positioning elements relative to the surface of the ground (EOB) and a source of electromagnetic radiation for generating a first and a second guide beam which is emitted with a prespecified opening angle in a fan-shaped manner. In this case, two legs (2a′) of the at least three adjustable legs (2a′, 2b) can be adjusted by an adjusting component in an associated manner in opposite directions.