  We describe two ground based observing campaigns aimed at building a grid of
approximately 200 spectrophotometric standard stars (SPSS), with an internal
~1% precision and tied to Vega within ~3%, for the absolute flux calibration of
data gathered by Gaia, the ESA astrometric mission. The criteria for the
selection and a list of candidates are presented, together with a description
of the survey strategy and the adopted data analysis methods. We also discuss a
short list of notable rejected SPSS candidates and difficult cases, based on
identification problems, literature discordant data, visual companions, and
variability. In fact, all candidates are also monitored for constancy (within
\pm5 mmag, approximately). In particular, we report on a CALSPEC standard,
1740346, that we found to be a delta Scuti variable during our short-term
monitoring (1-2 h) campaign.
