XY positions have been taken from the literature as far as possible, but many fields have been measured on a Hipad Plus digitizing tablet (Houston Instruments), which gave its name (hpd) to this datatype.
Magnitudes are usually V magnitude, but could be mpg or B, if nothing else
There is no reference for this kind of data, but information on the
data source and the unit scale, i.e. how many units to get one arc minute.