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, B or J, 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.