Coordinates: are either B1950 or J2000 and for images brighter than about 1 magnitude above the plate limit, the majority have an internal accuracy of approximately 0.1 arcsec. Exceptions are stars brighter than about 11th magnitude and fully resolved galaxies. The external accuracy is currently governed by the density and magnitude range of the PPM astrometric standards used and is limited by residual plate distortions and magnitude- dependent terms to an average of 0.5 arcsec. Edges and corners of plates are often worse than this (x2), central regions generally much better (/2). The coordinate system is that of the "red" plate even for "blue"-only images.
Magnitudes: are given as "red" and "blue" plate magnitudes. For UKST survey plates "blue" = Bj (ie. 3900A-5400A passband) whilst "red" = OR (ie. 5900A-6900A passband). For POSS1 survey plates "blue" = O (ie. 3200A-4900A passband) whilst "red" = E (ie. 6200A-6800A passband). Internal accuracy for the majority of images brighter than 1 magnitude above the plate limit are typically 0.1 magnitude, with the same exceptions as above. External accuracy is currently about 0.3 magnitudes for the fainter images but can be much worse than this (ie. 1+ magnitudes) for bright images. For bright galaxies we recommend using the semi-major axis size as a measure of brightness, since the magnitude system used is based on an internal calibration of stellar profiles. All magnitudes are in the natural photographic system.
Classification: information comes in two parts, an integer flag interpreted as: -1 stellar; 0 noise-like; 1 non-stellar; 2 merged objects; and a "sigma". This defines how far from the stellar locus an object is in units of sigma using an approximation to an N(0,1) distribution [caveat: the stellar tails are more extended than in a real N(0,1)].
Ellipse parameters: are derived from the 3 intensity-weighted second moments of the images. In order: the semi-major axis radius, in 0.5 arcsec units, is scaled such that the ellipse area is the same as the detection isophotal area; the ellipicity e = 1-b/a (ie. a circle has e = 0 and a line has e = 1); and the angle in degrees is that made by the semi-major axis in a N through E sense (ie. with due N at 0, due E at 90). Undetected images in one of the passbands are flagged with "zeros".