GLONASS uses a coordinate datum named "PZ-90" (Earth Parameters 1990 – Parametry Zemli 1990), in which the precise location of the North Pole is given as an average of its position from 1900 to 1905. This is in contrast to the GPS's coordinate datum, WGS 84, which uses the location of the North Pole in 1984. As of September 17, 2007 the PZ-90 datum has been updated to version PZ-90.02 which differ from WGS 84 by less than 40 cm (16 in) in any given direction.

And this: Civilian GLONASS used alone is therefore very slightly less accurate than GPS. On high latitudes (north or south), GLONASS' accuracy is better than that of GPS due to the orbital position of the satellites.

Of course either system is dependent on accurate charting... while you may know right where you are, if you don't know right where the brown crusty stuff is, you could be in trouble. Remember to always refer to Mark 1 eyeballs for final data confirmation.