VMT?
Did they use differential GPS? That would imply an astronomically determined local reference. Standard GPS doesn't have enough percision to compete with an astronomically determined meridian.
Gives a new meaning to "the meridian of God".
(The "Meridian of God" passes through Washington D.C. It refers to a small swath of the Earth's surface from which it would be possible, in principal, to construct a calendar based on simple arithemetic rules such that the Vernal Equinox would occur on the same calendar date for the forseeable future. A calendar that inserted leap years arbitarily to keep the VE on the same date doesn't count.)