At best GPS is around 3 feet off. BUT it can be as much as 100 Yards. This depends on the system and on the WAAS Land Towers staying operational ( Land towers mark the drift of the satellites as their orbit can be off). IF they go down the drift of the Satellites can be off. Weather can affect these land based towers as well I would imagine. That weather could be in New England or Texas Depending on where the Towers are that help the Satellites to transmit their signals..