Depends if you are navigating by magnetic north or true north.
If you were using magnetic bearings with your GPS then you will be just as lost as relying on the compass with GPS bearings.
If some dope seriously compromised the thermometer's accuracy, say buy putting it near a radiative surface, then accuracy is lost and you have to throw the data out.
You can't average out the error and just subtract it from all the numbers. The data is useless - you cannot repair inaccurate data.
But they aren't 'correcting' for a simple cases like what you gave anyway. They are correcting whole areas not individual thermometers.