Can someone please EXPLAIN Drift Correction to me?? PLEASE!!?

Question

My Text book isn't too hot, & I'm really having alot of trouble finding anything good online, I can't even really find a definition of "Drift Correction". What i'm really trying to answer, is an old midterm question for my geophysics class, The question is: What is the reasons for drift correction of (a) gravity measurements & b) magnetism measurements. Thank you ahead of time, will definitely choose best answer!!

Answer 1

http://paces.geo.utep.edu/grav_database/grav_db_getstart.shtml#dc
In the field, gravimeters need recalibration because they "drift" as they are affected by temperature variations, fatigue in the internal springs, and even gravity "tides". Check out the site above for a longer explanation. This site will help you with the rest of the topics you're studying besides.
Another definition: drift correction compensates for effects on the recording instrument caused by temperature fluctuations and hysteresis. For gravity meters a small amount of mechanical drift occurs in the spring which must be removed from the data, this is generally accomplished by carrying out base ties.http://www.arkgeo.com/data/gloss.html#d

Apparently, magnetometer readings need "drift correction" as well. See this article that refers to instrumentation effects on the validity of data in a study: http://www-odp.tamu.edu/publications/189_IR/appen/c8_4.htm
Temperature can be a cause of magnetometer drift: http://ourworld.compuserve.com/homepages/demerson/magtherm.htm
Sorry that these sites don't give actual algorithms for correcting drift.

Answer 2

Hi. I think they are talking about the drift of the magnetic North Pole. It moves a bit over time and effects magnetic navigation, so it must be 'corrected' for. http://www.physorg.com/news8917.html

Answer 3

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml