# Does Earth's magnetic field affect any datum definition?

I had an interview last week. One very interesting question was "does earth's magnetic field affect any datum definition?" I was sure it does not for geocentric datums. But I thought maybe the geodetic datums are affected by earth's magnetic field. But then, none of the physical parameters of a geodetic datum are related to Earth's magnetic field! And then I got confused!

Can anybody give the correct answer?

• Geodetic datums are determined ultimately by the earth's gravitational field, not its magnetic field. The two are (to an extremely high degree of precision) independent. – whuber Jul 10 '12 at 13:28
• @whuber Yes, I understand that a geodetic datum is defined from gravitational computations. But since its center is at earth's mass center, I am under the impression that the reversals of earth's magnetic field may affect earth's mass center position, which ultimately will lead to a subtle(?) change in a datum defining physical parameter. However that's only my guess. I could not find anything supporting this. May be because I am absolutely wrong!!! – thelastray Jul 11 '12 at 10:31
• Because the net magnetic charge on the earth is extremely close to zero, changes in the field will not measurably affect the earth's mass. The influence is actually in the other direction: movement of mass in the earth's core is believed to create its magnetic field and to change its dipole moment over time. Such movements may, very slightly, change the center of mass. – whuber Jul 11 '12 at 13:25

## 3 Answers

Other replies in this thread show that some specialized datums do depend on the earth's magnetic field. However, geodetic datums are determined ultimately by the earth's gravitational field, which establishes the "geoid" (an idealized "sea level," or contour shell of gravitational equipotential). The geoid is then approximated by an ellipsoid of revolution that is coaxial with the earth's axis of rotation and concentric with the earth's center of mass. The geoid is known to within a few centimeters and its ellipsoidal approximation is good everywhere to a few meters accuracy (and never worse than about 100 meters). (In contrast, the ellipsoid itself departs from a spherical model of the earth's surface by up to 23,000 meters in places.)

Because the net magnetic charge on the earth is extremely close to zero, any changes in the field will not measurably disturb the distribution of the earth's mass. The influence is actually in the other direction: electrical currents in the earth's outer core, the "geodynamo," are believed to create its magnetic field. Mass flow within the core can therefore change the magnetic field over time, including the direction of its dipole moment. It is possible that such movements in the core may, very slightly, change the earth's center of mass and thereby perturb the ellipsoid. I suspect that such movements are so slow and inconsequential that these effects cannot even be measured.

There are also geomagnetic datums (data?) out there which of course are affected by the movement of the magnetic poles. There was a related question about it just yesterday and it provides a concrete example:

How can I convert geomagnetic coordinates to geographic coordinates without doing the math myself?

• AFAIK there is nothing like a geomagnetic datum. They are Geomagnetic Coordinates. It is basically a Cartesian coordinate system with 3D. I also could not find any datum definition for Earth's geomagnetic Field on internet. At best I found International Geomagnetic Reference Field which is not a datum – thelastray Jul 11 '12 at 10:12
• @mmdemirbas What exactly you have edited? – thelastray Jul 12 '12 at 13:10
• he just made the link appear with the title instead of the url — you can see it for yourself if you click on the "edited x hours ago" link. – lynxlynxlynx Jul 12 '12 at 13:21
• Yes, a little readability added. – mmdemirbas Jul 12 '12 at 13:58

AFAIK PaleoLatitude is always measured with respect to the magnetic field, as inferred from paleomagnetism.