How does the Earth's magnetic field vary by altitude?

I’ve got a three-axis magnetometer and I read in the magnetic field strength in nanoTesla units in the x, y and z axis. I’m fully aware of magnetic declination (and compensate for it). I’m wondering how might the magnetic field vary by altitude?

All I’ve found is this: http://www.ngdc.noaa.gov/geomag/magfield.shtml

It seems to have the data but I’m looking for a general description of how it changes at a high level and access to the raw data so I can dynamically compute the values onboard an aircraft without an internet connection.