Subject: What is the Earth's magnetic field?

I have been looking into magnetic shielding for my research 
project, and in my reading I have come across several different 
values for the magnitude of the Earth's magnetic field, ranging 
from 0.1 to over 1 Gauss.  I am curious if this might be a 
result of position on the Earth.  Does magnetic field vary with 
latitude?  Also, does it vary with altitude?  Is there any 
formula to quantify the variation?  To your knowledge, what is 
the accepted value (in Gauss) of the Earth's magnetic field? (at 
sea level and 49 degrees North latitude)

Thanks.

Re: What is the Earth's magnetic field?