== Obsolete example == 

 "As a traveller cruises the east coast of the United States, for example, the declination varies from 20 degrees west (in Maine) to zero (in Florida), to 10 degrees east (in Texas)," 

 This example must be taken from a very old textbook or reference.  As the current NOAA map shows,   the magnetic declination anywhere on the coast of Texas is not 10 degrees east.    Around 60 years ago,  it was.