From MSDN
http://msdn.microsoft.com/en-us/library/aa578799.aspx
Latitude measures how far north or
south of the equator a place is
located. The equator is situated at
0°, the North Pole at 90° north (or
90°, because a positive latitude
implies north), and the South Pole at
90° south (or –90°). Latitude
measurements range from 0° to
(+/–)90°.Longitude measures how far east or
west of the prime meridian a place is
located. The prime meridian runs
through Greenwich, England. Longitude
measurements range from 0° to
(+/–)180°.
In your setter for latitude, check if the value being set falls between -90 and 90 degrees. If it doesn’t, throw an exception. For your longitude, check to see if the value falls between -180 and 180 degrees. If it doesn’t, throw an exception.