In the 70 cm band:
(1) No amateur station shall transmit from north of Line A in the 420-430 MHz segment. See § 97.3(a) for the definition of Line A.
Line A. Begins at Aberdeen, WA, running by great circle arc to the intersection of 48° N, 120° W, thence along parallel 48° N, to the intersection of 95° W, thence by great circle arc through the southernmost point of Duluth, MN, thence by great circle arc to 45° N, 85° W, thence southward along meridian 85° W, to its intersection with parallel 41° N, thence along parallel 41° N, to its intersection with meridian 82° W, thence by great circle arc through the southernmost point of Bangor, ME, thence by great circle arc through the southernmost point of Searsport, ME, at which point it terminates.
The definition of Line A in Part 97 originates from a 1965 treaty between the USA and Canada, and there is established precedent for specific coordinates to be used for the cities mentioned in § 97.3(a).
The FCC's Office of Engineering Technology has several web pages giving information about Line A, including a 1989 memo posted at transition.fcc.gov/oet/info/maps/canline/linea.html which recommends that "all agencies of the Federal Government" use the following coordinates:
|Westernmost Pt||46-37-00 N||125-00-00 W|
|Aberdeen, WA||46-58-00 N||123-50-00 W|
|Duluth, MN||46-36-00 N||92-10-00 W|
|Bangor, ME||44-46-00 N||68-47-00 W|
|Searsport, ME||44-27-00 N||68-55-00 W|
|Easternmost Pt||44-00-00 N||68-40-00 W|
Also, transition.fcc.gov/oet/info/maps/canline/ gives a download link for canline.zip which contains a list of coordinates tracing Line A, including the great circle arcs. The coordinates shown on the map here come from ./LINE.A/Line_A.MID inside of canline.zip.
Regarding Great Circle Arcs
For Line A, there is a potential for confusion involving the difference between rhumb lines and great circle arcs.
The issue is that on Mercator projection maps, straight lines are rhumb lines, not great circles. Rhumb lines on Mercator maps are useful for navigating by following a constant compass bearing, and that's a big part of why the Mercator projection is so popular. The catch is that the rhumb line between two points can be considerably longer, and follow a very different course, than the corresponding great circle arc.
For Seattle and Line A, the area bounded on the north by the great circle arc from Aberdeen to 48° N 120° W, and on the south by the single corresponding rhumb line, encloses a substantial chunk of the metro region. The FCC's Line A coordinates given in canline.zip approximate the great circle arc from Aberdeen as two rhumb lines, with the intermediate point located north of Redmond and west of Duvall. If you were to approximate the great circle arc from Aberdeen with a single rhumb line, it would pass south of the Sea-Tac Airport, near Kent.
For more background information on rhumb lines, map projections, and such, I recommend "The American Practical Navigator", which is available as a free download from the United States National Geospatial Intelligence Agency through their Maritime Safety Information portal.
The excerpts of US Title 47 CFR § 97 (Part 97) came from www.eCFR.gov and were current as of November 9, 2012.