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<TITLE>Convergence & Declination FYI</TITLE>
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<P><FONT SIZE=4 FACE="Arial">As most of you who work in the FDEP Albers Projection know, true north and grid north (the north on the map sheet) only agree at 84° longitude. Since the Albers Projection is a conic projection, at other longitudes the map's "north" is tilted slightly. I wanted to be able to show true north, grid north and magnetic north on some of my maps but didn't know how to calculated the convergence or declination for these other "norths".</FONT></P>
<P><FONT SIZE=4 FACE="Arial">It turns out that the declination for magnetic north is difficult to calculate, but the NOAA has a website that will give you the correction factor and the annual rate of change for anywhere in the world. This is found at:</FONT></P>
<P><A HREF="http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp"><U><FONT COLOR="#0000FF" SIZE=4 FACE="Arial">http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp</FONT></U></A>
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<P><FONT SIZE=4 FACE="Arial">It will calculate it for a given zip code or lat/long.</FONT>
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<P><FONT SIZE=4 FACE="Arial">Next, I thought that the true north -- magnetic north would also be hard to calculate. But after playing with it for awhile I found out that, in fact, it is a constant ratio per degree of shift from the Albers' "Central Meridian" of 84°. It turns out that the convergence factor for the FDEP Albers projection is 0.464631517995°/° of longitude. The convergence is negative (towards the west) if the longitude is < 84° and positive (towards the east) if the longitude is > 84°.</FONT></P>
<P><FONT SIZE=4 FACE="Arial">So for a city located on the east coast at 81° longitude, true north would be at an angle of 1.393894554° west of grid north. Don't forget that the convention is usually to make these corrections for the center point of your map.</FONT></P>
<P><B><FONT SIZE=5 FACE="Monotype Corsiva">-- John</FONT></B>
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