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67 lines
2.6 KiB
67 lines
2.6 KiB
<sect1 id="ai-geocoords">
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<sect1info>
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<author
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><firstname
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>Jason</firstname
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> <surname
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>Harris</surname
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> </author>
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</sect1info>
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<title
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>Geographic Coordinates</title>
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<indexterm
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><primary
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>Geographic Coordinate System</primary
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></indexterm>
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<indexterm
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><primary
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>Longitude</primary
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><see
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>Geographic Coordinate System</see
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></indexterm>
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<indexterm
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><primary
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>Latitude</primary
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><see
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>Geographic Coordinate System</see
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></indexterm>
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<para
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>Locations on Earth can be specified using a spherical coordinate system. The geographic (<quote
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>earth-mapping</quote
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>) coordinate system is aligned with the spin axis of the Earth. It defines two angles measured from the centre of the Earth. One angle, called the <firstterm
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>Latitude</firstterm
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>, measures the angle between any point and the Equator. The other angle, called the <firstterm
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>Longitude</firstterm
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>, measures the angle <emphasis
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>along</emphasis
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> the Equator from an arbitrary point on the Earth (Greenwich, England is the accepted zero-longitude point in most modern societies). </para
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><para
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>By combining these two angles, any location on Earth can be specified. For example, Baltimore, Maryland (USA) has a latitude of 39.3 degrees North, and a longitude of 76.6 degrees West. So, a vector drawn from the centre of the Earth to a point 39.3 degrees above the Equator and 76.6 degrees west of Greenwich, England will pass through Baltimore. </para
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><para
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>The Equator is obviously an important part of this coordinate system; it represents the <emphasis
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>zeropoint</emphasis
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> of the latitude angle, and the halfway point between the poles. The Equator is the <firstterm
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>Fundamental Plane</firstterm
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> of the geographic coordinate system. <link linkend="ai-skycoords"
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>All Spherical Coordinate Systems</link
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> define such a Fundamental Plane. </para
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><para
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>Lines of constant Latitude are called <firstterm
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>Parallels</firstterm
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>. They trace circles on the surface of the Earth, but the only parallel that is a <link linkend="ai-greatcircle"
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>Great Circle</link
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> is the Equator (Latitude=0 degrees). Lines of constant Longitude are called <firstterm
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>Meridians</firstterm
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>. The Meridian passing through Greenwich is the <firstterm
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>Prime Meridian</firstterm
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> (longitude=0 degrees). Unlike Parallels, all Meridians are great circles, and Meridians are not parallel: they intersect at the north and south poles. </para>
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<tip>
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<para
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>Exercise:</para>
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<para
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>What is the longitude of the North Pole? Its latitude is 90 degrees North. </para>
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<para
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>This is a trick question. The Longitude is meaningless at the north pole (and the south pole too). It has all longitudes at the same time. </para>
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</tip>
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</sect1>
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