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