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Chapter 4: Geometry

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4.19.2.10 Summary of solution of oblique spherical triangles<br />

Given Solution Check<br />

Three sides Half-angle formulae Law of sines<br />

Three angles Half-side formulae Law of sines<br />

Two sides and<br />

included angle<br />

Two angles and<br />

included side<br />

Two sides and<br />

an opposite<br />

angle<br />

Two angles and<br />

an opposite side<br />

Napier’s analogies (to find sum and difference<br />

of unknown angles); then law of sines (to find<br />

remaining side).<br />

Napier’s analogies (to find sum and difference<br />

of unknown sides); then law of sines (to find<br />

remaining angle).<br />

Law of sines (to find an angle); then Napier’s<br />

analogies (to find remaining angle and side).<br />

Note the number of solutions.<br />

Law of sines (to find a side); then Napier’s<br />

analogies (to find remaining side and angle).<br />

Note the number of solutions.<br />

4.19.2.11 Haversine formulae<br />

Ú ½ Ó× <br />

×Ò ¾<br />

¾<br />

¾<br />

Ú´ µ · ×Ò ×Ò Ú <br />

×Ò´× µ ×Ò´× µ<br />

Ú <br />

×Ò ×Ò <br />

Ú Ú´ µ<br />

<br />

×Ò ×Ò <br />

Ú½¼ Æ ´ · µ℄· ×Ò ×Ò Ú <br />

Gauss’s<br />

formulae<br />

Gauss’s<br />

formulae<br />

Gauss’s<br />

formulae<br />

Gauss’s<br />

formulae<br />

4.19.2.12 Finding the distance between two points on the earth<br />

To find the distance between two points on the surface of a spherical earth, let point<br />

È ½ have a (latitude, longitude) of ´ ½ ½ µ and point È ¾ have a (latitude, longitude)<br />

of ´ ¾ ¾ µ. Two different computational methods are as follows:<br />

1. Let be the North pole and let and be the points È ½ and È ¾ . Then the<br />

spherical law of cosines for sides gives the central angle, , subtended by the<br />

desired distance:<br />

Ó×´µ Ó×´µÓ×´µ · ×Ò´µ×Ò´µ Ó×´µ<br />

where the angle is the difference in longitudes, and and are the angles of<br />

the points from the pole (i.e., ¼ Æ latitude). Scale by ʨ (the radius of the<br />

earth) to get the desired distance.<br />

2. In ´Ü Ý Þµ space (with ·Þ being the North pole) points È ½ and È ¾ are represented<br />

as vectors from the center of the earth in spherical coordinates:<br />

¢ £<br />

v ½ ¢ ʨ Ó×´ ½ µÓ×´ ½ µ ʨ Ó×´ ½ µ×Ò´ ½ µ ʨ ×Ò´ ½ µ £ <br />

v ¾ ʨ Ó×´ ¾ µÓ×´ ¾ µ ʨ Ó×´ ¾ µ×Ò´ ¾ µ ʨ ×Ò´ ¾ µ <br />

(4.19.3)<br />

© 2003 by CRC Press LLC

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