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### 3. Azimuth

Figure F-2 Azimuth Observation |

Equation F-4 is the angle of a line from the meridian in terms of a its coordinates.

Equation F-4 | |

A: |

The angle from the meridian can be to the right (+) or to left (-) depending on the coordinate differences signs. This places the angle in one of the four quadrants, Figure F-3.

Figure F-3 Angle from Meridian |

The value of Q_{AT}, added to the angle to obtain the azimuth, depends on the quadrant. Table F-1 shows the sign of α, its quadrant and Q_{AT} value.

Table F-1 Azimuth for a Line ij by Quadrant |

Partial derivatives of Equation F-4 are taken with respect to each coordinate of both points. The partial derivative for N_{F} is:

Remaining partial derivatives are:

Equation F-5 is the azimuth observation equation, evaluated at initial coordinate approximations.

Equation F-5 | ||

Equation F-6 |