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## 3. Parallax Equations

### a. Ground Coordinates

The ground coordinate system is similar to that of a single vertical photo. The origin and axes directions are defined by the left photo, Figure E-9, using the actual flight direction.

Figure E-9 Ground Coordinate System |

Coordinates and elevation of ground points can be determined from parallax measurements using equations E-4 through E-6.

Equation E-4 | |

Equation E-5 | |

Equation E-6 |

### b. Elevations by Parallax

Figure E-10 shows the basic parallax and elevation geometry for two points.

Figure E-10 Parallax and Elevation Difference |

If we can measure the parallax of both points, we can determine one point's elevation based on the other, Equation E-7.

Equation E-7 |