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### 3. Distance Reduction

An EDM measures the line of sight distance between the instrument and reflector. This is a *slope distance*. Figure F-7, and not horizontal unless the EDM and reflector are at the same elevation.

Figure F-7 Slope Measurement |

In order to determine a horizontal or vertical distance additional information is needed. Combining an EDM with a digital theodolite results in a *Total Station Instrument* (TSI). The TSI measures the slope distance and a zenith angle from which it computes a horizontal and vertical distance, Figure F-8.

Figure F-8 Slope Reduction |

H = S x sin(Z) | Equation F-2 |

V = S x cos(Z) | Equation F-3 |

It's a *little* more complex than this and we'll discuss a refinement in a bit.