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.

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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.

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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.