3. Distance Reduction

An EDM measures the line of sight distance between the instrument and reflector. This is a slope distance. Figure A-8, and not horizontal unless the EDM and reflector are at the same elevation.

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Figure A-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 A-8..

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Figure A-8
Slope Reduction


H = S x sin(Z)         Eqn (A-2)
 V - S x cos(Z)        Eqn (A-3) 


It's a little more complex than this and we'll discuss a refinement in the Errors chapter..