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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 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) | Equation A-2 | |
| V - S x cos(Z) | Equation A-3 |
It's a little more complex than this and we'll discuss a refinement in the Errors chapter.