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.
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..
|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..