Complex sentence: Tacheometry is the science of measuring distances by measuring the angle between two ends of an object with a known size, and was sometimes used prior to the invention of EDM where rough ground made chain measurement impractical.
Split into simpler sentences: Tacheometry is the science of measuring distances by measuring the angle between two ends of an object with a known size. It was sometimes used prior to the invention of EDM where rough ground made chain measurement impractical.