Free SAT Practice Question

Map historian Idris Camara has argued that the coastal charts drawn by surveyors in the 1740s were far more accurate than their modest reputation suggests. ______ when he laid the old charts over present-day satellite imagery, the hand-drawn coastlines tracked the true shoreline to within a few dozen meters along nearly their whole length — closer, in places, than charts made a century later with better instruments.

Which choice completes the text with the most logical transition?