The transit of Venus is an astronomical phenomenon. But geography was needed to make use of it.
Astronomers had to determine not just when a transit would take place, but also from where it could best be viewed. The more widely-spaced the observing stations, the better the resulting value for the Earth-Sun distance.
When observers on expeditions finally reached remote sites, much of their work was to establish exactly where they were. The necessary astronomical instruments were at least as important as those used to view the transit itself.
Both latitude and longitude had to be determined with extreme care. The longitude was particularly demanding, requiring the independent observation of eclipses, lunar distances and the moons of Jupiter.