The concept of mapping the Earth using two sets of parallel lines, one running from north to south and the other from east to west, was first utilized by the Greek Eratosthenes. Hipparchus, another Greek, was the first to use these lines as coordinates for specific locations.
Hipparchus was a sharp critic of Eratosthenes' geography and improved upon his grid system, using trigonometry to plot exact locations upon the grid. Both Greek cartographers were indebted to the ancient Phoenicians, for they were the first to determine latitude, or distance from the Earth's poles. The Phoenician method was based on astronomical observations. Longitude cannot be determined via such observations, so the problem of accurately determining longitude from any location on Earth continued right up until 1762, when English inventor John Harrison developed a method for determining longitude based upon highly precise timekeeping.