Here's a short story:

A geographic coordinate system is a coordinate system used in geography that enables every location on Earth to be specified by a set of numbers, letters or symbols. The coordinates are often chosen such that one of the numbers represents a vertical position, and two or three of the numbers represent a horizontal position. A common choice of coordinates is latitude, longitude and elevation. 

To specify a location on a two-dimensional map requires a map projection. 

The invention of a geographic coordinate system is generally credited to Eratosthenes of Cyrene, who composed his now-lost "Geography" at the Library of Alexandria in the 3rd century BC. A century later, Hipparchus of Nicaea improved on this system by determining latitude from stellar measurements rather than solar altitude and determining longitude by timings of lunar eclipses, rather than dead reckoning. In the 1st or 2nd century, Marinus of Tyre compiled an extensive gazetteer and mathematically-plotted world map using coordinates measured east from a prime meridian at the westernmost known land, designated the Fortunate Isles, off the coast of western Africa around the Canary or Cape Verde Islands, and measured north or south of the island of Rhodes off Asia Minor. Ptolemy credited him with the full adoption of longitude and latitude, rather than measuring latitude in terms of the length of the midsummer day. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: What is a common choice of coordinates?
Guess: latitude, longitude and elevation

Question: Define a geographic coordinate system?
Guess: a coordinate system used in geography that enables every location on Earth to be specified by a set of numbers, letters or symbols.

Question: and how are the coordinates chosen?
Guess: 1 number is for vertical, 2-3 for horizontal

Question: What is required to specify a location in a 2d map?
Guess: map projection

Question: Who is credited for the invention of the coordinate system?
Guess: Eratosthenes

Question: In what century?
Guess: 3rd century BC

Now answer the following question in the same format.

Question: Who improved it later?

Hipparchus