The Mercator map projection is the basic map projection, where there is minimal distortion at and around the equator but countries like Greenland and Antarctica look very large.  This invention may seem obvious, but in fact it is not.  It is specifically centered on the equator, so the Chinese would have had to know the exact location of the equator and their location relative to it.  This is an extremely difficult task, given that their instruments for measuring were far less accurate than those we have today.

Date of Invention:
940 AD

Inventor: 
Unknown

Importance to Chinese civilization:
The projection demonstrated advanced geographical knowledge.  The maps that it was used to produce helped oceanic travelers plot and follow courses to wherever they wanted to go.  This helped increase trade and encourage exploration.

Importance to other civilizations:
Was used to produce maps that helped other civilizations in the same way they helped the Chinese.