Geographical Map Pattern Java

I'd like to implement a grid (or layer or pattern) over a geographical map, somehow like that:

Grid of map of NYC

The goal is to get a fairly big amount of coordinates from the database and to assign those to a square of this pattern/grid. The map is not shown to the user afterwards, so it doesn't need to be pretty. What's the easiest and best way to accomplish that regarding performance and usability (for me, not the user)?

Thanks for any help!