PostGIS is an extension of PostgreSQL. It claims that it has some geography features. I think it is not really related with a database. It brings only a function (HAVERSINE) that gets distance of two points on earth. It is couple of lines of code. It is not really a software project, but a detail about how the earth coordinates are calculated, and I think it is a total made up story. The real computed thing works like math.sqrt function.