Detecting whether a GPS coordinate falls within a polygon on a map

Question

As stated in the title, the goal is to have a way for detecting whether a given GPS coordinate falls inside a polygon or not.

The polygon itself can be either convex

Answer 1

If you have WGS84 coordinates on the sphere, then your polygon divides the sphere into two areas - how do we know which area is 'inside' and which is 'outside' the polygon? The question is essentially meaningless!

For example, suppose the polygon formed the line of the equator - is the Northern hemisphere 'in' or 'out'?