how to find whether a person is inside a building using gps co ordinates

I am working on a iphone GPS stuff. I have the 4 GPS co ordinates of a building. Is there a way to find out whether a particular GPS coordinates is inside this 4 co ordinates. That is to find out whether a p开发者_运维技巧erson is inside the building.

C code http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html

for google service http://www.mobileorchard.com/hello-there-a-corelocation-tutorial/

It's a pretty simple geometry problem. You know how to test if a point is within a triangle, right? The quadrilateral is just two triangles stuck together.