Location Finder - Longitude and latitude

I have an API with information giving me the longitude and latitude of lot's of services.

I want to be able to choose a region of where to see these services, and then I want to be able to get a map only of this small region where the location of these services are marked.

Since these regions are kind of small neighborhoods I will have to draw the outlines of the regions myself.

So what I was wondering was, draw up some regions, on google maps or similar service, then somehow get the measurements of what I have drawn, and then I could make calculations on the API information I get, to see which region the service lands based on its latitude and longitude.

Any ideas of how to implement this final step?

Finding the measurements of these "regions" and what calculations I could use to figure out if the long-lat ends up inside that region?

Thanks,