title: Given a polygon in latitude and longitude and a set of points, determine which points lie inside
body: <p>Given a set of latlon points that make up a polygon, and a set of latlon points, how can I determine which points lie inside. The polygon may be up to 100km across and the error could be a few hundred meters (i.e. points inside or outside can fail or be included incorrectly at the edges). The polygons and points won't be near the poles. Can I treat the latlon points as 2d, or do I need to convert them to a projection of some kind? Circles are easy but I wonder if the error will be too great for a 100km wide polygon?</p>
  
  <p>I plan to do this in C++ but the language doesn't matter.</p>