title: Calculating area of a polygon drawn on google map
body: <p>I am looking for an accurate algorithm or a service to calculate surface area on earth where points are calculated on the basis of GPS Coordinates.</p>
  
  <p>I am  using Google Map Api version 3 and are drawing polygons based on recorded coordinates but I don't think the standard ways of calculating area of the polygon will take into account  of slopes(hills). Do I need to work on contours for such thing?</p>
  
  <p>Are there any third party services may be ArcGis or some other that takes into account of slopes as well.</p>