GPS app with polygon area calculations

Basically I want the app to be able to import a KML file with a number of polygons on it. Once it has been imported then it will need to display where the user is on the map of polygons and the area of the polygons. Main concept for the app is to be able to divide an area as quickly and as easily as possible. The way I think would be the most user friendly is to allow them to run a finger over an outline of the polygon and have the app draw a 90 degree straight line across to the other side, displaying the area either side of the line. Users would also need to be able to adjust the line on either side of the polygon. I have a lot more ideas for additions and would also like it as an iPhone app in the future if all goes well with this. But for now I would just like to have a working version as basic as possible in order to test the concept to see how popular it would be.