The accuracy with GPS becomes better and better. Right now there is an accuracy down to three meter. That means that you can mark control positions very good. Also the development of the european satellite navigation system Galileo (http://www.esa.int/esaNA/GGGMX650NDC_galileo_0.html
), that is compatible with GPS, will further improve the accuracy. At the same time there is a continuing development of GPS-sport-clocks that will improve the runners ability to log their route choices.
Because you can give control positions very accurate it arises the possibility to verify that the runner using GPS has visited the control positions. This application uses that fact.