Thank you for your answer. It's 3D GPS.
If I understand, the application isn't really compatible with a stock Iphone 3G. I wish it was more clear when I bought the app.

Hello, I was driving at LeMans yesterday and I tried Harry Laptimer Professional for the first time. Apparently I couldn't get a decent gps signal and it didn't recorded any lap. My 3G connexion was fine but I had only 1 red bar on the application with the internal gps. I drive a Lotus Elise and eve...