Thanks a lot man i will find the sensor and start testing.
I'm starting my first real professional project my client wants to have a map of a parking lot in a mobile app and see through the app wich ones are available and wich ones aren't.
At first i thought to use a water resistant ultrasonic sensor (hcsr04t) to measure from top to ground but this is a very ugly solution because of the million poles i would have to put over all of the parking spots to hold the sensor, so that´s why i'm looking for a ground solution.
Any other comments would be greatly appreciated.