Location-based applications require a user's movements and positions to provide customized services. However, location is a sensitive piece of information that should not be revealed unless strictly necessary. In this paper we propose a procedure that allows a user to control the precision in which his location information is exposed to a service provider, while allowing his location to be certified by a location verifier. Our procedure makes use of a hash chain to certify the location information in such a way that the hashes of the chain correspond to an increasing level of precision.