Method and system for storage and fast retrieval of digital terrain model elevations for use in positioning systems

The present invention relates to methods and apparatuses to generate compressed and indexed elevations of Digital Elevation Models. The invention provides a method to store elevation data, which includes the steps of compressing elevation data of a first portion of a Digital Elevation Model (DEM) to generate first compressed elevation data; storing the first compressed elevation data in a storage location pointed to by a first index; and storing the first index. The invention also provides a machine readable media containing executable program instruction which when executed by a digital processing system cause said system to store elevation data according to the above method. The invention further provides a digital processing system to store elevation data and a machine readable media containing a data stream representing a Digital Elevation Model.