Multi-level altitude map matching

An apparatus comprising:

• at least one processor;
• and
• at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform and/or to control:
• obtaining or gathering sample measurements, wherein one or more of the sample measurements are or comprise Global Navigation Satellite System, GNSS, available sample measurements, the one or more GNSS-available sample measurements respectively comprising information indicating that at least one GNSS signal is or was available at a respective location;
• matching, with a GNSS-availability altitude map, one or more of the GNSS-available sample measurements, wherein one or more of the respective locations are represented by the GNSS-availability altitude map;
• and
• based at least on the matching, determining or correcting altitude information respectively associated with one or more of the sample measurements.

A tangible computer-readable medium storing computer program code, the computer program code when executed by a processor causing an apparatus to perform and/or control:

• obtaining or gathering sample measurements, wherein one or more of the sample measurements are or comprise Global Navigation Satellite System, GNSS, available sample measurements, the one or more GNSS-available sample measurements respectively comprising information indicating that at least one GNSS signal is or was available at a respective location;
• matching, with a GNSS-availability altitude map, one or more of the GNSS-available sample measurements, wherein one or more of the respective locations are represented by the GNSS-availability altitude map;
• and
• based at least on the matching, determining or correcting altitude information respectively associated with one or more of the sample measurements.