Progressive map maintenance at a mobile navigation unit

A system, comprising:

• a hardware-based processing unit;
• and
• a non-transitory computer-readable storage device comprising code that, when executed by the processing unit, causes the processing unit to perform operations comprising:
• receiving baseline map data, including a baseline number of prioritized map features for each of a plurality of road segments, from a remote map-data source;
• receiving first supplemental map data, wherein the first supplemental map data includes a supplemental number of prioritized map features for at least some of the plurality of road segments, wherein the at least some of the plurality of road segments are selected for receiving the supplemental number of prioritized map features based on a measure of visitation of a mobile navigation unit to the at least some of the plurality of road segments being above a threshold amount of the measure of visitation;
• determining whether prioritized map features necessary for a present mobile navigation-unit function along selected ones of the plurality of road segments are found in at least one of the baseline map data and the first supplemental map data, yielding a map-sufficiency determination;
• using, if the map-sufficiency determination is affirmative, the baseline map data and the first supplemental map data for the present mobile navigation-unit function along the at least some of the plurality of road segments;
• and
• if the map-sufficiency determination is negative:
• determining navigation-unit movement characteristics;
• obtaining, based on the navigation-unit movement characteristics, second supplemental map data;
• and
• using at least the second supplemental map data for the present mobile navigation-unit function.