some considerations:
- allow multiple activity types
- adapt KMZ format to lexicon: this will allow blobs to be stored separately from location data, and appviews to aggregate data without extra parsing
atprotocol-community has done some good work on standardizing geo types: https://wiki.atprotocol.community/en/working-groups/atgeo/experiments, but the KML format specifies a coordinate quite simply as a (lat, long, alt) triple.