Google Maps Data API (via) I’m disappointed by this one—it’s really just a CRUD store for the KML files used in Google MyMaps. It would be a lot more useful if it let you perform geospatial calculations against your stored map data using some kind of query API—a cloud service alternative to tools like PostGIS.
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What kind of geospatial queries would you want to run? I'de be interested in building a service like that. It fits nicely inside the union of my day job (gogrid.com) and personal project (transitandtrails.org).