Databases that support spatial queries are often limited to three dimensions, but the requirements increase. You might want to query in more dimensions, for time ranges or other attributes like trajectories. Documents are represented as JSON. The values that will be stored in the index can be extracted from anywhere within such a JSON document. Even conversions like reprojections are possible. Apache CouchDB and Couchbase are document databases, hence belong to the non-relational space which is also known as “NoSQL”. One of the strengths of Apache CouchDB is the (multi-master) replication. You can keep the data from several different instances easily in sync, even if you change the data on different instances. The replication isn't limited to Apache CouchDB, but it's a whole ecosystem. It's even possible to sync with your web browser and store it in its offline storage. This way the user can access the data offline, without the need to be always connected to the server. In contrast Couchbase has its strong point in working at scale. The data gets automatically sharded across machines. Adding and removing servers at a later stage can be performed through a simple web interface. If a server goes down the system can still work without any interruptions. GeoCouch, Apache CouchDB and Couchbase are open source and licensed under the Apache License 2.0.