GeoRSS (geo-extended RSS feed)

RSS is an old and still very much alive way to subscribe to updates: a program periodically fetches the feed and shows what is new. GeoRSS adds coordinates to the feed's items, and the feed turns into a stream of events on a map. That is how the USGS earthquake feeds and the NOAA storm warnings work.

Export to GeoRSS is chosen when a set of features is meant not so much for analysis as for distribution: showing points on a website, handing them to subscribers, opening them in Google Earth. It is the simplest way to give out geodata through a familiar web mechanism.

The file is an RSS 2.0 feed in which every feature is an item with a title and a description, and the geometry is written in the GeoRSS Simple encoding: georss:point, georss:line, georss:polygon. The coordinate order there is "latitude first, then longitude" — the reverse of what GeoJSON accustoms you to; the coordinate system is WGS84. Several layers are packed into a ZIP, one feed per layer.

The size limit is dictated by common sense: an XML feed with tens of thousands of items becomes unmanageable. GeoRSS is good for dozens and hundreds of features; for a whole city take GeoJSON.