I like to think I'm relatively well-versed in datums, projections, and coordinate reference systems.
OpenStreetMap data is stored in WGS84 Lat/Lon (EPSG:4326). This CRS is geographic and therefore intended to store locations on a 3D globe. When I see this map what am I looking at?
Looking at Reykjavik it appears that the longitude lines are tending towards the North Pole at the Greenwhich Meridian (or maybe they planned the city in perspective view??) so I assume 0,90 is top-centre of the map.
Does this class as a projection, and if so which one?
Is there one projection that is commonly used for lat/lon data?
I have data stored in Oracle using EPSG:4326 and when rendered by GeoServer without a specified projection it also displays these characteristics.
Answer
OpenLayers uses the term 'EPSG:4326' to mean the Plate Caree projection. Referring to 'WGS84' and EPSG:4326 as a projection has been common for so long that it is a source of confusion. This short-hand has been going on since before Google and OpenLayers came on to the scene. For instance, ESRI have been fudging the terms for as long as I can remember. OpenLayers does not reproject to EPSG:900913 on the fly unless you tell it to, which you have to if you want to mix data with Google base maps because the Google API only uses 900913 (which was invented by Google - the numbers being vaguely reminiscent of the word 'google').
No comments:
Post a Comment