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I like to think I'm relatively well-versed in datums, projections, and coordinate reference systems, so if this is a stupid question I will suffer eternal shame.

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.

Any input much appreciated

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I actually wondered exactly the same quite recently. And it's not only for OSM but for any GIS software... What do ArcGIS, Qgis... do when they display non-projected data? What are we looking at? My guess (but feel free to correct) is that they do not care about it! They display the coordinates as if they were a cartesian reference system. On earth, the meridians are converging towards the poles, which is also reflected in the fact that Reykjavik appears distorted towards NE. Now, if this is correct, can it be considered as an implicit projection? If yes, what type of projection would that be? –  Stéphane Henriod Mar 7 '12 at 8:12
    
Did you notice the coordinate display way down at the lower right? A quick look at this will readily settle your question. –  whuber Mar 7 '12 at 15:35
    
@whuber I see the coordinate readout but it doesn't really help - without imposing the latitude and longitude lines over the display the numbers only give me a vague notion of how positions are mapped to the image, and don't directly explain why northern or southern cities tend to the centre –  tomfumb Mar 7 '12 at 17:17
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When displaying data in a geographic coordinate reference system, ArcGIS software treats latitude/longitude as if they're 2D Cartesian values. Plate Carree scales lat/lon values by converting them to radians and multiplying by the semimajor axis. I say that ArcMap (for instance) uses a pseudo-Plate Carree projection. –  mkennedy Mar 7 '12 at 21:51
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2 Answers 2

up vote 8 down vote accepted

Open Layers 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. Open Layers 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').

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Hah! Found it. Christopher Schmidt first suggested using 900913. crschmidt.net/blog/archives/243/google-projection-900913 –  mkennedy Mar 7 '12 at 21:47
    
This turns out to be incorrect. EPSG:4326 defines both a datum that uses WGS84, and the projection that is LongLat/Plate Caree projection. See spatialreference.org/ref/epsg/4326/proj4js –  Chad Dec 4 '13 at 6:45
    
Not quite. WGS84 is a name that is used for both a projection and a datum (same name but two different things). EPSG:4326 is merely the EPSG code for a projection commonly called 'WGS84'- which also uses the WGS84 datum (see the Official EPSG site), so the answer is not wrong –  MappaGnosis Dec 4 '13 at 8:06
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EPSG:4326 is not a projection; it’s “unprojected”. Reykjavik is indeed at an angle relative to cardinal directions: http://osm.org/go/e0UtZkUl-, and when you look at your linked map you are seeing the lines of latitude and longitude mapped to a 2:1 rectangle that's 360° wide and 180° tall. The top and bottom edges are both points on the surface of the earth, the north and south poles.

On the web, a common projection used for lat/lon data is EPSG:900913 (or 3875), the Google spherical web mercator projection. That’s mostly just from the past few years when this projection became ubiquitous through web maps.

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This projection has been common for a LONG time before web maps and therefore before the creation of EPSG:900913. EPSG:900913 uses a different datum to EPSG:4326 (to be exact, the former uses a perfect sphere and the latter an oblate spheroid) –  MappaGnosis Mar 7 '12 at 8:46
    
sorry I should have been more specific in my question: I wasn't asking it 4326 was a projection, but rather if the map I was looking at was using a projection to display 4326 data. I know that this particular map is not using spherical mercator as I am used to seeing Reykjavik without the visual skew –  tomfumb Mar 7 '12 at 17:15
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In response to your comment, tomfumb, it’s probably more accurate to say that the map you are looking at is using a transformation. Projections are typically trigonometric functions designed to preserve some aspect of the geography in preference to others: equal areas, conformal shapes, compass bearing, etc. Transformations are linear functions that move a set of coordinates into a given view, for example by addition to move down and right or multiplication to make 360° fit into 600 pixels. –  Michal Migurski Mar 7 '12 at 20:16
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You make useful distinctions, but your terminology is unconventional. Your 'transformation' is usually called an "affine transformation" to distinguish it from the far more general sense of transformation in mathematics. Also, few projections are actually given by trigonometric functions (although such functions can be part of the process). So, in fact, Sylvester Sneekly is correct to identify this map as using a Plate Carree projection even though that projection does not involve any trig at all. It is not an affine transformation; that concept applies only to Euclidean spaces (maps). –  whuber Mar 7 '12 at 20:25
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