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I'm using QGIS in order to re-project a shapefile I have into "Mercator" so that something like protoviz understands my co-ordinates. Protoviz tells me "Mercator" (http://vis.stanford.edu/protovis/ex/projection.html) while QGIS has a list of many different types of Mercator projections for me to play with. Is there a standard Mercator projection?

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I second. The combination of projections, coordinate systems and datums can be really confusing. –  jvangeld Mar 24 '11 at 20:47
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If you're coming from a programming perspective, I think the easiest way to think of projections is like character encodings: its a simple concept with complex implications. –  scw Mar 24 '11 at 21:29
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up vote 8 down vote accepted

Yes, there is a standard Mercator projection. However, as with (almost) any other projection designed for the earth's surface, it can use different ellipsoidal models and has different aspects:

  • The ellipsoidal model describes an idealized shape (and size) of the earth. Most projections have formulas that apply to any ellipsoid of rotation, whose shape is determined by its "flattening" (or an equivalent parameter such as the eccentricity).

  • The aspect describes the orientation of the ellipsoid. The standard Mercator projection orients north to the top and is centered at the equator and prime meridian. You can vary these three parameters by applying the projection formulas to the rotated ellipsoid. This is not straightforward when the ellipsoid is not a sphere, because if you rotate the axis away from north-south the ellipsoid's shape with respect to the plane on which it is projected actually changes slightly. However, some rotations are relatively easy to handle: any rotation about the axis just shifts the meridians east and west. A rotation that places the axis along the (former) equator--that is, a 90 degree rotation--is usually easy to handle. Such an aspect is called "transverse." All other aspects are known as "oblique."

Therefore, in choosing any projection, you usually get to choose not only the projection itself, but also the earth's shape and the aspect of the projection. The Mercator is commonly used in its standard (equatorial) and transverse aspects with either a sphere or a slightly flattened spheroid (such as the WGS 84 spheroid). It is rarely seen in an oblique aspect. No single one of these choices is universal: as always, the decision depends on your accuracy needs and on how you plan to use the projection.

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This reply avoids the word "datum" on purpose: it is used in several overlapping senses and, properly interpreted, is not needed here. A clean separation occurs between a datum and a projection at the ellipsoid level. A projection is a mathematical transformation from an ellipsoid (or other model of a planetary body) into the mapping space (usually a Euclidean plane). A datum establishes the correspondence between physical locations and points on the ellipsoid (a purely mathematical construct). –  whuber Mar 24 '11 at 21:54
    
+1 Good to avoid datum. With so many to choose from, I worry that the more persnickety among us will speak of them in their correct plural form: data. –  Kirk Kuykendall Mar 24 '11 at 22:15
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When it comes to web-mapping systems, EPSG:4326, or Mercator with the WGS84 spheroid is as close to a 'standard Mercator projection' as it gets.

Google chose it for google maps a long time ago, and in some way, picked the 'default' for a lot of software to come later. It is also what many other web-mapping systems, such as Leaflet, Bing, OpenStreetMap, and others use. Sometimes, it is also known as EPSG:900913 and is also referred to as "Google Mercator" or "Web Mercator."

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Answering my own question after two years worth of experience and learnings. –  prabhasp Aug 28 '13 at 4:25
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EPSG:4326 is not a Mercator projection, but a geographic coordinate system (GCS) in degrees. What Google uses to render their maps is EPSG:3857, but they show coordinates in EPSG:4326 degrees. Don't worry, there is a lot more to learn... –  Andre Joost Aug 28 '13 at 5:33
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