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I try to understand the idea behind the different Spatial Reference Identifiers. I understand that the different SRIDs identify different projection views on the earth and that they are important for ensure comparability. This StackOverflow Post also told me:

All of these [projections] have trade-offs between accurately measuring distance, area or direction -- you can't preserve all three

So I wonder which are the most important SRIDs and for which requirements should I use which ones. Some SRIDs I frequently stumble across are WGS84/EPSG:4326 (Lat/Lng), EPSG:3857(Web Mercator) [former EPSG:900913].

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    Just some clarification: EPSG:4326 is WGS84. EPSG:900913 is not Mercator but Web Mercator and also deprecated, one should use EPSG:3857 instead. – bugmenot123 Oct 2 '15 at 14:25
  • Well, did I mention that I'm pretty confused by all that specifications? Thanks for clarification, I edited my question – nik Oct 2 '15 at 15:56
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SRIDs are used as a means to distinguish between the many different coordinate systems. Often there are several coordinate systems that will be suitable for the task you are trying to achieve.

The first decision you need to make is whether to use a geographic (latitude/longitude) coordinate system or a projected (x/y) coordinate system. If you are working with areas, distances, directions, or shape and want to carry out any spatial analysis you will want to use a projected coordinate system. To highlight this, consider a geographic coordinate system using degrees as units - what is area measured in? Degrees squared? Which is nonsense really - you must have a projected coordinate system in, for example, metres.

Next you will want to further consider which property is most important to your task, as you will have to choose a projected coordinate system which preserves it.

  • Conformal projections preserve shape;
  • Equal Area projections preserve area;
  • Equidistant projections preserve distance;
  • Azimuthal (or True-Direction) projections preserve direction.

There are some projections that preserve several of these properties, though some accuracy is sacrificed as a result.

A further consideration is the scale at which you are working - is it global, national, or regional? Some coordinate systems will be accurate for a specific area only, others will be accurate for a larger area.

The topic of which specific coordinate systems to use under which circumstances is a very broad one but hopefully this will help you understand some of the considerations to make when deciding.

For further reading, the following link discusses projections and coordinate systems in much more detail:

https://courses.washington.edu/gis250/lessons/projection/

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Not really an answer, but important:

An SRID like EPSG:4326 or EPSG:3857 is just a reference number to access a commonly used projected coordinate reference system (CRS).

A projected CRS consists of a geographic CRS (geodetic datum, ellipsoid/spheroid, prime meridian), a projection method and parameters, linear unit, and (optionally, for the PROJ.4/GDAL-based software) a datum shift to the WGS84 datum.

The EPSG (now IOGP's geodesy subcommittee) has collected and published a great number of CRS in its database. The codes used by the EPSG database have been picked up by GDAL for a unified access number to be used in databases like spatialite and postgis.

As an alternative, Esri (a major software company that is not based on Proj.4), has introduced the .prj file for shapefiles. These contain a WKT definition of the CRS. This format is not limited to the CRS that are in the EPSG database, which would have an Esri SRID, not an EPSG SRID. Some of these can be used in PROJ.4 and GDAL as well if you use the complete proj.4 projection string, or the WKT definition.

  • A projection should really include a datum as well. – Mintx Oct 2 '15 at 17:13
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    AndreJ - I hate to edit other people's answers, but...do you mind if change I some terminology: projected CRS not projection, geographic CRS (with a datum that uses an ellipsoid) not a spheroid. EPSG does NOT have all CRS in usage! etc. – mkennedy Oct 2 '15 at 18:03
  • @mkennedy just go ahead. You have surely the better terminology at hand. – AndreJ Oct 3 '15 at 7:07
  • @Samane: I tried to use the EPSG database. It's easy to filter geo-related, but I don't find information about which properties they preserve (shape/area/distance/direction). My concrete example should be a projected CRS of whole Germany equidistant. – nik Oct 4 '15 at 9:41
  • In the EPSG database, there is no equidistant projection defined for the whole of Germany. So you can either take UTM 32N (covring most of Germany) for the whole country, or create a custom equidistant conical projection on a center point in the middle of the country, like 10°E 51°N: +proj=eqdc +lat_0=51 +lat_1=50 +lat_2=52 +lon_0=10 – AndreJ Oct 5 '15 at 5:06

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