Difference between projection and datum?
What's the difference between a projection and a datum?
This is going to be one of the highly viewed questions on this stackexchange.
the answer using text from the Manifold docs is rated higher because one doesn't have to click through to get the meat. Personally I don't find the projection page that illuminating, the datum one one the other hand I like, especially for the graphic: http://help.arcgis.com/en/arcgisdesktop/10.0/help/003r/GUID-E94B3A5F-D997-4E6F-B400-BC15C4B0DCDC-web.gif
I wonder if this would be best as a community wiki, where we could collectively roll all of the answers up into a single, combined best answer. I'm personally not especially fond of broad questions where the answers are easily found on wikipedia, etc.
I think someone should mention the distinction between the two possible interpretations of "Map Projection" - namely "Projected CRS", which includes the datum and is what wwnick seems to be describing (http://gis.stackexchange.com/questions/664/whats-the-difference-between-a-projection-and-a-datum/722#722) and "Projection Method", which is what is described in dev's answer (http://gis.stackexchange.com/questions/664/whats-the-difference-between-a-projection-and-a-datum/665#665).
Great question for beginners! Here is another great explanation of the differences between datums, projections, coordinate systems, etc. with some illustrations (This is linked to in the PostGIS documentation): http://www.sharpgis.net/post/2007/05/Spatial-references2c-coordinate-systems2c-projections2c-datums2c-ellipsoids-e28093-confusing.aspx
There won't be a single answer for this as "datum" in GIS can be one of at least three different things e.g. geodetic datum (a reference from which measurements are made), a single reference point (often sea level as in "Ordnance Survey datum" = mean sea level at Newlyn in Cornwall, UK) and a reference ellipsoid (which is probably how most GIS people use the term. Finally here's a fourth meaning for the sake of pedantic completeness - datum = singular of data (so any single piece of information is a datum) :)
I have rolled back the revision of the title, since a datum doesn't always mean a Geographic Coordinate system, nor does a projection always mean projected coordinate system.
I wrote an in-depth article on this on my blog here: http://www.sharpgis.net/post/2007/05/05/Spatial-references2c-coordinate-systems2c-projections2c-datums2c-ellipsoids-e28093-confusing It covers all these concepts in a hopefully easy to understand manner.
Geographic coordinate systems (lat/long) are based on a spheroidal (either truly spherical or ellipsoidal) surface that approximates the surface of the earth. A datum typically defines the surface (ex radius for a sphere, major axis and minor axis or inverse flattening for an ellipsoid) and the position of the surface relative to the center of the earth. An example of a datum is NAD 1927, described below
Ellipsoid Semimajor axis† Semiminor axis† Inverse flattening†† Clarke 1866 6378206.4 m 6356583.8 m 294.978698214
All coordinates are referenced to a datum (even if it is unknown). If you see data in a geographic coordinate system, such as GCS_North_American_1927, it is unprojected and is in Lat/Long, and in this case, referenced to the NAD 1927 datum.
A Projection is a series of transformations which convert the location of points on a curved surface(the reference surface or datum) to locations on flat plane (ie transforms coordinates from one coordinate reference system to another).
The datum is a integral part of the projection, as projected coordinated systems are based on geographic coordinates, which are in turn referenced to a datum. It is possible, and even common for datasets to be in the same projection, but be referenced to different datums, and therefore have different coordinate values. For example, the State Plane coordinate systems can be referenced to either NAD83 and NAD27 datums. The transformations from geographic to projected coordinates are the same, but as the geographic coordinates are different depending on the datum, the resulting projected coordinates will also be different.
Also, projecting data may result in a datum conversion as well, for example, projecting NAD_1927 data to Web Mercator will require a datum shift to WGS 84. Similarly, it is possible to convert data from one datum to another without projecting it, as with the NGS's NADCON utility, which can shift coordinates from NAD27 to NAD83.
Example of a point's coordinates referenced to different datums
Coordinates referenced to NAD_1927_CGQ77
19.048667 26.666038 Decimal Degrees Spheroid: Clarke_1866 Semimajor Axis: 6378206.4000000004 Semiminor Axis: 6356583.7999989809
Same point referenced to NAD_1983_CSRS
19.048248 26.666876 Decimal Degrees Spheroid: GRS_1980 Semimajor Axis: 6378137.0000000000 Semiminor Axis: 6356752.3141403561
This answer seems somewhat misleading to me. It suggests that datum's are GCSs. "Datum's i.e. Geographic coordinate systems..." A datum is used to reference coordinates (both geographic and projected) in space. See my explanation below.
Just a small comment on the first line "Datums, i.e. Geographic coordinate systems". It sounds like a Datum is a geographic coordinate system. That's not the case. Datums are part of a simple ellipsoidal earth model. Geographic coordinate systems uses a datum as part of it's definition. But so does geocentric and projected coordinate systems.