Do tag excerpts need some disambiguation?
For some time I too have been troubled by the usage of those two and another tag:
- coordinate-system (with spatial-reference, crs and srs as synonyms) - 1,850 questions
- projection (with projections as a synonym) - 1,672 questions
- projection-conversions (with reprojection and reproject as synonyms) - 829 questions
I know that I am not on top of when precisely each should be used, and as a result, I just use coordinate-system for questions about coordinate systems and conversions between them.
I suspect that a large percentage of our users are in a similar position, but maybe they add projection instead.
I think a large part of the confusion for many users is that the term "projection" is being used both as a property of a dataset and a method that can be performed on a dataset i.e. in ArcGIS terms "Define Projection" vs "Project" (FAQ #2: Layers with same coordinate system should align/overlap but do not?).
Rather than working on improved tag excerpts and then trying to train our users in any subtle (or obvious only to the experienced) differences between them, I would favour making the other two tags become synonyms of coordinate-system. I think this is the pragmatic way to bring these questions to the attention of those most likely to be able to provide quick and accurate answers to them.
Reminded by another answer, I would be comfortable making transformation (404 questions) a synonym of coordinate-system too. Synonymising (and later merging, if consensus to do so is clear) all four tags in this way would give us coordinate-system with 3,820 questions and make it the 7th most common tag on the site. To me that does not seem out of proportion to how frequently our users ask about something to do with coordinate systems.
I can tell you how I would dice up the tags for questions, but as a "power user" in this area, I do not use the terminology in the same way that most general users do and not even as most colleagues understand them.
Note: correct ISO/OGC (International Standards Organization/Open Geospatial Consortium) usage for the georeferencing information of a dataset is "coordinate reference system" (CRS). In ISO-speak, "coordinate system" includes the axis names, directions, order, units, abbreviations only.
projection/projections - My personal usage is that it means the algorithm for a map projection like Albers or transverse Mercator. General usage is as a synonym for a coordinate reference system. Sometimes used specifically as a synonym for a projected CRS. (See Note near end of answer)
coordinate-system - If I have my EPSG Registry or standards hat on (meaning that I'm speaking about or for those groups), as mentioned in the Note above, coordinate system is the axis names/order/directions/units/abbreviations only.
Ex. EPSG::4400, Cartesian 2D CS. Axes: easting, northing (E,N). Orientations: east, north. UoM: m.
Otherwise, if my Esri hat is on, it's a synonym for coordinate reference system and is widely (solely) used in the Esri ecosystem.
coordinate reference system / crs - The standards name for a complete definition that describes the georeferencing of a vector dataset. A projected CRS would include at least a partial geographic CRS plus information on the linear unit of measure, map projection (algorithm) and any parameters needed by the map projection.
Ex. EPSG::32611, WGS 84 / UTM zone 11 North
Here's the WKT2 version:
PROJCRS["WGS 84 / UTM zone 11N", BASEGEODCRS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS 84",6378137,298.257223563,LENGTHUNIT["metre",1.0]]]], CONVERSION["UTM zone 11N", METHOD["Transverse Mercator",ID["EPSG",9807]], PARAMETER["Latitude of natural origin",0,ANGLEUNIT["degree",0.01745329252]], PARAMETER["Longitude of natural origin",-117,ANGLEUNIT["degree",0.01745329252]], PARAMETER["Scale factor at natural origin",0.9996,SCALEUNIT["unity",1.0]], PARAMETER["False easting",500000,LENGTHUNIT["metre",1.0]], PARAMETER["False northing",0,LENGTHUNIT["metre",1.0]]], CS[cartesian,2], AXIS["easting (E)",east,ORDER], AXIS["northing (N)",north,ORDER], LENGTHUNIT["metre",1.0], ID["EPSG",32611]]
This is even more complicated than how I described it above. The main difference is that the map projection algorithm and parameters are part of a "conversion".
spatial-reference - I think this was originally an Esri term, or least it ended up adopted by us for a specific purpose. When I use it, I mean a construct that's uniquely Esri. A spatial reference includes a CRS (Esri "coordinate system") plus some storage and processing values used mainly by geodatabases although the values are used in general. For example, ArcMap has a spatial reference that's usually set by the first layer added. The other values are:
Storage XY resolution Z resolution M resolution Processing XY tolerance Z tolerance M tolerance
The resolution values determine the precision at which coordinate values are stored within the geodatabase. The tolerance values (usually 10 times as large as the resolution values - but must be at least 2 times as large) are used when calculating topological relationships like intersections and overlaps. It's been overtaken by "coordinate system" in general as we downplay the tolerance and resolution values in the general documentation. I have seen it used elsewhere, outside of Esri where it is generally a synonym for CRS.
srs - spatial reference system. Not used much in Esri. I would expect that it's normally a synonym for a CRS.
reprojection / reproject - The act of changing the coordinate system via mathematics of some sort (not just redefining it). Is used generically, not just when converting between two projected CRS or to/from a projected CRS to a geographic CRS. So even though the process may be converting between two geographic CRS, someone might still say that they reprojected the data. Used a lot within Esri/Esri software.
projection-conversions - I never use this. I would either use reprojection or geographic/datum transformation. The latter is specifically used between two geographic/geodetic CRS. I assume it's meant as a more generic term for reprojection/conversion/transformation.
- Esri (I'm an employee) probably caused a bunch of this by using projection when "coordinate system" or "coordinate reference system" was more correct. That was only partially corrected in ArcGIS v8 (about 15 years ago) and we still used "define projection" and "project" over my protests.
I think also agree with PolyGeo to just synonymize the various tags.
In the same way that it is almost inevitable that different folks will ask similar questions using totally different terms, it is inevitable that tags will be used inconsistently.
Coordinate systems and projections are related but different phenomena. Coordinate systems allow numerical positioning of points in two- or three-dimensional spaces. Projections allow mapping between 3D and 2D coordinate spaces. Not mentioned above but relevant here, transformations allow mapping between two 2D coordinate spaces.