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I have two layers in a QGIS project: "Layer 1" and "Layer 2".

In their "Layer Properties" under "Coordinate Reference System" I see this:

Layer 1 enter image description here

Layer 2 enter image description here

I'm trying to understand the differences between the CRS nomenclature.

Layer 1 is obviously projected, since it says UTM zone 20, but I'm left wondering what the datum is that it's based on?

Layer 2 tells me its datum = WGS 84, but I'm left wondering what its projection is?

It appears to me the various CRSs shown in both screen shots use various combinations of datums and projections terminology. This appears needlessly confusing. Only the bottom one appears to clarify both its datum (NAD83) as well as its projection (UTM).

New Question /Task:

Top marks to the one who can accurately describe the meaning of every component (word, number, punctuation mark, etc) in all 4 CRSs appearing in Layer 1, particularly so that the datum and coordinate system are clearly defined.

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  • As per the Tour there should be only one question per question.
    – PolyGeo ♦
    Commented Aug 18, 2017 at 12:42
  • If you see a forward slash ("/"), the name before it is usually the geographic coordinate reference system ("datum"). EPSG is the authority for the well-known ID that follows. 4326 is WGS 84, a 2D (latitude-longitude) geographic coordinate reference system.
    – mkennedy
    Commented Aug 18, 2017 at 18:19
  • Thanks for the helpful hint. And so anything after the "/" will be the projection system, yes? Also, I like thinking about the "geographic coordinate reference system" and "datum" as the same thing... if indeed that is true...? I ask because in the video suggested by Basile below (coursera.org/learn/spatial-analysis/lecture/6q7Wb/datums), at 6:38 the narrator speaks as if they are two separate things.
    – Waterman
    Commented Aug 26, 2017 at 19:45

1 Answer 1

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You can always find all corresponding information about certain projection by its standard EPSG code on spatialreference.org.

For example, your Layer 1 has EPSG code of "2203", so here is all relative information on this coordinate system in different formats. In particular, if you check "Human-Readable OGC WKT" link, you will find things like datum, projection (if coordinate system is projected), units etc:

DATUM["Red_Geodesica_Venezolana",
...
PROJECTION["Transverse_Mercator"]

The same info can be found for your Layer 2 here:

DATUM["WGS_1984",

Here is no projection information though, because the coordinate system here is geographic and not projected.

Here is a good video-explanation on Datums, Geographic and Projected coordinate systems.

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    The official website for the EPSG authority is epsg-registry.org. spatialreference.org is not up-to-date.
    – mkennedy
    Commented Aug 18, 2017 at 18:17
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    ... and epsg.io tries to connect both ;-)
    – AndreJ
    Commented Aug 19, 2017 at 14:34
  • Great answer. As an addendum, would you agree with mkennedy's suggestion that anything before a "/" is the geographic coordinate reference system (GCRS) and anything after a "/" is the projected coordinate reference system (PCRS)? e.g. my Layer 1 starts off with "EPSG:2203", which is the GCRS, and then when you visit the site you provided you find out the datum it's based on is "Red_Geodesica_Venezolana"?
    – Waterman
    Commented Aug 26, 2017 at 20:14
  • But the next thing I notice is that sometimes the GCRS appears to contain the datum in its name already. e.g. the last option in Layer 1 starts off with "EPSG:26920 - NAD83". Is this just an oddity that certain GCRS' tell you the datum they're based on and others make you search further for that information?
    – Waterman
    Commented Aug 26, 2017 at 20:14

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