I would like to resolve some general confusion of mine concerning the terms (and concepts) of "coordinate reference system" and "projection". The way I initially understood these terms is that a CRS defines a set of properties, such as the spherical model used to approximate the shape of the earth and a datum that defines some fix point to which geographic latitudes and longitudes can relate to. A projection then is a mathematical function that is able to map these geographic 3D coordinates to a 2D plane. But after reading more about the topic my understanding gets more and more blurry.
Let's take for example the most common code
EPSG 4326. My understanding was that this would be the part that defines the spherical model etc. in order to define lat/lon data points. This data could then be run through a projection such as a "Albers Equal Area" projection which would result in points on a 2D plane. So far I think my idea holds up to the test.
My understanding began to crumble when I started to work more with QGIS and looked more into EPSG codes. When I for example open a shapefile that was defined in
EPSG 4326 in QGIS, it automatically displays the data on the 2D computerscreen without having to additionally specify how the data should be projected. Does QGIS just pick a default projection or does that mean that in
EPSG 4326 there's a 2D projection defined as well? Similarly, when working with visualisation libraries such as D3, GeoPandas or Cartopy often times one only has to define a projection such as
'albersUsa' in order to draw a 2d map. Does this mean that these tools just assume as a default that the data was recorded using
EPSG 4326 and we can then project them on a 2D plane using some other EPSG code that defines a projected coordinate system?
So to summarise my confusion and question: How do the terms "projected coordinate system", "geographic/geodetic coordinate system" and "coordinate reference system" relate and what of them is defined in an epsg code? Are they sometimes manually combined (
EPSG 4326 + alberUsa projection) in order to create a 2D projected map and sometimes already combined within one single EPSG code such as for example
EPSG 102008 (North America Albers Equal Area Conic)?