[Proj] latlon projection

[Even Rouault]

On samedi 17 novembre 2018 21:03:06 CET Pavel Hančar wrote:
>   Hello,
> I am trying to understand some basic concepts of map rendering, but I've
> got really confused by the fact, that "longlat" or "lonlat" are often
> mentioned as a name of a projection. For example:
> +proj=longlat +datum=WGS84 +no_defs
> 
> AFAIK longitude and latitude stay for a coordination system, not for a map
> projection. Thus I thought it was a kind of simplification and the actual
> projection must have been the Equidistant Cylindrical. But I found the
> Equidistant Cylindrical being determined by "eqc" not "longlat".  So what
> is the "longlat"? I ended up running `proj -lP`, but found just this:
> 
> lonlat : Lat/long (Geodetic)
> latlon : Lat/long (Geodetic alias)
> 
> The only explanation is, that it is geodetic. But to me it seems it is not
> any explanation, but a further specification saying it is not geocentric,
> but the angles are taken from normals of the ellipsoid.
> 
> So, what projection is the "lonlat"? Or how are longitude and latitude
> projected to the flat map when nothing else is specified?

I'm not going to bring really new information as you pretty much figured out 
everything by yourself. Just to confirm that the 'longlat' 'projection' is not 
a projection in the official meaning of the term. In PROJ, it is basically a 
passthrough operation that doesn't change input coordinates:
- if longitudes are in the [-180,180] range, otherwise they will be by default 
wraped to it)
- and if you don't add some qualifiers that might alter its default behaviour, 
like 'pm', 'axis', 'lon_0', ...

The 'proj' keyword has even been abused/extended in PROJ 5 to designate a 
number of non-projection coordinate operations, or build transformation 
pipelines.

Even

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