[Proj] latlon projection

[Pavel Hančar]

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?

Thank you very much.

Best wishes,
Pavel Hančar

[Even Rouault]

Post by Pavel Hančar
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
+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
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

[Luí­s Moreira de Sousa]

Hi Pavel,

that equates to Marinus of Tyre's projection [0]. Even though PROJ itself takes it as a geodetical coordinate system, pretty much all other software (Qgis, GRASS, etc) treats it as cartographic. You should keep this in mind when computing distances or areas in this system.

Regards.

[0] https://en.m.wikipedia.org/wiki/Plate_carr%C3%A9e_projection

Sent from ProtonMail mobile

-------- Original Message --------

Post by Pavel Hančar
Hello,
+proj=longlat +datum=WGS84 +no_defs
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?
Thank you very much.
Best wishes,
Pavel Hančar