[PROJ] Creating projection *not* from full init string?

[Even Rouault]

Le 09/03/2021 à 19:18, Matthew Woehlke a écrit :
> On 09/03/2021 13.04, Even Rouault wrote:
>> Le 09/03/2021 à 18:59, Matthew Woehlke a écrit :
>>> I'm trying (again) to port some geodetic conversion code from the 
>>> PROJ 4 API.
>>>
>>> Our system uses EPSG identifiers to specify coordinate systems. 
>>> (Yes, yes, I'm — vaguely — aware we should also be using epochs, but 
>>> frankly it's a struggle to get our users to even be *aware* that 
>>> there are different coordinate systems, and more often than not we 
>>> have to assume 4326 because we don't know *anything*. Getting 
>>> anything more is simply not practical.)
>>>
>>> This was manageable in PROJ 4. With 6.3.2, however, I can't seem to 
>>> create a *usable* projection except by specifying a complete init 
>>> string. Calling `proj_create(0, "EPSG:4326")` gives back a non-null 
>>> pointer, but any calls to proj_trans fail, proj_info gives back an 
>>> empty definition, and proj_angular_{in,out}put all return 0.
>>
>> Yes, `proj_create(0, "EPSG:4326")` creates a CRS object, not a 
>> coordinate operation one. proj_trans() requires a coordinate 
>> operation and will not work on a CRS. You need to use 
>> proj_create_crs_to_crs() to gt an object usabled with proj_trans()
>
> Okay, I don't understand that at all. 

>
> In PROJ 4, 4326 is "+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84". 
> If I use that with proj_create, I get something usable. What's the 
> difference?

PROJ >= 6 now makes a difference between a CRS and a coordinate 
operation. A PROJ string without +type=crs is assumed to be a coordinate 
operation, and with +type=crs to be a CRS.

The old PROJ.4 definitions without +type=crs depending on the context 
could be used as a CRS definition or a coordinate operation. PROJ >= 6 
now requires to be explicit to avoid that confusion. You can't use a CRS 
like "EPSG:4326" with proj_trans(). That doesn't make sense. A CRS 
doesn't define a coordinate operation by itself. If you want to project 
coordinates to EPSG:32631 for example, you can use the coordinate 
operation returned by proj_create_crs_to_crs(ctx, "EPSG:4326", 
"EPSG:32631", NULL) as an argument of proj_trans()

>
> p.s. How do I get that definition in PROJ 5? In PROJ 4, I could use 
> pj_get_def. (See also https://stackoverflow.com/questions/66533885.)
Use proj_as_proj_string(). That can work on both CRS and coordinate 
operation (when there's a single coordinate operation actually contained 
in proj_create_crs_to_crs() )

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