[MapServer-dev] Custom Projections in MapServer 8 / Proj 6+

[Seth G]

Thanks Even for the details. 

I've been trying to get the text file approach working, but always seem to end up with the following error when using:

http://localhost/mapserver/?map=./wxs/ows_wms2.map&SERVICE=WMS&VERSION=1.1.0&REQUEST=GetMap&SRS=EPSG2:42304&BBOX=2258982,-70747.9140625,2615354.5,495480.9375&FORMAT=image/png&WIDTH=300&HEIGHT=200&STYLES=&LAYERS=road

msProcessProjection(): Projection library error. proj error "projection not named" for "EPSG2:42304"

I copied the epsg2 file which contains a proj string for 42304 from msautotests [2] to the folder set in PROJ_LIB. 
The LAYER projections are fine with this setup, and there are no errors if I make a request using a well-known projection e.g. 

http://localhost/mapserver2/?map=./wxs/ows_wms2.map&SERVICE=WMS&VERSION=1.1.0&REQUEST=GetMap&SRS=EPSG:4326&BBOX=-67.5725,42.3683,-58.9275,48.13&FORMAT=image/png&WIDTH=300&HEIGHT=200&STYLES=&LAYERS=road

So it looks as though there is a lookup based on the projection code sent in the SRS parameter, and this doesn't pick up any custom projection files, even though they are correctly used when reprojecting a layer in a custom projection to a well-known projection. 

I'll debug further, and try to get an msautotest setup with a GetMap request using a custom projection. 

Also, I've created a pull request to add in GDAL and PROJ versions [1].

Seth

[1] https://github.com/MapServer/MapServer/pull/6794
[2] https://github.com/MapServer/MapServer/blob/main/msautotest/wxs/data/epsg2



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On Sat, Jan 14, 2023, at 12:45 PM, Even Rouault wrote:
> Seth,
>
>> In Proj4 adding new projections was as simple as adding a new string to the epsg text tile in the PROJ_LIB directory. With Proj6+ the proj.db Sqlite database is used. I believe the text files are no longer used at all
>
> +init=foo:bar should still try to use a "foo" text file. And if you have 
> an "epsg" text file, it will have precedence over proj.db for 
> +init=epsg:XXXX codes (although I don't recommend this, because proj.db 
> will be completely ignored for any epsg entry, so you'd better make sure 
> your text file is complete enough)
>
> (although the GIS Internals proj7/share includes an other.extra file in 
> the same format - is this also ignored?).
>
> I presume +init=other.extra:XXXX should still work
>
>>
>> So to add a new projection would require adding a new record to proj.db.
>> The docs at https://proj.org/apps/projinfo.html refer to a `--dump-db-structure` parameter:
>>
>>>     New in version 8.1.
>>>
>>>     Outputs the sequence of SQL statements to create a new empty valid auxiliary database.
>>> 	This option can be specified as the only switch of the utility.
>>> 	If also specifying a CRS object and the --output-id option, the
>>> 	definition of the object as SQL statements will be appended.
>> There is also a new environment variable (see Even's comment at [3]) - PROJ_AUX_DB that allows this to be read by Proj.
>>
>> - Is this the recommended approach for MapServer users? (Import any new projections into a new proj_extra.db and setting this as an environment variable).
> The mechanism technically works but it has been reported in 
> https://github.com/OSGeo/PROJ/issues/2904 that there are at least in 
> some use cases with significant performance degradation. I'm not 
> completely sure if it would affect Mapserver use cases though, and the 
> degradation would only affect the setup of reprojection objects, so it 
> should likely be OK for FastCGI scenarios where they are cached and thus 
> an extra cost can be amortized.
>> - What is the recommended approach for MapServer deployments with PROJ >=6 and <=9?
> Not easy to answer this. To be honest nothing has beaten yet the 
> simplicity of old PROJ.4 string. If you have not too complex 
> requirements regarding datum shifts expressed by those and aren't 
> mapping at a sub-metric level, they are probably good enough for most 
> use cases. If you want to be able to express geodesy-grade datum 
> transformations, you'd better setting up proper CRS and transformation 
> records, either by patching proj.db or using an auxiliary database.
>> - Is PROJ_AUX_DB a setting that can be added to the MapServer CONFIG file?
> Probable not. I assume that currently, it should be set by the HTTP 
> server itself, before it forks mapserver CGI. In a CONFIG block, this 
> would only go through CPLSetConfigOption() but PROJ reads through 
> getenv() or if it was set with proj_context_set_database_path(). I 
> presume the msSetConfigOption() logic should have a special case similar 
> to the PROJ_DATA one so that setting PROJ_AUX_DB calls 
> OSRSetPROJAuxDbPaths() (GDAL >= 3.3)
>> - Would it be worth adding  the PROJ and GDAL versions to the mapserver -v output as several of these options rely on newer versions of PROJ?
>
> why not
>
> Even
>
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>
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