g.proj: add JSON support

[kritibirda]

Use parson to add JSON support to g.proj. Currently, only works with the -p flag.

{
    "name": "Lambert Conformal Conic",
    "proj": "lcc",
    "datum": "nad83",
    "a": "6378137.0",
    "es": "0.006694380022900787",
    "lat_1": "36.16666666666666",
    "lat_2": "34.33333333333334",
    "lat_0": "33.75",
    "lon_0": "-79",
    "x_0": "609601.22",
    "y_0": "0",
    "no_defs": "defined",
    "srid": "EPSG:3358",
    "unit": "Meter",
    "units": "Meters",
    "meters": "1"
}

I tried to add support for some other flags as well but it seems the SHELL/PLAIN output needs to be parsed manually to obtain the data and there is no direct way to obtain the data directly, for instance for w flag.

Also, some floats are represented as strings at the moment and need to be fixed.

[cwhite911]

For the proj4 format flag -j and the WKT format flag -w you can just return the the flattened (flag -f) string to the json.

{
   "wtk": "string_value",
   "proj4": "string_value"
}

[kritibirda]

@cwhite911 I have made the requested changes.

[cwhite911]

From @florisvdh on issue https://github.com/OSGeo/grass/issues/3020

Adding here that the preferred JSON encoding for g.proj output would be the PROJJSON specification: https://proj.org/en/latest/specifications/projjson.html. It's a JSON encoding of WKT2:2019.

[kritibirda]

@cwhite911 I tried to update the output to use projjson specification, however there seem to be a lot of cases to handle. IIUC, there are 3 different major versions (4/5/6) of proj4 that need to be handled separately and another case of not having libproj4 but using internal Grass GIS functions. FWIW, libproj4 starting with v6 exposes a function to generate the projjson representation directly.

While reading the code in get_proj.c and related files, I saw some comments about a future cleanup of proj4 versions. I think it would be better to handle add the proper JSON representation of proj4 here at that stage.

[cwhite911]

@kritibirda26

@cwhite911 I tried to update the output to use projjson specification, however there seem to be a lot of cases to handle. IIUC, there are 3 different major versions (4/5/6) of proj4 that need to be handled separately and another case of not having libproj4 but using internal Grass GIS functions. FWIW, libproj4 starting with v6 exposes a function to generate the projjson representation directly.

While reading the code in get_proj.c and related files, I saw some comments about a future cleanup of proj4 versions. I think it would be better to handle add the proper JSON representation of proj4 here at that stage.

@kritibirda26 that sounds good to me. Thanks for looking into it!

[echoix]

Are we even running build that use proj4? I only remember seeing proj 5+ api at the end of configure, and now Proj is at 9.4 now.

[neteler]

Are we even running build that use proj4?

According to configure.ac we support "PROJ.4.4.6 or later".

[echoix]

Do we want to go forward finishing up and merging the existing json PRs kinda soon? since some work is being done to implement on other modules too.

[cwhite911]

@echoix same issue here. I am no longer able to resolve conversations.

[echoix]

Ok, resolved them. Do you think the tests failures are transient and I should update the PR, or there's some real changes to do?

[echoix]

Because of the open comments, when doing my rounds of PRs to merge, I didn't act on it.

[cwhite911]

@kritibirda26 will you please accept these changes so we can merge this PR.

[cwhite911]

Ok, resolved them. Do you think the tests failures are transient and I should update the PR, or there's some real changes to do?

Real changes. Nice catch @echoix!

[nilason]

grass_parson needs to be added as dependency to:

https://github.com/OSGeo/grass/blob/c855d997fd574d1b3d8cd7afdaff087a1ca5ffd6/general/CMakeLists.txt#L25

[NishantBansal2003]

Hey @cwhite911, can you clarify a bit about the PROJJSON specification? I'm looking for an example of what it should look like with the existing data, as I didn't completely understand what and how things should be added to the JSON format. Or should I just use the existing JSON structure, add tests, and update the docs?

[petrasovaa]

Hey @cwhite911, can you clarify a bit about the PROJJSON specification? I'm looking for an example of what it should look like with the existing data, as I didn't completely understand what and how things should be added to the JSON format. Or should I just use the existing JSON structure, add tests, and update the docs?

I think the PROJJSON is out of scope.

[cwhite911]

@NishantBansal2003 Don't worry about the ProjJSON component now. Just do what was done here already and add the grass_parson dependency.

[kritibirda]

Hi @cwhite911! Sorry for the very late reply. I will reach out in an email explaining it later but for now I am available to finish this PR.

[petrasovaa]

Hi @kritibirda26, good to see you back! @NishantBansal2003 has been tying the loose ends of the open JSON PRs. He opened already a PR (#5419) for this (with added documentation) so we will probably continue there and close this one. But I see you were active in the other 2 open PRs, so @NishantBansal2003 please leave these for @kritibirda26 to finish. I am aware we are due reviews there, bare with us.

[NishantBansal2003]

This can be closed, as it was completed in https://github.com/OSGeo/grass/pull/5419