libjson converts doubles to strings with low precision

[molnar]

See ogr\ogrsf_frmts\geojson\libjson\json_object.c

json_object_double_to_json_string() uses "%f" as conversion format for snprintf(). This defaults to a precision of 6 digits, which can result in loss of precision for many doubles.

This situation has been improved in the most recent release of json-c (In case it's interesting, here is the commit: ​https://github.com/json-c/json-c/commit/06450206c4f3de4af8d81bb6d93e9db1d5fedec1 and the discussion: ​https://github.com/json-c/json-c/pull/59).

For my own purposes, it is easy to patch around this issue by incorporating a few lines of code from v0.12 json-c. I just thought I'd bring it to the attention of the GDAL/OGR maintainers.

PS. In case it is of interest: this issue was discovered while round-tripping data to/from Elasticsearch.

[Even Rouault]

trunk r32952 "GeoJSON: use '%.17g' formatting by default for floating-point numbers and add SIGNIFICANT_FIGURES layer creation option (#6291)"

[Jukka Rahkonen]

I think that @goatbar has been fixing typos everywhere but I am not sure about comments. There is one in r32952 on line 2387:

# then it alos applies to coordinates

Comments in ogr_autotest.py and drv_geojson.html claim:

If explicitly specified, and COORDINATE_PRECISION is not, this will also apply to coordinates

However, in ogrgeojsonwriter.cpp it is written:

• If SIGNIFICANT_FIGURES is defined, it will override COORDINATE_PRECISION.

Last comment makes to think that SIGNIFICANT_FIGURES is overriding COORDINATE_PRECISION even if the latter had been explicitly set but I believe that it does not.

[Even Rouault]

trunk r32960 "GeoJSON: fix description of behaviour in comment of OGR_G_ExportToJsonEx(), fix logic in json_object_new_coord() to make COORDINATE_PRECISION win over SIGNIFICANT_FIGURES, fix typo in test and extend test to test behaviour when both options are specified (follow up to #6291, thanks to JukkaR for review)"