results.md

Results

The complete list of tests and results is available in this excel file. We found no bugs or issues in the GDAL implementation.

Geometry and place

The source data contained coordinates in the Dutch coordinate reference system RD. When converting to JSON-FG, by default GDAL:

• fills the place member with the coordinates as found in the source;
• fills the geometry member with WGS84 coordinates.

This conforms to the JSON-FG spec, but is not always what you want. Another option is to leave geometry empty (i.e. null value) and only fill the place member. You can instruct GDAL to do that, using the WRITE_GEOMETRY=NO option.

When to fill geometry, and when to give it a null value:

• When the JSON-FG is a payload of an API, this is clear from the response media type. The client probably asked specifically for JSON-FG, and therefore knows what it is and how to use it. In that case, it's unnecessary to enlarge the payload with geometry members containing converted coordinates. The client probably wants the specific CRS it asked for. If a client wants GeoJSON, with WGS84 coordinates, it can ask for that.
• When publishing JSON-FG somewhere outside the context of a webservice/API, it does make sense to fill geometry as well as place. In that case, the user has no context that helps figure out what the file is. There is no mime type or anything like that. In that case, it is good practice to have the fallback geometry, so that the file can be used in GeoJSON clients as well as JSON-FG clients.

Time

The time member initially was filled with a null value by GDAL, because it could not identify the temporal information to use in the source data. There is no specific option in GDAL to point to the right source element. It looks for an element called time, time_start, or time_end. If that's not what it's called in your input, you need to do some remapping. You can do this e.g. with an SQL statement:

ogr2ogr out.json test.xml -of jsonfg -sql "select cast (creationDate as date) as time, * from BAK"

In the example above, creationDate is the date element in the input we're interested in.

GDAL also writes the original property to properties, which is nice because the information is then not lost to GeoJSON clients (that only read geometry and properties).

featureType and schema

GDAL takes the featureType value from the element name in the case of GML, and from the column name in the case of GeoPackage.

There is no option to have GDAL write the featureSchema with a URL pointing to a JSON Schema for a feature. We did create a JSON Schema by hand (for BAK features). For purposes of creating a comprehensive example file, we add this manually.

3D geometries

GDAL converts the 3D geometry types without problems.

Sample output

A sample file converted from IMGeo GML, containing trash bins the area of the Binnenhof, The Hague is here. This has both place and geometry with coordinate values, time element with date values, and only contains 2D point geometries.

Client tests

We tested the sample output in different clients.

GeoJSON clients:

1. Github file preview. When geometry has coordinate values, this shows a map with point features. When geometry is null, it shows a world map without features. So this works as expected.

Github file preview, with coordinates in geometry

Github file preview, without coordinates in geometry

2. geojson.io (mapbox). Result is the same as [1].

geojson.io viewer, with coordinates in geometry

geojson.io viewer, without coordinates in geometry

3. QGIS also works the same; it recognizes features in the file only if geometry has coordinates. It reads properties and ignores JSON-FG specific members.

JSON-FG clients:

• A JSON-FG client by Ivan Sanchez from an earlier code sprint is used to demonstrate that the JSON-FG output from GDAL can be visualised in a web browser. This works as expected. The client visualizes the features based on the values of the place member, regardless of the presence or absence of coordinates in the geometry member. Note that this client only supports 2D geometries, we did not do client tests with 3D geometries.

JSON-FG client showing features based on coordinates in place member