geometry

[Slusep]

http://www.w3.org/ns/locn#Geometry

Description Shape and position characteristics of an object. Usage Describes these features using points, lines, polygons, and coordinates. Properties we have defined the following properties for this datatype: gml, wkt.

-) Shoudn't we want to support geojson as well?

[bertvannuffelen]

geoJson is a tricky beast. It means that the CRS is wgs:84. Because in Flanders/Belgium official geo-polygons are expressed as lambert72 or lambert2008, the use of geoJSON is limited. In that case GeoJSON is not the master data. GML and WKT are representations that can encode the master data.

[Slusep]

I'm confused. We need a standard that is interoperable so is WGS84. Lambert 72 would need re-conversion before usage that's why I think that GeoJson should be supported.

[GeertThijs]

You do not need to convert your data. The model does not impose any Coordinate Reference System (CRS) unlike GeoJSON does. In stead it requires you to specify the CRS that is used, in the case of WGS84 that would look like this (example of a Point geometry):

{
    "@type": "http://www.opengis.net/ont/sf#Point",
    "http://www.opengis.net/ont/geosparql#asGML": {
        "@value": "<gml:Point srsName="http://www.opengis.net/def/crs/EPSG/0/4326">
                           <gml:coordinates>51.2178744,3.229362</gml:coordinates><gml:Point>",
        "@type": "http://www.opengis.net/ont/geosparql#gmlliteral"
    }
}

Comments:

• As you can see the CRS is mentioned in de gml-string that serialises the geometry: http://www.opengis.net/def/crs/EPSG/0/4326 is a reference to the description of WGS84.
• In this description you would see that WGS84 is a geographic coordinate system so the coordinates will be geographic (latitude, longitude).
• In that manner the geographic data documents itself (it becomes so called smart data), rather than it is some agreement written in some specification detached from the data itself.
• Data in other CRS's can be exchanged in that way, eg in Belgium chances are that Lambert 72/50 is used, which is a cartographic coordinate system, the coordinates are x,y in stead of lat,long coordinate pairs.
• Conversion of data does not go away by using WGS84, especially in Europe. As a result of the origing of this CRS lying in the US and continental drift, coordinates in Europe have to be regurlary updated.
• It is true that GeoJSON would look cleaner as it is not some hybrid of JSON and xml, but I do not know of a version of GeoJSON where the geographic data describes its CRS.
• Lastly: GeoJSON supports not all existing types of geometry (only Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, and GeometryCollection), but it is true that you probably would need no other.

By the way, the GeoJSON of the example above would look like this, not so very different:

{
    "@type": "https://purl.org/geojson/vocab#Feature",
    "https://purl.org/geojson/vocab#geometry": {
        "@type": "https://purl.org/geojson/vocab#geometry:Point",
        "https://purl.org/geojson/vocab#geometry:coordinates": [
            51.2178744,
            3.229362
        ]
    }
}

[Slusep]

the latest spécifications for GML https://datatracker.ietf.org/doc/html/rfc7946 especially the CRS section https://datatracker.ietf.org/doc/html/rfc7946#page-12 and the following note (copied from the document ) _

Note: the use of alternative coordinate reference systems was specified in [GJ2008], but it has been removed from this version of the specification because the use of different coordinate reference systems -- especially in the manner specified in [GJ2008] -- has proven to have interoperability issues. In general, GeoJSON processing software is not expected to have access to coordinate reference system databases or to have network access to coordinate reference system transformation parameters. However, where all involved parties have a prior arrangement, alternative coordinate reference systems can be used without risk of data being misinterpreted.

_

indicates that indeed CRS is not supported anymore. It seems to me a sufficient reason to accept your note and reject my comment. Closing the issue