Hi Andreas,
thanks for the input. I have drafted the current text about geo-related
datatypes, but I am far from being an expert in this area. Our mapping
expert in Wikidata is Katie (Aude), who has also been working with
OpenStreetMap, but further expert input on this topic would be quite
valuable.
As in all areas, we need to find a balance between generality and
usability, so I am slightly in favour of committing to one SR for now
(as I understand, the data can be converted easily between SRs but -- as
opposed to other cases where people measure something -- most of the
world seems to be happy with one of them).
I have now included a link to this thread into an editorial remark in
the data model, so we do not forget about this discussion when working
out the details.
Markus
On 04/04/12 14:16, Andreas Trawoeger wrote:
Hi everybody!
As the guy who has to honor to shortly receive some funding from
Wikimedia Germany for handling spatial open government data [0] I
would like to make some remarks on the current geo definitions in the
Wikidata model:
1. Spatial Reference System Identifier (SRID [1]) definition is missing
Every GeoCoordinatesValue field should either have a corresponding
SRID field that defines the used spatial reference system (SRS [2]) or
mandate the use of a single SRS like WGS84 [3] which is currently the
standard used by GPS, OpenStreetMap and Wikipedia.
2. Geographic shapes should be defined in either Well-known text (WKT
[4]) or GeoJSON [5]
WKT is the defacto standard to store spatial data in a rational
database and GeoJSON is the defacto standard to access geo data via
web. Both formats can be easily transformed into each other. So which
one you choose pretty much depends on your preferred choice of SQL vs.
NoSQL database.
So in summary I would propose the following data model for spatial data:
Geographic locations
Datatype IRI: http://wikidata.org/vocabulary/datatype_geocoords
Value: GeoCoordinatesValue
Mandatory spatial reference system: EPSG 4326 (WGS 84/GPS)
Type: Decimal
Geographic objects
Datatype IRI: http://wikidata.org/vocabulary/datatype_geoobjects
Value: GeoObjectsValue
Type: GeoJSON [5]
Geographic objects SRID
Datatype IRI: http://wikidata.org/vocabulary/datatype_geoobjects_srid
Value: GeoObjectsSridValue
Type: EPSG Spatial Reference System Identifier (SRID [1])
That model would allow a structure where every spatial object can have
a complex geometry stored in its original geodetic system and still
have an easily manageable location in GPS format.
cu andreas
[0]
http://de.wikipedia.org/wiki/Wikipedia:Community-Projektbudget#2._kartenwerkstatt.at
[1] https://en.wikipedia.org/wiki/Spatial_reference_system_identifier
[2] https://en.wikipedia.org/wiki/Spatial_reference_system
[3] https://en.wikipedia.org/wiki/WGS84
[4] https://en.wikipedia.org/wiki/Well-known_text
[5] https://en.wikipedia.org/wiki/GeoJSON
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