Brontekst bekijken van Standards for linking geospatial data

== Coordinate reference systems ==
Coordinate reference systems (CRS) are to geo-information what character encodings are to text. If you don’t know which CRS is used, you can’t use the coordinates. Different CRSs exist for a reason: localized CRSs provide more precise coordinates for a certain part of the globe. It is not possible for a global CRS to be as precise, for example because the continental plates move a few centimetres every year. For large scale data and applications this continental drift could be very relevant over time. Take for example the boundary of cadastral parcels. If this drift is not taken into account, there could be issues if parcel boundaries that were established e.g. 10 years ago are overlaid over recently acquired aerial imagery with high accuracy (e.g. 10 cm). There could be visual differences, while the actual situation did not change.

Discussion is necessary on whether support for different coordinate reference systems, geographic as well as projected ones, is needed in linked geo data standards. The possibility to use different CRSs hinders interoperability (datasets using different CRSs cannot be easily combined, a complex transformation is necessary) but on the other hand this option is perhaps needed for use cases where a high precision of coordinates is important.

Even if this turns out to be necessary, the default should be WGS84 (lat/lon).

In GeoSPARQL it is possible to refer to a CRS, but the reference is part of the geometry literal. If this were a separate property it would be easier to use the CRS as a selection criterion (which is desirable, for example, when displaying data on a map: data which uses different CRS cannot be combined on a map).  In the GeoJSON object model a member ‘crs’ is defined. In GML there is a similar property, ‘srsName’, to indicate the coordinate reference system used. These are good examples of how it should be done.