[postgis-devel] Adding 4D support to asGeoJson

Alexandre Lessard alessard at mapgears.com
Wed Jan 31 13:39:57 PST 2018

If we take a look into the OpenGIS specification of a simple featue 
http://www.opengeospatial.org/standards/sfa Section the OGC 
describe the 4 possible dimensions of a geometry and the fourth 
dimension is describe as a measurement. Continuing on that road, ISO 31, 
31-1 and ISO 80000-3 define the units of measurements and they are all 
numerical (https://en.wikipedia.org/wiki/ISO_80000-3 Sorry for the 
wikipedia link I can only find the places to buy the standard 
documentation not the proper definition).

So knowing that we can now ask for an errata on the rfc7946 to define 
properly the forth dimension. We can even go deeper since the SFA also 
define that a 3 dimension geometry can be XYZ or XYM so we would need to 
add a way to distinct the 2 to respect that standard properly.

So i suggest to do the work now for the XYZM format and propose to the 
ietf modification to the RFC to respect the OGC SFA standard.


On 2018-01-31 11:12, Sandro Santilli wrote:
> On Wed, Jan 31, 2018 at 05:32:51AM -0800, Paul Ramsey wrote:
>> What, and then strip out extra dimensions for ISO GeoJSON? That seems needlessly byzantine. If people are getting 4d geojson from their 4d objects, they can just force3d them before calling asgeojson. I don’t think the standards differ enough to be worth the extra cruft.
> But the standard says you SHOULD NOT do this, so by NOT doing it
> we're respecting the standard, and in case a new version of the
> standard defines what to do with extra elements (making them
> unambiguous) we won't risk any semantic clash.
> On 2018-01-30 17:25, Paul Ramsey wrote:
>> https://tools.ietf.org/html/rfc7946#section-3.1.1
>> "Implementations SHOULD NOT extend positions beyond three elements
>> because the semantics of extra elements are unspecified and
>> ambiguous."
> --strk
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