RE: UCR issue: phrasing of CRS requirement(s)

Hi all,

OK, that could be the consensus within OGC, but the GeoJSON spec does describe a default CRS and I can understand this very well. Non-experts, i.e. people from outside the geospatial domain who are using or want to use geospatial data, often have no idea that there even *are* multiple coordinate reference systems.


Van: Peter Baumann []
Verzonden: vrijdag 15 mei 2015 13:01
Aan: Linda van den Brink; Frans Knibbe; SDW WG (
Onderwerp: Re: UCR issue: phrasing of CRS requirement(s)

Hi all,

FYI, there has been a vivid discussion in OGC on default CRSs on the occasion of JSON coming up with such an idea, and OGC very much and strongly agreed that this is not a good idea.

In general, a coordinate tuple should have exactly one CRS referenced which may include
- spatial horizontal (such as Lat/Long)
- time (possibly using different calendars)
- elevation
- anything else (eg, atmospheric sciences like to use pressure as a proxy for height)
- finally, planetary CRSs are more and more coming into play as well.
I sense that this is very much in alignment with the ideas that we are discussing here.

OTOH, it is indeed important to have one common mechanism of describing CRSs. As mentioned earlier, OGC has such mechanisms in place through CRS WKT plus the CRS Name Type Specification (maybe quite misleading in its title, it allows to describe CRSs by composing them from other ones, such as flatland + time, flatland + pressure, flatland + depth, flatland + geological time).

So definitely supporting Linda's observation on referencing vs describing.


On 05/15/15 09:40, Linda van den Brink wrote:
Hi Frans,

I noticed that a requirement related to this is in the spreadsheet but not (yet?) in the UCR document. It is this requirement:

“There should be a default CRS that is assumed when nog CRS is specified” (s/nog/no)

WGS84/lat lng is the de facto standard CRS for spatial data on the web. Both publishing and using spatial data on the web should be easy for non-experts, so this requirement of having a default CRS makes a lot of sense to me. The most common cases become more easy that way. I think this should be added to par. 5.6 of the UCR.

In this light (i.e. usability for non-expert users), the best practice should have information about how data owners should describe, how users can recognize and what tools they can use to transform non-WGS84 coordinate systems to the coordinate system they need.

A second point I’d like to make is that CRS should be suitable also for non-geographical reference systems (for non-Earth oriented applications).I think this is covered by 5.14, but the text of that paragraph is not completely clear to me. )“Standards for spatial data on the web should be independent on the reference systems that are used for data.”)

Finally, to answer the question in the issue, as I read it, req A is not replaceable by req B. Req A is about *referencing* a CRS, while req B is about *describing* a CRS – i.e. the description you get about the CRS when you dereference  a CRS reference.


Van: Frans Knibbe []
Verzonden: woensdag 13 mei 2015 14:20
Aan: SDW WG Public List
Onderwerp: UCR issue: phrasing of CRS requirement(s)

Hello all,

I have raised an issue for the UCR document: ISSUE-10<>.
All help in getting this issue resolved is very welcome.


Frans Knibbe
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1079 MB Amsterdam (NL)

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Dr. Peter Baumann

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Received on Friday, 15 May 2015 11:21:58 UTC