Dynamic CRS Datums. Was: UCR issue: phrasing of CRS requirement(s)

Bruce, Peter,

Actually, there is a pressing case in Meteorology – vertical CRSs such as sigma and eta coordinates use a ‘dynamic datum’ – actually a 3-D field/coverage over (x,y,t), a time varying 2-D coverage over (x,y).

Thus this is a second, different use case for dynamic datums.

A third allied use case is the aviation safety critical requirement for prediction of minimal surface pressures in air traffic control regions, used to calibrate physical altimeters. This then stops aircraft flying into the ground if electrical radar altimeters are defunct, as in a thunderstorm. These forecast pressures are called ‘QNH’

A few TCs ago we had a Vertical CRS Ad Hoc group meeting, and agreed to carry it forward in the (asleep if not dead) CRS DWG. We accumulated some info on the Met Ocean wiki at http://external.opengeospatial.org/twiki_public/MetOceanDWG/VerticalCRS


I have not had time to push this, but would be very supportive of whoever does.

Chris

From: Bruce Bannerman [mailto:B.Bannerman@bom.gov.au]
Sent: Monday, June 01, 2015 10:27 PM
To: Peter Baumann; frans.knibbe@geodan.nl
Cc: public-sdw-wg@w3.org
Subject: Re: UCR issue: phrasing of CRS requirement(s) [SEC=UNCLASSIFIED]

Agreed.

I find it difficult to see a pressing requirement in the meteorology, climate and hydrology domains. However, we will need to work through the issue in coming years.

However, geodicists do make a good case for precision use cases such as surveying, precision agriculture etc.

I suspect that we may end up with a new class of datum, algorithms etc., that co-exist with those currently in use. However this is some time away.

I don't think that we need to make a big thing out of this requirement for the immediate work at hand. We just need to leave the door open for its use in the future. I don't see the concept of a dynamic datum going away.

Bruce
________________________________
From: Peter Baumann <p.baumann@jacobs-university.de<mailto:p.baumann@jacobs-university.de>>
Sent: Monday, 1 June 2015 10:00:17 PM
To: Bruce Bannerman; frans.knibbe@geodan.nl<mailto:frans.knibbe@geodan.nl>
Cc: public-sdw-wg@w3.org<mailto:public-sdw-wg@w3.org>
Subject: Re: UCR issue: phrasing of CRS requirement(s) [SEC=UNCLASSIFIED]

Hi Bruce,

this is a mindblowing idea. I'll be curious about how to model this, and how to craft service definitions (like simple subsetting) in future.

-Peter

On 06/01/15 00:27, Bruce Bannerman wrote:
Hi Peter and Frans,

Just to add a little more complexity to this use case:

  *   There is an emerging concept of a dynamic datum, where the datum is regularly adjusted to take into account a number of factors, including: adjustment for continental drift; update of the geoid model as better data becomes available; fixes for errors; etc.

  *   In Australia, in my domain, we expect this to become an issue to address after the release of our GDA 2020 datum. Other domains may need to work more quickly.
We have an increasing number of people discussing the benefit of a dynamic datum, particularly in relation to an increasing number of precision location use cases such as precision agriculture.

Therefore it will be important to know 'when' locations were collected in addition to what SRS datum was used to constrain the coordinates. The 'when' will also need to be constrained by a temporal datum.

We are starting to think about the implications that this has for effective spatial data management and for the software and standards that we use with the data.

For a little more on this see from the Australian Intergovernmental Committee on Surveying and Mapping:

  *   [1], Note discussion on Stage 2 goals.
  *   [2], slides 31 - 32. The rest of the slide pack may also be of interest.
Bruce

[1] http://www.icsm.gov.au/geodesy/index.html

[2] http://www.icsm.gov.au/publications/PCG-SSSC_2013_Canberra.pdf


________________________________
From: Peter Baumann <p.baumann@jacobs-university.de<mailto:p.baumann@jacobs-university.de>>
Sent: Friday, 29 May 2015 10:07:28 PM
To: Frans Knibbe
Cc: SDW WG Public List
Subject: Re: UCR issue: phrasing of CRS requirement(s)

indeed, IMHO this is an excellent case showing how axis order is disambiguated. (There are reasons behind why OGC is doing it like this, but someone else should explain this).

But the term "OGC registry" is well defined by way of URL. It is just that OGC maintains a single point for multiple CRS families. For example, EPSG does not want to deal with time, so OGC is adding this. And more families may come in future. And yes, OGC takes care that it is always in sync with EPSG.

-Peter
On 05/29/15 14:01, Frans Knibbe wrote:
Hello Peter,

Your message made me realize that speaking of 'the OGC registry' could be considered ambiguous. The OGC provides CRS descriptions according to EPSG definitions and its own definitions. Take http://www.opengis.net/def/crs/OGC/1.3/CRS84 and http://www.opengis.net/def/crs/EPSG/0/4326. The first CRS definition (that happens to be the default CRS in GeoSPARQL) is an OGC definition of WGS84 where the axes have been switched with respect to the EPSG definition. The latter is the EPSG definition (as one can tell by looking at how the URIs are composed) of the WGS84 CRS.


Regards,
Frans



2015-05-29 13:31 GMT+02:00 Peter Baumann <p.baumann@jacobs-university.de<mailto:p.baumann@jacobs-university.de>>:
Hi again Frans,
On 05/29/15 13:10, Frans Knibbe wrote:
Hello Ed,

Yes, we could recommend using the EPSG descriptions in the BP. That is future work.

could be done right now, through URIs. OGC maintains a resolver which is "URI conforming" (concretely, URLs):
http://www.opengis.net/def/crs/EPSG/0/4326

...and it goes beyond EPSG. For example, (in agreement with EPSG) OGC is adding time support.


But looking ahead to that, I think the EPSG registry has some issues: CRS are not identified with a URI,

resolved by OGC


it is hard to automatically process CRS data,

behind the links are XML descriptions, based on a well-known schema.


and CRS do not have the same axis order.

this reflects reality. What's important is that the axis order is made explicit in these definitions, so a machine can understand coordinates expressed in such a CRS.


I think CRS registries like those made available by the OGC and IGN France are an improvement, at least in those respects.

I'd like to think so, but I am biassed :-)

Bottom line, using the OGC registry provides a low-hanging fruit.

-Peter




Do you agree with the proposed requirement?


Greetings,
Frans

2015-05-27 15:29 GMT+02:00 Ed Parsons <eparsons@google.com<mailto:eparsons@google.com>>:

Describing CRS attributes is a "can of worms" for is, I would rather we just point to the EPSG repository for simplicity. Perhaps we could in the BP point out a few well used CRS and their EPSG descriptions.

Ed

Ed Parsons
Geospatial Technologist, Google

Mobile: +44 (0)7825 382263<tel:%2B44%20%280%297825%20382263>
www.edparsons.com<http://www.edparsons.com> @edparsons
On 27 May 2015 13:49, "Frans Knibbe" <frans.knibbe@geodan.nl<mailto:frans.knibbe@geodan.nl>> wrote:
Hello all,

It is nice to see a good discussion with lots of outside references and proposals for meeting the requirements. But I would like to get back to the issue of phrasing the requirements.

Linda was right in noting that the requirement for a default or canonical has not been listed in the UCR document yet. I will add it. It will be very interesting to see if we can succeed in finding such a CRS. But that is something that will become clear when we work on the Best Practices.

As for the phrasing of the CRS requirement, I would like to propose the following:

"There should be a standard for publishing data about coordinate reference systems (CRS). It should be applicable to any 2D or 3D CRS, not only geographical reference systems. CRS descriptions should be referencable by HTTP URIs."

I think that given the context of this WG (spatial data on the web), the requirement that things should be referencable goes without saying, but I have made it explicit nonetheless.

I should note that when put this way, there is no requirement for having a registry of CRSs - anyone can publish a CRS defintion anywhere.

Would it be sensible to add some information about what kind of data we would expect when a CRS is described? There could be a need for having complete descriptions that allow automatic coordinate transformation. Or we could try to list a few CRS attributes that we consider essential for common usage (like axis order, units, whether it is spherical or flat plane, spatial coverage....)

Greetings,
Frans


2015-05-26 21:12 GMT+02:00 Krzysztof Janowicz <janowicz@ucsb.edu<mailto:janowicz@ucsb.edu>>:
Krzysztof, why is Java such a hot bed of linked data?!?

Good question. If you look at the detailed map here http://stko.geog.ucsb.edu/pictures/lstd_map.png you will see massive data errors. Based on our 14+ million sample, it looks like about 10% of all linked geographic data has some issues related to it. In many cases those issues were systematic and we notified the data providers, e.g., DBpedia. For instance, the dense block-like areas in China are in fact places from the US east coast. I will look into the data from Java and will let you know if I find something interesting.

Anyway, the systematic list of errors may turn out to be useful for our sdw group as it illustrates what typically goes wrong.

Best,
Krzysztof




On 05/18/2015 07:39 AM, Kerry.Taylor@csiro.au<mailto:Kerry.Taylor@csiro.au> wrote:
WGS84 is certainly widely used for linked data in practice, probably  heavily influenced by this http://www.w3.org/2003/01/geo/, commonly called "geo".

Oddly, perhaps, schema.org<http://schema.org> seems not to care about CRS at all: http://schema.org/GeoCoordinates


Can we take inspiration from the former one  (geo)  and admit alternative CRSs that must be identified by virtue of the ontology (and therefore namespace, assuming a 1-1 relationship) that is used?  We could perhaps develop a couple ourselves (perhaps a WGS84-like one, and another for a relative 3D system), and then allow any other to be used by virtue of reference to the intended vocabulary (as our best practice advice)?

Maybe this is a cop-out but it is a way of dealing with the common cases blindly, yet requiring a CRS to be implicitly identified, and also enabling the use of more complex or niche CRS whenever needed. We won't stop people making mistakes, whatever we do.

This could do for  *referencing* a  CRS without ever needing a "default". For the *description" of a CRS, I would vote to defer that to the OGC by its existing methods, and I see no reason why that description needs to have a linked data representation,  beyond an ontology that permits its use.




Krzysztof, why is Java such a hot bed of linked data?!?

Kerry

-----Original Message-----
From: Svensson, Lars [mailto:L.Svensson@dnb.de<mailto:L.Svensson@dnb.de>]
Sent: Monday, 18 May 2015 9:44 PM
To: Ed Parsons; janowicz@ucsb.edu<mailto:janowicz@ucsb.edu>
Cc: SDW WG Public List
Subject: RE: UCR issue: phrasing of CRS requirement(s)

On Monday, May 18, 2015 12:24 PM, Ed Parsons wrote:
In most cases I don't think they actually do mean WGS84 as in the
ellipsoid and datum.

I would guess it is usually shorthand for the the full spatial
reference system defined by EPSG4326 or more likely on the web
EPSG:3857
My fear is that in some cases the data providers don't really know what
their coordinates mean in terms of ellipsoid, datum and reference
system. They have some coordinates taken from geonames, Wikipedia or
some other source and haven’t really thought of that (geographic)
coordinates are not just coordinates but that there is a context to
that, too. To what extent we can assume that they mean CRS84, I don't
know.

So I think I'm on the same page as Linda on this.

Best,

Lars
On 16 May 2015 at 04:02, Krzysztof Janowicz <janowicz@ucsb.edu<mailto:janowicz@ucsb.edu>>
wrote:
right, so how can they be sure they mean WGS84?

Here is a funny example how this can go wrong and went wrong in the
past:
http://stko.geog.ucsb.edu/location_linked_data (See the Copernicus
crater)

Best,
Krzysztof




On 05/15/2015 04:27 AM, Peter Baumann wrote:
right, so how can they be sure they mean WGS84? if I copy-past
coordinates from web info about Germany then in the past this used to
be Gauss-Krüger, and several strips = sub-systems. Now let's talk
about height and SI vs imperial units etc - what default could we
agree on?
With a default, all coordinate info out there on the Web (flat,
height/depth, time, pressure, ...) will often be interpreted wrongly.
IMHO we should rather encourage, for m2m communication, that we
achieve informational completeness.

my 2 cents,
Peter

On 05/15/15 13:21, Linda van den Brink wrote:
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.

Linda

Van: Peter Baumann [mailto:p.baumann@jacobs-university.de<mailto:p.baumann@jacobs-university.de>]
Verzonden: vrijdag 15 mei 2015 13:01
Aan: Linda van den Brink; Frans Knibbe; SDW WG (public-sdw-wg@w3.org<mailto:public-sdw-wg@w3.org>)
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.
-Peter

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.

Linda

Van: Frans Knibbe [mailto:frans.knibbe@geodan.nl<mailto:frans.knibbe@geodan.nl>]
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.

Regards,
Frans


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