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RE: BP8: Note about axis order in WKT

From: Byron Cochrane <bcochrane@linz.govt.nz>
Date: Fri, 5 May 2017 09:27:28 +1200
To: "'andrea.perego@ec.europa.eu'" <andrea.perego@ec.europa.eu>, "'public-sdw-wg@w3.org'" <public-sdw-wg@w3.org>
CC: "'jeremy.tandy@gmail.com'" <jeremy.tandy@gmail.com>, "'l.vandenbrink@geonovum.nl'" <l.vandenbrink@geonovum.nl>
Message-ID: <666FB8D75E95AE42965A0E76A5E5337E1650674D9F@prdlsmmsg01.ad.linz.govt.nz>
I think, while it sounds a bit confusing and technical, it is important to note the difference between spherical coordinates (lat, long) and Cartesian coordinates (projected CRS's, x and y).  They, of course, are not the same and have different conventions.

In Cartesian space the order is almost always x, y.  In spherical, the convention is latitude, longitude.  Lat, Long order  is in fact mandated by ISO 6709.  It is true that many do not follow this order.  This is usually for practical reasons.  Software and systems often treat spherical coordinates as if they were Cartesian - they pretend the world is flat because it is more consistent with other systems and easier mathematically.

The flat earth conflation of spherical coordinate systems with Cartesian ones is where a lot of misunderstand and error creeps in.  Most importantly, Cartesian systems use fixed units of measurement such as meters, while spherical systems use degrees which are not of fixed length in distance.  In a practical way for practitioners using the data, this difference of order emphasis these differences.

I always think it is good to acknowledge the decisions of other standards bodies decisions and in this case the ISO 6709 should be mentioned.  If this document could educate the users a bit to this issue, that would be helpful and useful.

Cheers,
Byron

-----Original Message-----
From: andrea.perego@ec.europa.eu [mailto:andrea.perego@ec.europa.eu]
Sent: Thursday, 4 May 2017 7:46 p.m.
To: public-sdw-wg@w3.org
Cc: jeremy.tandy@gmail.com; l.vandenbrink@geonovum.nl
Subject: RE: BP8: Note about axis order in WKT

An additional consideration:

We are talking about coordinate axis-order across the BP document: should we add an entry in the glossary?

Andrea

----
Andrea Perego, Ph.D.
Scientific / Technical Project Officer
European Commission DG JRC
Directorate B - Growth and Innovation
Unit B6 - Digital Economy
Via E. Fermi, 2749 - TP 262
21027 Ispra VA, Italy

https://ec.europa.eu/jrc/

----
The views expressed are purely those of the writer and may not in any circumstances be regarded as stating an official position of the European Commission.

>-----Original Message-----
>From: andrea.perego@ec.europa.eu [mailto:andrea.perego@ec.europa.eu]
>Sent: Wednesday, May 03, 2017 7:58 PM
>To: public-sdw-wg@w3.org
>Cc: jeremy.tandy@gmail.com; l.vandenbrink@geonovum.nl
>Subject: RE: BP8: Note about axis order in WKT
>
>Dear all,
>
>The note about axis order in WKT is now included in the ED (thanks, Linda):
>
>http://w3c.github.io/sdw/bp/#bp-crs
>
>It's the last note under point #3. For your convenience, I include it below:
>
>[[
>It is worth noting that, in the [SIMPLE-FEATURES] definition of WKT,
>the coordinate axis-order is by default longitude / latitude,
>irrespective of the coordinate reference system used. The same applies
>to EWKT (Extended
>WKT) - a PostGIS extension to WKT supported also by other GIS tools -,
>which includes a parameter (SRID) for specifying the coordinate reference system.
>
>For this reason, whenever using WKT to encode geometries, it is
>important that the reference WKT specification can be unambiguously determined.
>]]
>
>Could you please have a check, and verify whether you think it's
>correct / accurate enough? If this is not the case, please propose revised text.
>
>In particular, I would like to ask your feedback on the last sentence:
>
>[[
>For this reason, whenever using WKT to encode geometries, it is
>important that the reference WKT specification can be unambiguously determined.
>]]
>
>Ideally, this text needs to be extended to provide guidance on how to
>make it unambiguous which is the WKT "flavour" used. When using RDF +
>GeoSPARQL's WKT, datatype geosparql:wktLiteral does the job. But what
>about other WKT flavours? And what to do when not using an RDF
>representation?
>
>Thanks In advance
>
>Andrea
>
>----
>Andrea Perego, Ph.D.
>Scientific / Technical Project Officer
>European Commission DG JRC
>Directorate B - Growth and Innovation
>Unit B6 - Digital Economy
>Via E. Fermi, 2749 - TP 262
>21027 Ispra VA, Italy
>
>https://ec.europa.eu/jrc/
>
>----
>The views expressed are purely those of the writer and may not in any
>circumstances be regarded as stating an official position of the
>European Commission.
>
>
>________________________________________
>From: PEREGO Andrea (JRC-ISPRA)
>Sent: 01 May 2017 00:08
>To: Jeremy Tandy; Linda van den Brink
>Cc: SDW WG Public List
>Subject: BP8: Note about axis order in WKT
>
>Jeremy, Linda,
>
>As agreed, I've prepared a draft note for BP8 about how axis order is
>used in (the different flavours of) WKT.
>
>The relevant pull request (including also a number of editorial changes
>mainly on Section 12.2.1 and Appendix A):
>
>https://github.com/w3c/sdw/pull/769
>
>Thanks
>
>Andrea
>
>----
>Andrea Perego, Ph.D.
>Scientific / Technical Project Officer
>European Commission DG JRC
>Directorate B - Growth and Innovation
>Unit B6 - Digital Economy
>Via E. Fermi, 2749 - TP 262
>21027 Ispra VA, Italy
>
>https://ec.europa.eu/jrc/
>
>----
>The views expressed are purely those of the writer and may not in any
>circumstances be regarded as stating an official position of the
>European Commission.



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Received on Thursday, 4 May 2017 21:28:10 UTC

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