RE: Request for input to the BP doc: an introduction to CRSs and related concepts

An alternate (starter for 10) approach to addressing CRS.  Very rough and needs validation of statements.  Trying to present it as an easy to use guide to publishers.

Still needs work and an intro explaining CRS’s and why there are so many different ones.  I am working on a rough outline of that intro.

Cheers,
Byron

Choosing a CRS

By default, most vector data should be shared in unprojected geographic coordinates WGS 84 – SRID 4326. Raster data, which is already projected by the nature of the format, should be shared by default in web Mercator  - SRID 3857.


Consider your audience

As a Publisher you must make decisions on behalf of your users.  Who are your primary and secondary users?  Target those whose requirements match the data.

Is your primary audience is known and work within a limited well defined spatial extent, of say an urban region?  Then it is likely that there is a local projected CRS by which data is commonly shared in that region.  Share your data in this projection.  Consider sharing your data in more than one projection.

Listen to your customers.  Metadata should provide contacts to publishers.  Encourage feedback particularly for new secondary users. This feedback may give insight to projections you should support.

[Web accessibility guidelines - ??]


Consider the Data

Vector data is quite easy for web mapping tools to project from geographic coordinates.  Raster data requires much more intensive computation for this task.  Raster is commonly published in a projected CRS instead of geographic coordinates.  Raster data should be published in the CRS in which it was produced and if possible and useful, also in web Mercator.


Consider the Use

The seemingly multitude of CRS’s exist because of different requirements.  Some projections are better for measuring area, some are better for measure distance, some are better for measuring angles, some simply look better when displayed and make for a better user interface.

Are the aesthetic and readability issues associated with the data?  What projections display the data to best convey the most information you wish to share? Which way is north is effected by the projection chosen.  Spatial data often are used to provide user interface via a map.  Some map projections may support this use better than others.

While it is likely true that the majority of users may be satisfied with WGS 84 and Web Mercator, the most value may be gained by those users requiring other projections.


Consider how the data is distributed (?)

Should there be something said about different requirements that may occur if the data is downloaded as a whole dataset or accessed via a web service? If so does this fall under Use?


From: Joshua Lieberman [mailto:jlieberman@tumblingwalls.com]
Sent: Tuesday, 2 August 2016 4:28 a.m.
To: Ed Parsons
Cc: Rob Atkinson; Linda van den Brink; SDW WG Public List; Jeremy Tandy
Subject: Re: Request for input to the BP doc: an introduction to CRSs and related concepts

Ed,

I’ll have a go at updating this if that’s alright with you. It seems to me it would be good to emphasize that spatial data users are usually projecting their data from WGS84, either explicitly (e.g. Web Mercator) or implicitly (e.g. haversine), in order to map or do calculations. So it may be more of a question of when to know what you’re doing to vary this, rather than projecting or not projecting.

—Josh

On Aug 1, 2016, at 10:10 AM, Ed Parsons <eparsons@google.com<mailto:eparsons@google.com>> wrote:

Turns out you can't do it in a paragraph, here is my first attempt...

https://docs.google.com/document/d/1EO70CvPJWg_dWcmgrzElUQMe6IK5OTry5S291ALQoKo/edit?usp=sharing


Feel free to comment, however I will not react to your valuable contributions until I return from my holidays in two weeks time.

Ed


On Mon, 1 Aug 2016 at 11:07 Rob Atkinson <rob@metalinkage.com.au<mailto:rob@metalinkage.com.au>> wrote:
what - we all have funny shaped heads?  :-)



On Mon, 1 Aug 2016 at 19:14 Ed Parsons <eparsons@google.com<mailto:eparsons@google.com>> wrote:
CRS are not standardised for the same reason we don't all speak English :-)  I will however give it a go..

ed


On Mon, 1 Aug 2016 at 07:39 Linda van den Brink <l.vandenbrink@geonovum.nl<mailto:l.vandenbrink@geonovum.nl>> wrote:
Hi,

I still remember well when I started working at Geonovum and first found out that there were many different coordinate reference systems. It seemed very silly to me that this was not standardized.

When one of you takes up Jeremy’s challenge, please see what is already written about CRS in http://w3c.github.io/sdw/bp/#ref-crs . It makes sense to move some of this to an introductory section in the front of the document, and any content making this section more accessible is welcome!

Linda

Van: Jeremy Tandy [mailto:jeremy.tandy@gmail.com<mailto:jeremy.tandy@gmail.com>]
Verzonden: zaterdag 30 juli 2016 10:06
Aan: SDW WG Public List; Ed Parsons
Onderwerp: Request for input to the BP doc: an introduction to CRSs and related concepts

Hi-

Thinking about the article that I just shared [1] on the mailing list it occurs to me that to the majority of webby folk, coordinate reference systems, projections, datums and the like will seem like some kind of dark art ...

All they will be used to is what every global mapping provider uses: spherical / web mercator EPSG:3857 [2] “google maps projection”

And likely they don't give it any thought at all.

Much in the same way that I have introduced features / spatial things and geometry in the front material of the SDW BP doc [3], I think we need a _short_ section to introduce the topic of coordinate/spatial reference systems (including non-earth CRS; e.g. cartesian used in imagery/microscopy) and projections so that readers:
1. know what the terms mean (a little bit - point them to references if they want to find out more)
2. why it matters

Please can a member of the Working Group provide a paragraph or two- and better still do it in GitHub and create a Pull Request with your proposal?

I'm thinking that this might be right up @eparsons' street (no spatial pun intended). I also recall that I've recently had conversations with @josh about CRS.

Please help.

Best Regards, Jeremy

(who is still thinking about work while I'm trying to pack holiday suitcases!)

PS: I also note that the section I have written on spatial references [4] is a little underwhelming. If anyone fancies re-working that section, I would appreciate it very much!

[1]: https://lists.w3.org/Archives/Public/public-sdw-wg/2016Jul/0336.html

[2]: https://epsg.io/3857

[3]: http://w3c.github.io/sdw/bp/#spatial-things-features-and-geometry

[4]: http://w3c.github.io/sdw/bp/#spatial-relations

--
Ed Parsons FRGS
Geospatial Technologist, Google
Google Voice +44 (0)20 7881 4501<tel:%2B44%20%280%2920%207881%204501>
www.edparsons.com<http://www.edparsons.com/> @edparsons
--
Ed Parsons FRGS
Geospatial Technologist, Google
Google Voice +44 (0)20 7881 4501<tel:%2B44%20%280%2920%207881%204501>
www.edparsons.com<http://www.edparsons.com/> @edparsons


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