Re: CRS "Gotchas"

Hi Andrea,

Yes, I think there is some relationship there, although I didn’t go into the specifics of CRSs for specific regions.

Cheers,Jon

On 03/08/2016 17:52, "Andrea Perego" <andrea.perego@jrc.ec.europa.eu> wrote:

    Thanks for this, Jon.
    
    I wonder whether your point (3) also relates to the analogy CRS - 
    locales discussed on the mailing list [1].
    
    Cheers,
    
    Andrea
    
    ----
    [1]https://lists.w3.org/Archives/Public/public-sdw-wg/2016Jul/0165.html

    
    On 03/08/2016 17:48, Jon Blower wrote:
    > Hi all,
    >
    >
    >
    > In the last telecon I agreed to draft a list of a few typical “gotchas”
    > regarding CRSs, which commonly catch out newcomers. This list is based
    > on what I would typically tell my software developers in their first
    > days on the job. So it doesn’t attempt to capture all possible issues.
    > I’m assuming that the reader might be involved in developing some
    > web-based mapping tool and knows how to program, but hasn’t come from a
    > GIS background.
    >
    >
    >
    > 1.       Longitude/latitude ordering. Make sure you know what axis order
    > your data uses (lon-lat or lat-lon). And then make doubly sure, because
    > not all data description documents are accurate!
    >
    >
    >
    > 2.       Understand why the Web Mercator projection is the way it is,
    > and its limitations, particularly the distortions that get worse at high
    > latitudes. Don’t use this projection if the relative area of landmasses
    > is important in your application. (Homework: understand map projections.)
    >
    >
    >
    > 3.       Understand time zones and pay attention to what to assume about
    > your data, should the time zone indicator be missing. (Homework: learn
    > about calendar systems.)
    >
    >
    >
    > 4.       Understand that height can be expressed in many different ways,
    > and that GPS (for example) usually gives you height above the ellipsoid,
    > not height above sea level or ground. (Homework: learn about geodetic
    > datums and the differences between different kinds of sea level.)
    >
    >
    >
    > Once they have got the basics we can move on to:
    >
    >
    >
    > 5.       Understand that “CRS” and “map projection” are not the same
    > thing. (Homework: learn the difference between Geographic, Geocentric,
    > Cartesian and Projected CRSs.)
    >
    >
    >
    > 6.       Understand why transforming between CRSs is difficult in
    > general, particularly if a datum shift is involved. (Homework: find out
    > the difference between a geodetic datum and an ellipsoid definition.)
    >
    >
    >
    > 7.       Understand the typical magnitude of different sources of error
    > or assumptions. (So that you can make a decision about what is
    > acceptable.) For example:
    >
    >
    >
    > a.       The difference between the WGS84 ellipsoid and sea level can be
    > tens of metres (maybe more, I can’t remember?)
    >
    > b.       Assuming the wrong datum in a lat-lon system can lead to
    > horizontal positioning errors of a few hundred metres.
    >
    > c.       Plate tectonics moves things around by a few cm per year.
    >
    >
    >
    > Hope this is helpful!
    >
    > Jon
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > *Jon Blower *| CTO, Institute for Environmental Analytics
    >
    >
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    -- 
    Andrea Perego, Ph.D.
    Scientific / Technical Project Officer
    European Commission DG JRC
    Directorate B - Growth and Innovation
    Unit B6 - Digital Economy
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Received on Wednesday, 3 August 2016 18:04:48 UTC