Re: CRS "Gotchas"

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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Received on Wednesday, 3 August 2016 16:53:24 UTC