RE: CRS "Gotchas"

Jon,

This is very helpful to me in my task of finding reference material.  Good list of topics that these reference should cover.

Thanks

Byron

From: Jon Blower [mailto:j.d.blower@reading.ac.uk]
Sent: Thursday, 4 August 2016 3:49 a.m.
To: public-sdw-wg@w3.org
Subject: CRS "Gotchas"

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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