Questions on projections, reference systems, map extents, scales

[Vortex]

Hello Ed!

On Sun, 24 Apr 2005 22:08:57 -0400
Ed McNierney <ed at TOPOZONE.COM> wrote:

> "Vortex" is a good choice - you're certainly spinning around rapidly!
:-]

> ask questions in the most appropriate places.  For example, there is a
> separate mailing list for PROJ and that's the best place to ask
> questions about it.
I'm afraid you are right. Your answers helped me a lot anyway and
i try to write some short comments.

> Yes, the "geographic" coordinate system refers to longitude/latitude
> coordinates.  Longitude and latitude are NOT "always the same" and must
> be expressed relative to a datum; 30 degrees North, 90 degrees West is
> NOT the same location in the NAD27 datum as it is in NAD83.  WGS83 and
Ok! So i suppose this comes from different equator/meridian definitions
inside the datums or something like that? (But you don't need to answer
this because i will follow your advice :-)

> NAD83 are datums - they are not coordinate systems.  Cartographic
> coordinate systems are based on datums.
Ok! I think i got this.

> If you have "pure" (most people would say "unprojected")
> longitude/latitude data you still need to know what datum is used.
Ok, i understand this now.

> If the output and input projections are identical there is no
> reprojection done.  Yes, this is faster, but vector reprojection is VERY
> fast and not worth worrying about (at least, not now).
Ok!

> You will have MUCH better luck getting answers if you limit your
> postings to one or a few related questions.  Long laundry lists of
> questions tend to make readers (a) feel they don't have time to answer
> all those questions and/or (b) don't know how to answer ALL of them, so
> they might as well not answer ANY of them.
Ok, i will consider this for my next posting.
Thank you for having the patience!

Klaus

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