[GRASSLIST:5419] Re: How to choose initial parameters?

[Ben Logan]

Thanks to all who answered.  As usual, the people here were very
helpful. :-)

On Thu, Jan 30, 2003 at 08:26:29AM -0500, John Gillette wrote:
> 
> I sympathize because I went through this.
> 
> Here's the answers:
> 
> (Q) How do I come up with all of that information 
>     necessary to creating a database?
> 
> (A) Check the Metadata for the data set.  The Metadata
>     should contain the projection, datum and ellipsoid
>     data.  See examples at bottom.
> 
> (Q)  Back when I first set my GRASS database up, I read 
>      somewhere that I should use the wgs84 ellipsoid.
>      How do you know which to use, and what happens if 
>      you use the wrong one?
> 
> (A)  The meta data may have the ellipsoid data.  However,
>      the ellipsoid is tied to the datum.  For a given datum,
>      a certain ellipsoid is used.  In the US, NAD83 datum
>      uses GRS80 ellipsoid and NAD27 uses Clarke66 (as in
>      1866).  Search the web and read about Datum and
>      Ellipsoid.  If you get it wrong, nothing happens.
>      Grass shows everything in its display as X,Y data.
>      The importance of getting it correct is if you
>      transform from one datum or projection to another 
>      or if you try to display 2 different datum or 
>      projections in the same mapset and expect them to 
>      line up.
> 
> (Q) Since I don't have a database setup yet, I can't 
>     run the GRASS commands to get information from the map data.
> 
> (A) Doesn't matter too much.  Just set up a location.
>     Let's say you have sdts data you want to import.
>     use m.sdts.read to read the files.  Poke around and
>     you will find the metadata you need.  If your location
>     isn't correct, stop grass and restart creating the
>     location you need.  No harm done.

Ahh, the metadata.  And as Yann Chemin pointed out, gdalinfo is very
useful when you don't have GRASS setup yet.  (There's also ogrinfo for
vector formats.)

I'm glad to know all this, because I've been using the wgs84
ellipsoid, and should have been using clarke66.  I think I'll start
over from scratch...I've been needing to clean out my GRASS database
anyway because it's eating up all my disk space. :-)
 
Thanks again,
Ben

-- 
Ben Logan
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