[Gdal-dev] saving GeoTiff

Ed McNierney ed at topozone.com
Tue Mar 18 10:18:31 EST 2003

Ayman -

1. The four mentioned "coordinate systems" are datums, not coordinate
systems.  The "units of the model system" depend upon the specific map
projection you are using.  For example, I live in Masschusetts, USA, and
locally we use the Massachusetts State Plane coordinate system, which
can be implemented in either feet or meters.  The majority of projected
coordinate systems use meters, but the correct value will depend on the
specific system you are using.

2. The top left corner coordinates and pixel size are all floating-point
numbers and can accommodate any plausible numbers you might have.

3. I don't think you have given us enough information here - can you
explain exactly how you changed the pixel size?

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: (978) 251-4242  Fax: (978) 251-1396
ed at topozone.com

-----Original Message-----
From: Ayman Kamal [mailto:kamal_jo at yahoo.com] 
Sent: Tuesday, March 18, 2003 10:03 AM
To: gdal-dev at remotesensing.org
Subject: Re: [Gdal-dev] saving GeoTiff

ok sir, I figured out that the pixel size decides how
long does the pixel represent in actual model system
units, as the scale factor for geograpich maps,

1. Now I just want to know, for the 4 mentioned
coordinate systems
"NAD27", "NAD83", "WGS72", "WGS84" what are the units
of the model system?? I want to ask the user to
explain this for the user.

2. What are the ranges for valid data the user can
provide for the top left corner of image & for pixel

3. I saved a certain image using GDAL and each time
changed the pixel size,and then opened this image
using GDAL and noticed that there was no difference in
how the image looked like, I am sure of that. Can you

--- Frank Warmerdam <warmerdam at pobox.com> wrote:
> Ayman Kamal wrote:
> > Then, if it can be derived, how can I do so, If it
> > can't, can you give me more explanation?
> > 
> > If you don't have enough time to answer me, can
> you
> > please givem me a link to a web page or whatsoever
> > resource that explains this. I need to understand
> this
> > point THOROUGHLY.
> Ayman,
> I do not know off hand of a resouce that discusses
> this thoroughly.  I would
> suggest you review the information in the GDAL data
> model document about
> "Affine GeoTransform".
> Best regards,
> -- 
> I set the clouds in motion - turn up   | Frank
> Warmerdam, warmerdam at pobox.com
> light and sound - activate the windows |
> http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial
> Programmer for Rent
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