[Gdal-dev] saving GeoTiff

Wed Mar 19 10:34:21 EST 2003

Now, after I saved several GeoTiff files using
"gdal_translate" source as a program to modify the
settings of Geo data, I don't know what is the value
of the geo data saved???

By changing the values of adfGeoTransform[] elemets: 
*I change the top left corner (longitude, lattitude)
*I change the pixel size

and when I open the image using different viewers, I
get the same image no matter what the values are, the
only change I see is the size and values of grids of
lattitude and longitude lines.

But how the imaage looks, or the height data that some
viewers show, are all unchanged.

Can you explain to me??

--- Ed McNierney <ed at topozone.com> wrote:
> Ayman -
> 
> Since you mention "latitude and longitude", I will
> assume your data is
> unprojected and in decimal degrees.  If so, then the
> "projection" is
> usually called "unprojected" or "geographic" - you
> need the datum and
> the latitude and longitude to define a location.
> 
> If your data is in decimal degrees, then the units
> of pixel size are in
> decimal degrees.  The values specified by the user
> should be the size of
> each pixel in degrees of latitude and degrees of
> longitude.  It is
> unusual for raster data to be in decimal degrees, so
> make sure that is
> correct.  Most raster data is projected.
> 
> 	- 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 10:42 AM
> To: gdal-dev at remotesensing.org
> Subject: RE: [Gdal-dev] saving GeoTiff
> 
> 1.
> sorry sir if I was wrong when calling "NAD83",.. 
> coordinate systems, but I referred to 
>
http://gdal.velocet.ca/projects/opengis/ogrhtml/osr_tutorial.html
> 
> where it mentions exactly:
> "The OGRSpatialReference has built in support for a
> few well known coordinate systems, which include
> "NAD27", "NAD83", "WGS72" and "WGS84....."
> 
> 
> 2. I changed the pixel size by changing the values
> of
> adfGeoTransform[1]//horizental pixel size
> adfGeoTransform[5]//vertical pixel size
> 
> in "gdal_translate" code.
> 
> I am sure that the pixel size was changed, as I
> opened
> the images in other softwares and noticed that the
> size of the grid cells has changed accordingly.
> 
> 3. Anyway, what I want simply is as follows:
> I want to show a dialog the user that contains the
> parametes he/she must fill to be able to generate a
> correct GeoTiff image,
> -I figuered out that I will ask him/her to choose
> one
> of "NAD83","NAD27","WGS72","WGS84"
> -Ask him/her about the latitude and longitude of top
> left corner
> -Ask him/her about pixel size, and I mention the
> unit
> of size, so what does DATUM mean, and if he chooses
> any of the DATUMs, what will the unit be?
> 
> 
> --- Ed McNierney <ed at topozone.com> wrote:
> > 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
> > size.
> > 
> > 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
> > explain?
> > 
> > 
> > --- 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".
> > > 
> > >   
> > >
> >
>
http://www.remotesensing.org/gdal/gdal_datamodel.html
> > > 
> > > 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
> > > 
> > > 
> > > _______________________________________________
> > > Gdal-dev mailing list
> > > Gdal-dev at remotesensing.org
> > >
> http://remotesensing.org/mailman/listinfo/gdal-dev
> 
=== message truncated ===

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