[Gdal-dev] saving GeoTiff

Wed Mar 19 12:55:53 EST 2003

Ayman -

You should ask that question to the developer of the viewing program you are using.  It's entirely up to that program to decide what data it uses or does not use when displaying an image.

	- Ed

-----Original Message-----
From: Ayman Kamal [mailto:kamal_jo at yahoo.com]
Sent: Wednesday, March 19, 2003 11:11 AM
To: gdal-dev at remotesensing.org
Subject: RE: [Gdal-dev] saving GeoTiff

Maybe I misleaded you with my words,
what I want to know clearly is the answer to this
question:

The data saved in the geotiff file (other than raster
data) is data that specifies the coordinate location,
and this can appear clearly in the grid lines of long
and lat that some viewers show,

BUT this coordinate data, doesn't affect by any means
the raster data or how it is viewed.

AND what is mentioned in the geotiff specification
about geo keys and geo data, is about the data that is
saved in the geotiff file which is extracted to show
the coordinate grids only.

--- Ed McNierney <ed at topozone.com> wrote:
> Ayman -
> 
> YOU need to take the time to understand the viewing
> software you are using.  Read the documentation and
> understand what it is supposed to do.  Then, if you
> find your images do not work as you expect, you can
> explain what the problem seems to be.
> 
> If you find that an image appears different using
> two different viewers, first take time to understand
> what those viewers do and how they display images. 
> It is certainly possible that two different viewers
> will display the same image two different ways;
> that's not surprising at all, and probably has
> nothing to do with GDAL.
> 
> 	- Ed
> 
> -----Original Message-----
> From: Ayman Kamal [mailto:kamal_jo at yahoo.com]
> Sent: Wednesday, March 19, 2003 10:50 AM
> To: gdal-dev at remotesensing.org
> Subject: RE: [Gdal-dev] saving GeoTiff
> 
> 
> Sir,
> What I want to make sure about is, if I don't care
> about showing a grid for coordinates(long,lat), then
> If I just take the raster data out of the geotiff
> file, this will be adequate?
> 
> If we say yes for the above question, then if I open
> a
> geotiff file and just read the raster data, I should
> get the same view I have when opening it using a
> GeoTiff viewer, 
> 
> BUT what I notices when I started search about 
> 
> GeoTiff, when opening geotiff files with ordinary
> image viwers(not supporting geotiff) they showed the
> image differnt from it appears in geotiff viewrs.
> 
> Can you PLEASE explain to me?
> 
> 
> --- Ed McNierney <ed at topozone.com> wrote:
> > Ayman -
> > 
> > What do you expect to see?  Many viewers will load
> > an image and display the entire image on the
> screen,
> > or display the upper-left portion at 100% zoom -
> one
> > pixel on the screen for each input pixel in the
> > image.  Both of these views are independent of the
> > geographic coordinates or the pixel size, so
> > changing those values won't change the image. 
> They
> > WILL change the coordinate information (for things
> > like the "size and values of grids", so your
> > description sounds exactly like what I would
> expect
> > to see.
> > 
> > 	- Ed
> > 
> > Ed McNierney
> > President and Chief Mapmaker
> > TopoZone.com / Maps a la carte, Inc.
> > 73 Princeton Street, Suite 305
> > North Chelmsford, MA  01863
> > ed at topozone.com
> > (978) 251-4242 
> > 
> > -----Original Message-----
> > From: Ayman Kamal [mailto:kamal_jo at yahoo.com]
> > Sent: Wednesday, March 19, 2003 10:34 AM
> > To: gdal-dev at remotesensing.org
> > Subject: RE: [Gdal-dev] saving GeoTiff
> > 
> > 
> > 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
> 
=== message truncated ===

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