[Gdal-dev] saving GeoTiff

Wed Mar 19 10:56:56 EST 2003

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
> 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
> 
=== message truncated ===

__________________________________________________
Do you Yahoo!?
Yahoo! Platinum - Watch CBS' NCAA March Madness, live on your desktop!
http://platinum.yahoo.com
_______________________________________________
Gdal-dev mailing list
Gdal-dev at remotesensing.org
http://remotesensing.org/mailman/listinfo/gdal-dev