[Gdal-dev] saving GeoTiff
Ed McNierney
ed at topozone.com
Tue Mar 18 10:47:49 EST 2003
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
>
>
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