[Gdal-dev] saving GeoTiff

Simon Perkins s.perkins at lanl.gov
Thu Mar 20 14:18:18 EST 2003

On Thu, 2003-03-20 at 09:05, Ayman Kamal wrote:
> Sorry, but your answer didn't help me.
> What I wanted to know simply is
> If I convert a GeoTiff file to a BMP file using
> "gdal_translate"
> Is the data that I will get in t the resulting BMP
> file is just the raster data found in the GeoTiff
> file, or does the geotiff data saved in the GeoTiff
> file affect how the resulting BMP file will look like?


It might help to clarify a few basic concepts:

- Image formats that support geo-referenced data (e.g. GeoTIFF, HFA,
etc) contain two pieces of information:

  * A "raster" containing one or more rectangular bands of image pixels.

  * Geo-reference data specifying such things as location of the corners
of the image in some specified coordinate frame.

- Image formats that don't generally support geo-referenced information
(e.g. GIF, JPEG, TIFF) contain just the raster data.

- A GeoTIFF file is simply a regular TIFF file, with additional special
keywords that indicate geo-referencing information. These keywords are
ignored by applications that understand TIFF files, but don't have any
special knowledge of GeoTIFF.

- In general mage viewing software may, or may not, make use of any
geo-referencing data (if present), when deciding how to display the
raster of pixels in an image file. So a geo-referenced GeoTIFF file may
look different in different viewers.

- If you convert from a geo-referenced format (e.g. GeoTIFF) to a format
that does not support geo-referencing (e.g. GIF) then you may well just
be copying the raster data from one file to another, and the
geo-referencing information will not change the raster information at
all. This is the default behavior for gdal_translate, but I think you
can do some image-reprojection with appropriate options.

- I believe that the BMP format under GDAL supports geo-referencing only
through the creation of a separate worldfile (but I could be way off
target here).

Your questions to date seem to reflect a misunderstanding of one or more
of the above points, so please try to reformulate your question in the
above terms, and there might be chance that someone can help you.



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