[Gdal-dev] saving GeoTiff

[Simon Perkins]

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?

Ayman,

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.

Cheers,

Sy