[Gdal-dev] Bands and overviews; BSB

[Frank Warmerdam]

Luis W. Sevilla wrote:
> Hi!
> 
>   I've two questions
>       - how must use overviews on bands (MrSIG, Erdas .img) ?

Luis,

You can build overviews for most file formats using the gdaladdo utility
or the corresponding BuildOverviews() method on the GDALDataset.  For most
formats the overviews are built as an external TIFF file given the extension
.ovr (a GDAL convention).

If you read data with the RasterIO() calls the overviews will be automatically
used to satisfy requests where your file window size is much larger than the
size of the buffer you are copying too.  You can also explicitly fetch the
overviews as GDALRasterBand objects and read from them.

>       - how must user Control Points on BSB (.kap) to obtain
>    georeferencing?

It is up to the application to decide how to use the GCPs to georeference
an image with GCPs.  Two common ways GDAL based applications approach this
are to derive an approximate affine transformation (with GDALGCPsToGeoTransform())
or to build higher order polynomials with the GDALCreateGCPTransformer().

For BSB files the approximate geotransform is usually ok if you accuracy needs
are not high.

Best regards,
-- 
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