[Gdal-dev] [Tiff] GDAL Unofficial Private TIFF Tags

[Frank Warmerdam]

Joris,

I promised to write an explanation of the GDAL private TIFF tags I have
been using.  I would like to stress again that these have not been allocated
by Adobe, though through no fault of theirs.  I never even asked.

I will say that I think both of these have some general utility and if there
was sufficient interest in the TIFF community I would be willing to come up with
a more generalized definition and to try and get properly allocated tags.

==================================================================================
GDAL_NODATA - 42113

Short Description:

Contains an ASCII encoded nodata or background pixel value.

Long Description:

Code: 42113
Name: GDAL_NODATA
Type: ASCII
Count: N
Default: None

Description:

In the geospatial image processing field especially (and in other fields) it is
common to use a special pixel value to mark geospatial areas for which no information
is available.  This is often called a "nodata" or "background" value.  Applications
often treat these pixels as transparent and they are often not included in spatial
statistics for the image.  Non-geospatial applications might still use the nodata
value to track a special value that should be treated as transparent since currently
TIFF palettes don't include an alpha value.

The GDAL_NODATA tag is intended to keep track of what pixel value is being
used for this background nodata value.  It is ASCII encoded so that if the pixel
value 255 was to be treated as nodata, then the tag would have the value "255".

If this tag is absent there is assume to be no nodata value in effect for the image.
If the image has more than one sample it is assumed that all samples have the same
nodata value.

This tag is currently only supported by the GDAL library.

See Also:
   GDAL Library: http://www.remotesensing.org/gdal/

==================================================================================
GDAL_METADATA - 42112

Short Description:

Holds an XML list of name=value "metadata" values about the image.   Used only
by the GDAL library.

Long Description:

Code: 42112
Name: GDAL_METADATA
Type: ASCII
Count: N
Default: None

Description:

Contains an XML fragment that looks like this example:
<GDALMetadata>
   <Item name="CEOS_LOGICAL_VOLUMNE_ID">RSAT-1-SAR-SGF</Item>
</GDALMetadata>

The <Item> names are the name of a metadata keyword, and the text contents of
the Item are it's value.  Normal XML escaping applies.  The GDALMetadata tag
may have zero or more Item sub-elements.  Generally speaking the item names
should be well behaved tokens (alpha-numeric + underscores) though that isn't
strictly enforced.  Also, normally the names are assumed to be unique.

This is an unregistered tag used only by the GDAL library and applications built
on it to hold "dataset" level Metadata about geospatial datasets.


See Also:
   GDAL Library: http://www.remotesensing.org/gdal/





-- 
---------------------------------------+--------------------------------------
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

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