|Private TIFF Tag GDAL_NODATA|
TIFF Tag Reference
TIFF Tag GDAL_NODATA
Used by the GDAL library, contains an ASCII encoded nodata or background pixel value.
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.