Image Metadata Guidelines

It is helpful if the GAIA images contain metadata giving details about what they represent (but entirely optional). We may add options for the user to query the image for information such as spatial extent and data limits, in which case it is necessary for some degree of standardisation.

Metadata values

All PNG files can include extra parameter name/values pairs, and this provides a means by which additional metadata can be included. The value is always a string. The following table describes the different metadata, and whether or not they should be included in keogram and thumbnail images.

You can add as many or as few of these as you choose, but please follow the conventions below.

Name	Value	Keogram	Thumbnail
PositionX	X value for the keogram	Y	N
DataLimitMin	Lower limit for data value (ie corresponging to 0 in the 8-bit image	Y	Y
DataLimitMax	Upper limit for data value (ie corresponging to 253 in the 8-bit image	Y	Y
DataQuality	Flag indicating quality issues which may affect the result in a negative way (eg Preliminary	Y	Y
DataUnits	The units for the data. Use raw for cases like ADC count.	Y	Y
DataValueMin	Minimum data value within the image (float)	Y	Y
DataValueMax	Maximum data value within the image (float)	Y	Y
DateTimeStart	The inclusive start time of the period that the image represents. Must be in YYYY-MM-DD hh:mm:ssZ format. Z indicates UTC.	Y	Y
DateTimeEnd	The exclusive end time of the period that the image represents. Must be in YYYY-MM-DD hh:mm:ssZ format. Z indicates UTC.	Y	Y
GaiaProject	The textual name (not abbreviation) of the GAIA project	Y	Y
GaiaStation	The textual name (not abbreviation) of the GAIA station	Y	Y
GaiaChannel	The textual name (not refname) of the GAIA channel	Y	Y
PositionUnits	The units for the position	Y	Y
PositionXMin	Minimum X value	N	Y
PositionXMax	Maximum X value	N	Y
PositionYMin	Minimum Y value	Y	Y
PositionYMax	Maximum Y value	Y	Y

Comments sought on GaiaProject, GaiaStation and GaiaChannel. Should we use GaiaProjectAbbrev, GaiaStationAbbrev, GaiaChannelRefName instead? -- SteveMarple - 23 Jan 2008

Adding metadata to PNG images

Matlab

Matlab allows the user to specify extra metadata when a PNG image is saved with imwrite. You can use a combination of imread and imwrite to add metadata to existing images.

Python

The Python Imaging Library (PIL) supports reading and writing PNG metadata. See also http://blog.client9.com/2007/08/python-pil-and-png-metadata-take-2.html. Image.load() loads the metadata into the Image.info dict. Extra effort appears to be required to write this dict back out (see previous link), although this may have changed by now.