# RFC 2: Spectral Properties¶

Author: Andreas Janz

Contact: andreas.janz@geo.hu-berlin.de

Started: 2021-12-23

Status: Proposed

## Summary¶

It is proposed to implement functions or methods for reading and writing spectral properties from/to QGIS PAM. Relevant items are: center wavelength, wavelength units, full width at half maximum (fwhm) and bad band multiplier values.

Both, QGIS and GDAL, don’t have a concept for spectral property management. In GDAL we have rudimentary support for ENVI files. We currently use the ENVI format as a conceptual base for spectral property handling in the EnMAP-Box.

The here proposed approach will integrate spectral property handling into QGIS PAM management, while honoring the well known ENVI format and naming conventions. This allows to set/update spectral properties for QgsRasterLayer objects, which is critical for GUI applications.

## Motivation¶

GDAL has rudimentary support for spectral properties specified inside ENVI *.hdr header files:

ENVI
...
wavelength units = Micrometers
wavelength = {0.46, 0.465, 0.47, ..., 2.393, 2.401, 2.409}
fwhm = {0.0058, 0.0058, 0.0058, ..., 0.0091, 0.0091, 0.0091}
bbl = {1, 1, 1, ..., 1, 1, 1}


GDAL will dump all items into the ENVI dataset domain, including relevant spectral properties like the list of center wavelength, the wavelength units, the list of full width at half maximum (fwhm) values, and the list of bad band multipliers (bbl):

<Metadata domain="ENVI">
...
<MDI key="fwhm">{ 0.005800, 0.005800, 0.005800, ..., , 0.009100, 0.009100, 0.009100}</MDI>
<MDI key="wavelength">{ 0.46, 0.465, 0.47, ..., 2.393, 2.401, 2.409}</MDI>
<MDI key="bbl">{ 1, 1, 1, ..., 1, 1, 1}</MDI>
<MDI key="wavelength_units">Micrometers</MDI>


GDAL will store the wavelength units inside the default dataset domain:

<Metadata>
<MDI key="wavelength_units">Micrometers</MDI>


For each band, GDAL will store center wavelength and wavelength units inside the band default domain:

<PAMRasterBand band="1">
<MDI key="wavelength">0.46</MDI>
<MDI key="wavelength_units">Micrometers</MDI>
</PAMRasterBand>
<PAMRasterBand band="2">
<MDI key="wavelength">0.465</MDI>
<MDI key="wavelength_units">Micrometers</MDI>
</PAMRasterBand>
...


Given that ENVI specific behaviour, we propose the following approach for fetching band-specific spectral properties.

## Approach¶

Band-wise spectral properties like wavelength, wavelength units, full width at half maximum (fwhm), and bad band multiplier values, are fetched with the following priorisation:

1. Look at QGIS PAM band default domain. This is mainly relevant for GUI applications, where we need to set/update spectral properties using QgsRasterLayer objects:

wavelengthUnits = layer.customProperty('QGISPAM/band/42//wavelength_units')
wavelength = layer.customProperty('QGISPAM/band/42//wavelength')
fwhm = layer.customProperty('QGISPAM/band/42//fwhm')


2. Look at GDAL PAM band default domain. This follows the behaviour of the ENVI driver, that sets wavelength and wavelength units to this location. We consequently also look for fwhm and bad band multiplier here:

wavelengthUnits = gdalDataset.GetRasterBand(42).GetMetadataItem('wavelength_units')


3. Look at GDAL PAM dataset ENVI domain. This follows the behaviour of the ENVI driver, that sets lists of wavelength, fwhm and bad band multipliers (bbl) and the wavelength units to this location:

wavelengthUnit = gdalDataset.GetMetadataItem('wavelength_unit', 'ENVI')
wavelength = parseEnviString(gdalDataset.GetMetadataItem('wavelength', 'ENVI'))[42 - 1]
fwhm = parseEnviString(gdalDataset.GetMetadataItem('fwhm', 'ENVI'))[42 - 1]


4. Look at GDAL PAM dataset default domain:: This follows the behaviour of the ENVI driver, that set the wavelength units to this location. We consequently also look for lists of wavelength, fwhm and bad band multipliers (bbl) here:

wavelengthUnit = gdalDataset.GetMetadataItem('wavelength_unit')


Note that the parseEnviString function is assumed to parse ENVI list string like ‘{1, 2, 3}’ into Python lists like [1, 2, 3].

Guide line 1:
If you need to set band-wise spectral properties in a processing algorithm: set it to the GDAL PAM band default domain. This way, i) the information is accessible with the GDAL API, and ii) consecutive band subsetting via gdal.Translate and gdal.BuildVrt can easily copy the band domains to the destination dataset.
Guide line 2:
If you need to set/update metadata in a GUI application: set it to QGIS PAM. This is most flexible and secure. The spectral properties are i) available as custom layer properties, ii) stored in the QGIS project, and iii) can be saved to QML layer style files.
Guide line 3:
Do not update GDAL PAM *.aux.xml file, while the corresponding source is opened as a QgsRasterLayer in QGIS. QGIS will potentially overwrite any changes, when closing the layer.

## Implementation¶

Technically, we don’t need any new functions or methods, because we fully rely on QGIS PAM.

But, the handling of property keys, and the assurance of fetching priorities, can be tedious and should be encapsulated in util functions or methods. An example implementation is given by the RasterReader class.

To query spectral properties for band 42 (in nanometers), we can use:

from enmapboxprocessing.rasterreader import RasterReader



The destination wavelength units (default is nanometers) can also be stated:

wavelengthInNanometers = reader.wavelength(42, 'nanometers')

bandNo = reader.findWavelength(850)