One aspect of the axis coordinate system which has not yet been discussed is the property of axis normalisation, which is indicated by a logical normalisation flag associated with each axis. This flag does not affect the interpretation of the axis information itself, but instead determines how the NDF's data and variance arrays should behave when the associated axis information is modified.
If the normalisation flag for an NDF axis is set to .TRUE., then it indicates that the NDF's data values (and by implication its variance values) are normalised to the pixel width values for that axis. To give an example, suppose that a spectrum contains data values representing energy accumulated per unit of wavelength, with each pixel having a known spread in wavelength. In this case, the sum of each pixel's data value multiplied by its width will give the total energy in any part of the spectrum. This is an important property which may need to be retained if the axis width values are altered for any reason (e.g. to apply an instrumental correction, or to allow for red-shift).
The axis normalisation flag indicates whether this type of normalisation should be preserved. If it is set to .TRUE., and the associated axis width values are modified, then each NDF data value should be multiplied by an appropriate factor so that its data width product remains unchanged. If present, the variance values should also be corrected by multiplying by the square of this factor. In cases where more than one axis normalisation flag is set to .TRUE., the correction factors for each axis must be applied in turn.
If all the axis normalisation flags are set to .FALSE. (the default situation), then no changes to the data or variance components will be necessary if the axis width values are modified.