### Choice of Reference Point

When writing a FrameSetFrameSet to a FitsChanFitsChan, the pixel coordinates of the reference point for linear axes (i.e. the CRPIXj values) are chosen as follows:

- If the FrameSet is being written to a FitsChan which previously contained a set of axis descriptions with the same identifying letter, then the previous CRVALjvalues are converted into the coordinate system of the FrameFrame being written (if possible). These values are then transformed into the pixel Frame, and the closest integer pixel values are used as the CRPIX keywords.
- If the above step could not be performed for any reason, the central pixel is used as the reference point. This requires the image dimensions to be present in the FitsChan in the form of a set of NAXISj keyword values.
- If both the above two steps failed for any axis, then the pixel reference position is set to a value of 1.0 on the pixel axis.

The pixel to WCS MappingMapping is then used to find the corresponding CRVALjvalues.

Again, the above procedure is over-ridden if the Frame is a SkyFrameSkyFrame or a SpecFrameSpecFrame, in which case the conventions of papers II and III are used as described below.