## Networks of Coordinate Systems

Mappings and Frames may be connected together to form networks called FrameSets, which are used to represent sets of inter-related coordinate systems (Figure 7).

Figure 7: A FrameSet is a network of Frames inter-connected by Mappings such that there is exactly one conversion path, via Mappings, between any pair of Frames.
[width=0.65]sun211_figures/frameset

A FrameSetFrameSet may be extended by adding a new FrameFrame to it, together with an associated MappingMapping which relates the new coordinate system to one which is already present. This process ensures that there is always exactly one path, via Mappings, between any pair of Frames. A function is provided for identifying this path and returning the complete Mapping.

One of the Frames in a FrameSet is termed its base Frame. This underlies the FrameSet's purpose, which is to calibrate datasets and other entities by attaching coordinate systems to them. In this context, the base Frame represents the “native” coordinate system (for example, the pixel coordinates of an image). Similarly, one Frame is termed the current Frame and represents the “currently-selected” coordinates. It might, typically, be a celestial or spectral coordinate system and would be used during interactions with a user, as when plotting axes on a graph or producing a table of results. Other Frames within the FrameSet represent a library of alternative coordinate systems which a software user can select by making them current.

Further reading: For a more complete description of FrameSets, see §13 and §14. Also see the FrameSet entry in Appendix D.