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https://github.com/pydata/xarray/issues/3620#issuecomment-565622345 https://api.github.com/repos/pydata/xarray/issues/3620 565622345 MDEyOklzc3VlQ29tbWVudDU2NTYyMjM0NQ== 1197350 2019-12-13T21:53:55Z 2019-12-13T21:53:55Z MEMBER

Thanks for reaching out Erik! We’d love to find a way to better support Astro data in xarray. Before digging deeper, I just want to ask a clarification question. When you say “arbitrary complex mathematical functions”: what are the arguments / inputs to these functions?

Presumably they have to be evaluated at some point, ie for plotting. Can you describe what happens when the time comes to turn the arbitrary functions to actual numbers?

Your answer will help us respond more accurately to your question.

On Dec 13, 2019, at 3:51 PM, Erik Tollerud notifications@github.com wrote:

 @Cadair and I are from the solar and astrophysics communities, respectively (particularly SunPy and Astropy). In our fields, we have a concept of something called "World Coordinate Systems" (WCS) which basically are arbitrary mappings from pixel coordinates (which is often but not necessarily the same as the index) to physical coordinates. (For more on this and associated Python/Astropy APIs, see this document). If we are reading correctly, this concept maps roughly onto the xarray concept of "Non-dimension coordinates".

However, a critical difference is this: WCS are usually expressed as arbitrary complex mathematical functions, rather than coordinate arrays, as it is crucial to some of the science cases to carry sub-pixel or other coordinate-related metadata around along with the WCS.

So our question is: is it in-scope for xarray non-dimensional coordinates to be extended to be functional instead of having to be arrays? I.e., have the coordinate arrays be generated on-the-fly from some function instead of being realized as arrays at creation-time. We have thought about several ways this might be specified and are willing to do some trial implementations, but are first asking here if it is likely to be

Easy Hard Impossible PR will immediately be rejected on philosophical grounds, regardless? Thanks!

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