[PROJ] RFC4: last chance to comment before motion

[Even Rouault]

Hi Nyall,

> My understanding of the current API leads me to believe that this
> wouldn't be possible in the current RFC4 API. The missing bits would
> be:
> 
> 1. Public api to query if the preferred operation between two crses
> requires a grid download for a certain area
> 2. (Thread safe) API to manually download a grid for a certain area
> 
> What's your thoughts on this?

I appreciate your suggestions but I can see some difficulties:

- "requires a grid download for a certain area". Imagine that the case is
  to reproject the 4 corners of Continental US. You just need (part of) the  
  grids at those points, not for the whole continent. More generally, this
  depends on the 'density' of points on a given area with respect to the
  resolution of grid(s).

- there is also the fact that the local cache is of limited size, so if for
  the whole set of points to coordinates you'd need more than the cache
  maximum size, eviction will occur. For that point, that could be potentially
  avoided by making the cache size such that it never happens. I arbitrarily
  decided for a default of 100 MB. The total size of GeoTIFF grids is close
  to 500 MB currently. So if we set the default to a bit more than the total
  size of grids, eviction would never occur. That said, this point is probably
  only theoretical. The 2 largest files currently are egm08_25.tif (77 MB) and
  BWTA2017.tif (70 MB), so even if you reprojected entire Baden-Wurttemberg
  and referenced EGM08 height that would still below 100 MB.
   
So perhaps an easier & robust solution for your use case could be:

- for the transformation of interest, use the PROJ_GRID_AVAILABILITY_IGNORED 
flag to have the operations returned in the ideal case. Then for each point to 
transform, you check the first candidate operation whose area of use 
intersects with the point. That's the one that would be selected in an ideal 
world. If it references a grid that proj_coordoperation_get_grid_used() 
indicate as not available, take note of it, and use the next operation that 
doesn't need a grid or for which grids are available, to perform the 
approximate coordinate transformation.

- at the end of this stage, collect the set of missing grids and download them 
(entirely), and restart the transformation.

Which can be done with existing (non-RFC4) API.

Even

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