[PROJ] "Fallback" support for transformations

[Even Rouault]

> Another constraint
> for gsb files is that the resolution of the sparse parent grid should be a
> multiple of the resolution of the dense subgrid and that a subgrid should
> cover the area of an integer number of parent grid cells.

For what is worth, the Geodetic TIFF grids (GTG) format resulting from RFC4 
doesn't have those NTv2 specific constraints: a subgrid must just fall inside 
the extent of its parent.

Even

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com