[PROJ] Early vs Late binding

[Nyall Dawson]

On Thu, 1 Aug 2019 at 17:01, Martin Desruisseaux
<martin.desruisseaux at geomatys.com> wrote:
>
>
> "Early binding" ≈ hub transformation technique.
> "Late binding" ≈ hub transformation technique NOT used, replaced by a more complex technique consisting in searching parameters in the EPSG database after the transformation context (source, target, epoch, area of interest) is known.
> The problem of hub transformation technique is independent of WGS84. It is caused by the fact that transformations to/from the hub are approximate. Any other hub we could invent in replacement of WGS84 will have the same problem, unless we can invent a hub for which transformations are exact (I think that if such hub existed, we would have already heard about it).
>
> The solution proposed by ISO 19111 (in my understanding) is:
>
> Forget about hub (WGS84 or other), unless the simplicity of early-binding is considered more important than accuracy.
> Associating a CRS to a coordinate set (geometry or raster) is no longer sufficient. A {CRS, epoch} tuple must be associated. ISO 19111 calls this tuple "Coordinate metadata". From a programmatic API point of view, this means that getCoordinateReferenceSystem() method in Geometry objects (for instance) needs to be replaced by a getCoordinateMetadata() method.
>
> Said otherwise, the solution to misalignment problem involves two parts: dynamic datum and late-binding implementation. Dynamic datum is enabled by replacing association to CRS by association to {CRS, epoch} tuple in all client applications (geometries, rasters, etc.). Late-binding is about knowing the context in which the transformation will be applied, and is more an implementation issue (largely solved in PROJ 6).

Just wanted to extend my thanks for this clear, well reasoned
explanation. It's much appreciated.

Nyall

>
> Regards,
>
>     Martin
>
> [1] https://www.iogp.org/bookstore/product/geospatial-integrity-of-geoscience-software-part-1-gigs-guidelines/
>
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