[PROJ] Static/Dynamic Webmapping Problem version 2.0

[Martin Desruisseaux]

Hello Cameron

Le 15/07/2019 à 01:59, Cameron Shorter a écrit :

> * We are currently using the dynamic WGS84 datum as if it were a
> static datum (locked to a different epoch in different regions).
> * This appears to be suitable for current web mapping requirements.
> * Realigning static and dynamic datums (as started by Australia, with
> others to follow) is tripping a map misalignment problem.

I'm not sure which "dynamic WGS84 datum" we are talking about. If we are
talking about EPSG::4326, in my understanding this is now an "ensemble
datum", i.e. a group of many different WGS84 datum (there is at least 6
different WGS84 datum today) to be considered as equivalent for
applications that do not need an accuracy better than a few meters. An
"ensemble datum" is not the same as a "static datum" in the sense that
it does not mean "a datum that does not move", but rather "we don't
really know which datum it is, but we don't need to care if the desired
accuracy is only a few meters".

I'm also not sure what me mean by "suitable for current web mapping
requirements". Do we we mean "an accuracy of a few meters is
sufficient"? If yes, then this requirement is addressed by above-cited
"ensemble datum", which is a new concept introduced in ISO 19111:2019.


> Question to techies:
> * Is it technically possible to create a reference frame realisation
> (datum) which is fixed to different epochs in different regions?

One implication of this approach would be that coordinate operations
would need to identify in which zone (e.g. using a R-Tree) are located
the coordinate values to transform, which is potentially a costly
operation. Another issue is that it would introduce discontinuities at
the frontier between different zones. It is possible that those
inconvenient would bring bigger problems than the problem we are trying
to solve.

    Regards,

        Martin