Constrain IWXXM to 2D spatial CRSs

[marqh]

#120 was discussed in the Joint MIE MRI meeting in November 2017

A consensus was reached that IWXXM shall constrain use to 2D CRS definitions

An implementation plan to provide this constraint is now required

[marqh]

One implementation proposal I have on this topic is to mandate the use of
srsDimension

within IWXXM.

srsDimension is optional within GML, with a constraint that if it is stated it must be consistent with the canonical information in the defined CRS, referenced by the srsName

By mandating that IWXXM messages use the srsDimension and that it's value is always set to srsDimensions=2 then IWXXM is enforcing the use of 2D horizontal CRS definitions within ElevatedPoint and other elements.

This proposal avoids the need for a whitelist of allowable srsName values, which may be an overly restrictive constraint for ongoing development.

@blchoy how would this look to you as a verifiable constraint?

[blchoy]

srsDimension is optional within GML, with a constraint that if it is stated it must be consistent with the canonical information in the defined CRS, referenced by the srsName

The (embedded) schematron rules for GML are in this file but it doesn't seem to have the constrains you've mentioned. Seems to me that we will have to implement the CRS checking directly.

The checking is further complicated that srsName could be inherited from an ancestor as described here.

[marqh]

If we were to take this path, I think that we would need to link the requirements, making one conditional on the other.

We could perhaps state:

If and srsName is defined on an element, then that element shall also define
  - an srsDimension 
  - define srsDimension=2

This may be able to be added by us, but it would have to be part of IWXXM

We may want to add a note indicating why this is done, and highlighting that the srsDimension value must be consistent with the definition referenced by the srsName

This is extra work for us to manage inside IWXXM

[blchoy]

Let me recap what we would like to do. Correct me if I have interpreted it wrongly:

1. Indicate in our documentation that we SHALL only use 2D CRS in all features (we need to confirm this later on) making reference to the coordinate system
2. Introduce schematron rules to ensure (1) is complied in IWXXM reports

There are certain ways to implement (2), including:
a. Mandate the use of 2D CRS by directly checking srsName
b. Mandate the inclusion of srsDimension=2 when srsName is specified, but expect the document creator to ensure the srsName is consistent with srsDimension

My gut feeling is that implementing (b) may not be reliable enough to tell people one has violated (1). Having said that, enforcing (1) with (a) at this moment could be too restrictive, especially as we still need to figure out how to manage the list of 2D CRS to be allowed.

[marqh]

question: should elevation in aixm:ElevatedPoint mandate that a vertical datum is defined

[blchoy]

This was implemented in IWXXM 3.0.0RC3.