Spatial referencing of a PNG image; pHYs unsuitable

[svgeesus]

Raised by: Adrian Custer
Original date: 12 Jun 2012

I am working as part of the Open Geospatial Consortium (OGC) on a web
service standard which exchanges PNG datastreams which are cartographic
maps (or tiles thereof). As part of that work, I am having to establish
how a PNG decoder determines the spatial referencing of a PNG image,
both the initial orientation (where I rely on the orientation of the
'reference image') and the various geospatial referencing elements. For
the latter, I claim that PNG provides no information but this forces
me to dismiss the 'pHYs' chunk of the standard as descriptive and also,
possibly, to address currently registered extensions such as 'oFFs'.

[ProgramMax]

Indeed, the pHYs chunk and oFFs are not suitable for geospacial information.

This is related to #15. In fact, they're two separate pieces of the original post.
The original post suggests that a new chunk may or may not be needed to represent geospacial information.
Let's determine that in #15 and then update this issue as needed.