[QGIS-Developer] Projects without a CRS

[Nyall Dawson]

On Mon, 1 Jul 2019 at 16:46, Andreas Neumann <a.neumann at carto.net> wrote:
>
> Hi,
>
> I am wondering about projects without a CRS. While a new project now always comes with a CRS (depending on the global CRS settings), the user can still explicitly disable a definition of a CRS in Menu "Project" --> "Properties" --> "CRS"
>
> I have the following questions about projects without a CRS:
>
> - what are the restrictions/limitations of projects without a CRS?

Coordinates are all just treated as "pure numbers". Distance and area
measurements are unit-less. Scale values have no real-world meaning.
Time and space break down into a singularity where "up" is left and
"right" is tuesday, July (and sometimes never). (ok, everything but
that last one!)

>
> - assuming that a layer has a CRS (e.g. a Postgis layer that has a CRS) is loaded into a project that explictly has no CRS set - does QGIS still know the CRS of the Postgis layer or would it treat it as a CRS-less layer?

It still knows the CRS, and will show it e.g. in layer properties, but
the project doesn't do anything with this information. It's still
utilised by layer-specific operations though (e.g. saving the layer to
another format).

> - does it still make sense to support projects without a CRS? If yes, what are the use cases?

Yes, for projects which don't use real-world coordinates. E.g.
projects using some local invented coordinate system, buildings,
mapping on tiny scales, mapping off-world objects, creating maps of
fantasy novels or video game worlds,...

> - will it still be possible to have projects without a CRS or will this one day be deprecated

I don't think it should ever be deprecated, there's a demonstrated need for it.

> I personally do understand that it is not a good idea to have projects with no CRS defined, but I am trying to understand and convince my colleagues at my workplace that projects without a CRS are an absolute nogo. But maybe I am wrong and it still makes sense to support projects without a CRS.

It's basically the same argument required to convince CAD designers to
use real world coordinate systems and not just some paper-based
reference system. Fortunately, I think that battle has more-or-less
been won, so maybe the same arguments can be used here...

The other thing to consider is that CRS-less projects are only half
the solution. We also should add concepts of CRS-less layers, which,
when loaded into a project WITH a CRS set, are still always just
treated as raw numbers and not projected in any way.

Nyall


> One use case I could think of for projects without a CRS, is using QGIS for something that is planar, but not geographic in nature, e.g. a non-GIS topic, such as examining an image (let's say the enlarged view of an electron microscope of some small thing) where the user might want to "map" certain characteristics of the image, but the object itself is "mobile" (can move around) and is not fixed to a fixed position of the earth.
>
> What are your thoughts about projects without a CRS?
>
> Greetings,
>
> Andreas
>
>
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