[Qgis-user] QGIS project with no projection

[Lee Hachadoorian]

Simon,

You *must* specify a projection, but within some limits, the projection
doesn't matter. Basically, if the GeoTIFF uses units of meters and you want
your distances in meters, you can load the GeoTIFF in *any* projected CRS
that uses meters. An example would be to use any of the UTM zones. You
cannot use a lat-long CRS such as the default WGS 84, because QGIS will
convert (naively, *not* using great circle distances) from decimal degrees
to meters.

Once the layer is loaded, all geometry calculations are done in a Cartesian
plane.

I answered a similar question on GIS.SE regarding someone who wanted to use
QGIS to map indoor location data.

http://gis.stackexchange.com/questions/187699/how-to-create-a-qgis-map-of-unprojected-data

Best,
--Lee

On Fri, May 6, 2016 at 10:12 AM, Jonathan Moules <
jonathan-lists at lightpear.com> wrote:

> Hi Simon,
> If you know the real-world width and height of the area covered on the
> image, could you not geo-reference it to have lower-left at 0,0, and then
> the top-right at the applicable location in the projection of your choice,
> and then work from there?
>
> The problem is picking an applicable projection. I can't see any generic
> "X,Y" projection, let alone one that is suited to equal-area or
> equal-distance in the projections list. They may exist, I'm just failing to
> find them. They all seem to assume they're "somewhere" in the world (which
> can be a problem if the data is spatial but not geographical).
> Personally if it's a relatively small area (a few km across), I'd probably
> just use a standard Mercator for the applicable part of the world probably.
>
> Cheers,
> Jonathan
>
>
>
> ---- On Fri, 06 May 2016 13:33:47 +0100 *simonc8<simoncrombie at onetel.com
> <simoncrombie at onetel.com>>* wrote ----
>
> I want to use QGIS to make measurements such as distances and angles on a
> flat scaled image which I have as a georeferenced tiff. I don't want to use
> any projection - just a straightforward orthogonal linear scale in metres.
> QGIS seems to require that I choose a projection. What would be the best
> choice for my needs? Can I make a custom projection with no ellipsoid or
> datum?
>
> If I import my geotiff and don't specify a projection QGIS assumes my units
> are in degrees rather than metres, which will make measurement of distances
> and angles inaccurate.
>
> Grateful for assistance.
>
>
>
> --
> View this message in context:
> http://osgeo-org.1560.x6.nabble.com/QGIS-project-with-no-projection-tp5264924.html
> Sent from the Quantum GIS - User mailing list archive at Nabble.com.
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-- 
Lee Hachadoorian
Asst Professor of Geography, Dartmouth College
http://freecity.commons.gc.cuny.edu/
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