[Qgis-user] QGIS project with no projection

[Jonathan Moules]

There have been some more knowledgeable responses than mine, but they do make me ask:
Why doesn't QGIS have a generic Coordinate System for X/Y in meters (and maybe one in feet for the less developed nations ;-) )? It would be particularly useful for non-geographical datasets and would open QGIS up to being used more readily in other fields.

Similarly, it could have generic projections for equal area and equal distance calculations too. I know these are all possible because FME has all these features.
After all, there's no reason spatial data has to be GEOspatial data.

Cheers,
Jonathan



---- On Fri, 06 May 2016 16:28:45 +0100 Lee Hachadoorian<Lee.Hachadoorian+L at gmail.com> wrote ---- 

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> 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. 



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-- 
Lee Hachadoorian
Asst Professor of Geography, Dartmouth College
http://freecity.commons.gc.cuny.edu/
 
 




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