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-li...@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<simoncrom...@onetel.com > <simoncrom...@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. > _______________________________________________ > Qgis-user mailing list > Qgis-user@lists.osgeo.org > List info: http://lists.osgeo.org/mailman/listinfo/qgis-user > Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user > > > > > _______________________________________________ > Qgis-user mailing list > Qgis-user@lists.osgeo.org > List info: http://lists.osgeo.org/mailman/listinfo/qgis-user > Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user > -- Lee Hachadoorian Asst Professor of Geography, Dartmouth College http://freecity.commons.gc.cuny.edu/
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