[Qgis-user] QGIS project with no projection
Andrew Harfoot
ajph at geodata.soton.ac.uk
Tue May 10 04:43:01 PDT 2016
QGIS is configured into a non-earth or local CRS mode by doing the
following (in 2.14):
1. Turn off 'On the Fly CRS transformation' (Project > Project
Properties > CRS)
2. Ensure that the measurement ellipsoid is set to 'None / Planimetric'
(Project > Project Properties > General)
3. Set the coordinate, distance and area units to 'Map Units' (Project >
Project Properties > General)
All of these can also be set in the application level settings as the
defaults. QGIS can also be prevented from prompting for a CRS when
loading data that does not have one (QGIS still assigns a CRS, but
essentially it is ignored, so this isn't quite what Simon was asking
for, but close)
The LocalCoordinateSystemSetter in FME creates custom CRS for features
passing through it, optimising them for equal area or distance. The same
customised CRS could be implemented in QGIS, what is missing is a tool /
plugin that would automatically generate them.
Cheers,
Andy
On 06/05/2016 17:03, Jonathan Moules wrote:
> 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 <http://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
> <mailto: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
> <mailto: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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--
Andy Harfoot
GeoData Institute
University of Southampton
Southampton
SO17 1BJ
Tel: +44 (0)23 8059 2719
Fax: +44 (0)23 8059 2849
www.geodata.soton.ac.uk
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