[Qgis-user] Non Earth projection (Alex Mandel)

Tue Sep 30 12:58:19 PDT 2014

Priscilla,
you have an easy choice, the georeferencer; you may find under the raster
menu. Use it against a map with known CRS. Il will "almost" become a map in
that system. If it's not in the same projection you will have barrel or
cushion distorsion, but a map is not a picture, it's something you should
use (I think as an historical source, beside others), so some distortion is
bearable.
The alternative is knowing something more about your map. It's not on the
moon, you can do some research about it's origin (in both senses, author
and geodetical measure), if you care about its "shape".
QGIS (and other GIS systems)  reference raster images in a very different
way compared to MapInfo. The latter keep the image as reference and change
the overlaid vectors without reprojecting the raster, but doing so impedes
superimposing two raster images with different projections. You can only
use one reference at a time.
With QGIS - or I may say with GDAL(warp) - you alter the reference. If you
want to keep it intact and accept waiting a little (ehm, something more for
big images), you can "wrap" your image with a virtual envelope
(GDALbuildvrt) and then do any pesky alteration to this envelope without
really touching it.
All the best.
c

On Tue, Sep 30, 2014 at 9:26 PM, Priscilla Maisie <adwoa.maisie at gmail.com>
wrote:

> Its a scanned image I'm trying to digitize for a large scale map. I have
> my coordinates on the map but the challenge is which projection to use for
> georefrencing
> On Sep 30, 2014 6:10 PM, "Carlo A. Bertelli (Charta s.r.l.)" <
> carlo.bertelli at gmail.com> wrote:
>
>> Hello Alex,
>> you took the question too seriously, "Non Earth" is MapInfo parlance for
>> "I dunna 'but it" or "My user and I do not care about CRS". Nothing to do
>> with the moon and the stars, sometimes a space which is not georeferenced
>> (your cellar and bottles, a library with books, etc.).
>> Sometimes this is just related to "paper coordinates" (pixels are not so
>> bad as a unit) or unknown CRSs (better nothing than wrong).
>> Anyway the solution could be from very easy to very difficult. Frequently
>> the user has a way to recognize the right CRS or infer it by
>> time/place/numbers or simply reading what's written on the original (I
>> know, reading the map is more than reading the words in it, but this is a
>> good start...), sometimes it's just a matter of affine transformations
>> (there is a plugin for this, sometimes the map has to be georeferenced
>> again. In these cases there is a plugin to georeference vectors (MapBender,
>> if I remember, I write on my mobile).
>> If Priscilla tells us more about what type of non-earth-projection are
>> these, we could be more helpful.
>> c
>>
>> On Tue, 30 Sep 2014 09:06:40 -0700 Alex Mandel wrote:
>> > From: Alex Mandel <tech_dev at wildintellect.com>
>> > To: Priscilla Maisie <adwoa.maisie at gmail.com>,
>> >         qgis-user at lists.osgeo.org
>> > Subject: Re: [Qgis-user] Non Earth projection
>> > Message-ID: <542AD510.2070808 at wildintellect.com>
>> > Content-Type: text/plain; charset=windows-1252
>> >
>> > On 09/30/2014 03:10 AM, Priscilla Maisie wrote:
>> > > Hi,
>> > > I need to work in a non earth map projection system. Can't locate the
>> > > option on qgis. Is there a way I can create this?
>> > >
>> > > Regards,
>> > > Priscilla
>> > >
>> >
>> > Yes you can create your own projection in Settings->Custom CRS.
>> > You'll need to create your own proj4 compatible string. Comparing a few
>> > similar projections should help you figure out what you need.
>> >
>> > Note in recent versions of QGIS if you look in the Projection on the Fly
>> > dialog, there are several Mars projections and Coordinate systems.
>> >
>> > Need more help, please provide more information about the planet
>> > size/shape and type of projection.
>> >
>> > Other references, http://epsg.io and http://spatialreference.org
>> >
>> > Thanks,
>> > Alex
>>
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>
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