[Qgis-user] Non Earth projection

[Alex Mandel]

If that's the case then you are correct, simply ignore the projection
system and everything will be in relative units to what you load. This
works fine for scanned/photo/hand-drawn maps, assuming you photographed
at 90 to the plane with a short lens that doesn't distort at the edges.
You can later georeference and reproject your local unknown coordinates
to match something else if you want.

I have done this for scanned maps, and it works fine. There is no need
to create a "Projection" for it at all.

If you're really dealing with somewhat imprecise paper drawings you may
want to georeference it first with Thin Plate Spline (which does
variable distortion correction). The digitize from the result in a known
CRS/SRS.

Thanks,
Alex


On 09/30/2014 10:08 AM, Mats Elfström wrote:
> Hi!
> I think that Priscilla does not mean another celestial body by 'non earth',
> but rather the old MapInfo concept of a non projected flat coordinate
> system. This is called a cartesian system, and would be useful in case QGIS
> is used to digitize scanned paper drawings for instance.
> In MapInfo, a cartesian table can be set up to a high resolution since such
> projects typically covers very small areas (printed drawings).
> Many years ago, when QGIS was still a toddler, I solved the problem by
> simply disregarding the projection factor since it would make a very small
> difference for a small area.
> But I did spend some time trying to create a flat projection. This failed,
> being a contradiction in terms.
> 
> Regards, Mats.E
> 
> 2014-09-30 18:06 GMT+02:00 Alex Mandel <tech_dev at wildintellect.com>:
> 
>> 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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-- 
Alex Mandel
http://wildintellect.com