[Qgis-user] Re: Unusual application of QGIS

[Rainer M Krug]

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On 07/05/12 17:27, Gerhardus Geldenhuis wrote:
> Hi

Hi

> I was wondering if QGIS could be used in a slightly different way.

As nobody has answered, I'll give it a try.

> 
> I do EMPA ( Electron probe micro-analyzer ) of microscope slides. When it is in the device I
> can note down x,y,z co-ordinates when taking a photo or doing a sample analysis. The problem is
> when you remove the slide and put it back in again these previous x,y and z components are now
> meaningless because the device that holds the slide is not accurate enough to achieve the same
> positioning again. I was thinking that if I had known reference points on the slide I could
> "geo-reference" all pictures and scans I did for a specific session and then when I have follow
> up photos and point analysis data, taken in a separate session, I could "geo-reference these
> and use QGIS to map out all the photos and pin point analysis as different layers making it
> very easy to correlate data. As long as I have the co-ordinates I should then be able to add
> any other microscope ( eg. petrographic images ) data as a separate layer and match it up.


Sure - you can "geo-reference" whatever image you have against whatever system. In your case, yo
should be choosing the right co-ordinate system and the right projection. This is something I
can't help you with at the moment, but it should be easy enough to find out. If in doubt, ask
specifically for this again, as it is of uttermost importance for measuring distances, areas,
calculations and so on.

One definitely has to be aware, that the space of the GIS, which is usually the surface of the
earth (or a section of it) can be anything. It can simply be a piece of paper or, in your case,
the area *flat* area which is seen in the EMPA. And you want to reference your slide to the area,
so that a specific point of the slide is always at the same location - look at it as satellite
pictures: the earth is your slide, and the satellite is your EMPA (OK - the earth is not flat, but
this is where the projection comes in).

> 
> If I could get it working I could also write a plugin that could tell me co-ordinates of a
> point on the "map" based on the new x,y co-ordinates when I re-insert the microscope slide. 
> Just in case it is not clear let me explain: When I insert a slide into the EMPA machine a
> certain spot will have co-ordinates of 255,234,40. When I re-insert the slide it might now have
> co-ordinates of 100,2000,80. However it is still the same point but because I am measuring in
> nano meters small changes can make big differences.

Just to be sure - you have three co-ordintes here? then it might be a problem - but if you only
want to geo-reference the *picture* of the slide into a two dimensional flat surface, that should
work easily.

> 
> My question is thus, does this sound possible and have anyone else done something similar? The
> area being mapped is actually flat and I am not sure if that would make a difference.

1) The area being mapped is flat
2) are all areas parallel?


> 
> Regards

Cheers and ask again if you have further questions,

Rainer


> 
> -- Gerhardus Geldenhuis
> 
> 
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