[Qgis-user] Re: Unusual application of QGIS

[Gerhardus Geldenhuis]

Thanks for the reply

On 8 May 2012 12:11, Rainer M Krug <r.m.krug at gmail.com> wrote:

>
>
> > 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.
>

All true, but at least it sounds possible. The images I am mapping is
square so another thing to consider in my co-ordinate and projection
system.


>
> 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.
>

I do have 3 co-ordinates but for the work that we do at least for now the
y-co-ordinate is not really important so should not be a problem.


>
> >
> > 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?
>
>
Yes the area being mapped is flat for practical intents and purposes. and
everything should be parallel. Although some images might need rotation
since it is not possible on a micron meter scale to align up a microscope
slide exactly parallel.

I am planning on doing a lot more work on this later in the summer and will
report back to the list of results and any more questions which I might
have.

Regards

-- 
Gerhardus Geldenhuis
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