[Qgis-user] scaling bug?

[Bernardo Rossi Doria]

Hi
I already discuss some of my problems about projections.
Now I looked carefully to this thread.

The  general information from Carson Farmer looks as interesting. So  
I  did follow your suggestions and information.

As I already  told to the list I have Vectors in UTM32 (Municipalities  
borders in italy),  and in WGS84 (reference frame for maps of an  
italian  region), and I have Geotiff maps of the same region in WGS84.

I tried first to set the project In decimal degrees,  WS84 and  
checked   flight ptojection. Imported vector reference frame and  
geotiff maps. They perfectly overlap  of course,   but the UTM32 does  
not.

Second I tried to set meters and UTM32 and flihgt projection. the UTM  
file comes out. Then I can see both vector and  e geotiff WGS84 in the  
legend but they disappear in main window.

Third  I tried the same as second with degrees instead of meters  UTM  
comes out and also geotiff but the do not overlap.

So I am still to the starting point.

as final non expert user i cannot give suggestions, but I hope  that  
this problem will be clarified.

the best to all.

bernardo




Il giorno 22/ott/08, alle ore 16:03, Carson Farmer ha scritto:

> Hi all,
>
> Here is a little bit of general projection information:
>
> First things first: If you want your QGIS project to be in a  
> particular coordinate system (such as M34) , then do this in the  
> project properties dialog. I think this point is fairly clear...
> But you still have to turn on 'on the fly projection' if you want  
> layers in different coordinates systems (or projections) to line up...
>
> The other thing to keep in mind is the meta-data that you get with  
> your layer (your spatial data SHOULD have some meta-data, if it  
> doesn't try to find out who, what, when, why, how, and in what  
> coordinate system the data was created). If your layer is indeed  
> already in some projection or another, it should have this  
> associated with it already (ex. .prj file for shapefiles), if it  
> doesn't, this is where you will have to specify it's projection in  
> the layers properties.
> However, you have to keep in mind that changing the projection of a  
> layer in it's properties doesn't actually (re)project the layer to  
> the specified coordinate system, it only tells QGIS which coordinate  
> system that layer is supposed to be in. So if you get this wrong,  
> your layer won't display where it's supposed to be, and measurements  
> based on that layer will likely be wrong.
>
> In terms of which is better as a default, metres or decimal degrees,  
> consider this:
> Most freely available spatial datasets come as shapefiles, and are  
> most often in a geographic coordinate system such as WGS84. This is  
> because this covers the entire globe, and anyone, anywhere can use  
> this data, and if need be project it to their locally used  
> projection system.
>
> So when you have data in your QGIS project, it is important to  
> insure that A) your project is in the correct coordinate system, B)  
> you are using the correct scale measure for the coordinate system  
> you are using, and C) that all your layers have their coordinate  
> system specified correctly. If all of the above are done properly,  
> your layers will line up, your scales will be correct, and your  
> analysis will be enlightening and life changing.
>
> A good rule of thumb is to make sure that all your layers are  
> projected to the same coordinate system right off the bat, and this  
> coordinate system should be the same as your project coordinate  
> system.
>
> In short, I don't think there are any bugs here, and IMHO the  
> default settings do make sense for most users.
> I don't know if this clears anything up, but that's my two cents ;-)
>
> Carson
>
>
> -- 
> Carson J.Q. Farmer
> PhD Fellow
> National Centre for Geocomputation (NCG),
> John Hume Building,
> National University of Ireland, Maynooth,
> Co. Kildare,
> Ireland.
> http://www.carsonfarmer.com
> http://www.ftools.ca
>
> _______________________________________________
> Qgis-user mailing list
> Qgis-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/qgis-user

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