[Qgis-user] scaling bug?

[Carson Farmer]

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