[Qgis-user] scaling bug?

[Magnus Homann]

Werner Macho skrev:
> Hi again!
> Ok I must miss something somewhere..
> I open QGIS .. Set the project projection to M34.
> Load the shape file as a vector layer - set the Layer projection to 
> M34 (missed that before) .. but .. the same happens
> Project has +proj=tmerc +lat_0=0 +lon_0=16.33333333333333 +k=1 
> +x_0=750000 +y_0=0 +ellps=bessel 
> +towgs84=577.326,90.129,463.919,5.137,1.474,5.297,2.4232 +units=m 
> +no_defs
> Layer CRS is +proj=tmerc +lat_0=0 +lon_0=16.33333333333333 +k=1 
> +x_0=750000 +y_0=0 +ellps=bessel 
> +towgs84=577.326,90.129,463.919,5.137,1.474,5.297,2.4232 +units=m 
> +no_defs
>
> Beat me but where can i find the "coordinate transformation turned 
> on?" switch?
>
The last part is the key to the entire mystery! :-)

When you install QGIS, degrees is the default unit, and it tells QGIS 
that any coordinate in your layer is in degrees. As you have large >>360 
values of coordinates, the scale calculation breaks down. Calculating 
scale is done by projecting coordinates sin the center of the canvas
to an ellipsoid. Hairy stuff. This projection is not reliable with 
unrealistic values. Remember QGIs thinks that your coordinate (6848, 
272169) is in degrees.

Now, you can either change this to meter manually in the option dialog 
OR you can turn on projection (top left corner in dialog where you 
select project CRS in project properties). In the last case, the unit is 
set to what the project projection ("+units=m") says.

Hope this explains it all.

magnus