[Qgis-user] Coordinates system south-oriented

[Alexia Mondot]

Hello,

I've got a non georeferenced raster with GML quality masks.
By default, when we open the raster in QGIS, my image is displayed 
correctly (north up, south down) but the coordinates are something like 
(i,-j), which seems to be QGIS default behaviour, and I can not overlay 
the GML masks on top, because the mask is displayed in coordinates 
(i,j). i,j are the pixel, line coordinates of the image.

I've tried to load the mask and the raster using a .vrt header. For the 
raster, QGIS default behaviour seems to work the same as if I write :

> <SRS> +proj=tmerc +lat_0=0 +lon_0=0 +k=1 +x_0=0 +y_0=0</SRS>
> <GeoTransform>0.0, 1.0, 0.0, 0.0, 0.0, -1.0</GeoTransform>

As GeoTransform is not a field for OgrVrt, I've been trying to define a 
coordinate system south-oriented to use in QGIS.
I thought I could use the "axis" keyword in the definition of the 
coordinate system. I've found some mention of "axis" in several 
discussions about south-oriented projections.
I've tried to load my raster using a .vrt header with something like :

> <SRS> +proj=tmerc +lat_0=0 +lon_0=0 +k=1 +x_0=0 +y_0=0 +axis=esu</SRS>
> <GeoTransform>0.0, 1.0, 0.0, 0.0, 0.0, 1.0</GeoTransform>

The axis keyword does not seem to be taken into account by QGIS. Is it 
correct?

Is there a way to define a coordinate system with X increasing East and 
Y increasing South?

If not, is there a simple way to "inverse" the GML masks to display 
(x,-y) rather than (x,y) using a .vrt rather than preprocessing all the 
masks and transforming each point one by one.

Best regards,
Alexia Mondot