[Qgis-developer] QGIS and South Oriented CRS

[Zoltan Szecsei]

Hi Devs,
Gavin & I have swapped a few emails discussing this, and would like to 
get a wider opinion as to whether it is a QGIS error, or not.
QGIS version in use by me, as per attached screenshot.

*The problem:*

    If I read in a CSV layer that uses South Oriented CRS, then QGIS
    displays this layer "mirrored" even after telling QGIS what the
    applicable layer CRS is.
    Furthermore, when I move the mouse downwards or to the left, the
    coordinate readout values decreases instead of increases.
    In short, if I work in a South Oriented CRS, I must deal with an
    inverted picture or an incorrect coord readout.
    Is this correct behaviour for QGIS?

*_To test:_*
*
Fixed test parameters:*

  * Always start with a "new" QGIS so that only default project settings
    are "set". (EPSG:4326 but 'on the fly CRS' off)
  * Only load the CSV file so that there are no other map layers to to
    contend (CRS wise) with.
  * The absolute position (on earth) of the test data is not important -
    we are looking to see that QGIS displays the points
    correctly_relative to each other_, and that the coordinate readout
    is correctly increasing South and Westwards.
  * Use only the two following CRS (note the only difference is the axis
    parameter)
    *EPSG:2048* is      +proj=tmerc +lat_0=0 +lon_0=19 +k=1 +x_0=0
    +y_0=0 *+axis=wsu* +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
    *USER:100004* is +proj=tmerc +lat_0=0 +lon_0=19 +k=1 +x_0=0 +y_0=0
    *+axis=enu* +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
      o *Testdata:*
        The coords below should be EPSG:2053 - in other words, the South
        Africa Land Survey coord system, which is essentially a distance
        from the central meridian (19E in this case) and a distance from
        the Equator (some 3769Km in this case). Note that the further
        South you go, the larger the '3769xxx' value gets (ie: South facing)

            A,19961.49,3769105.30
            B,19902.64,3769135.18
            C,19899.91,3769132.99
            L,20095.87,3769783.70

        The relative positions of these 4 values ("in real life") are: 
        A at the top, B and C  Eastish of A (but B below [South] of C)
        and L far below A and slightly to the West of A (see attached
        'correct_orientation.jpg')

*Test results:*

  * Open QGIS, add delimited text layer and specify its CRS to be
    EPSG:2048, keep Project CRS on the fly "off".
    Result:

        Image is inverted (A is South of L) and B & C are West of A.
        Coordinate values are both +ve numbers and mouse position coords
        decrease to West and South

  * Using above data, Set Project CRS (on the fly = YES) to same EPSG:2048
    Result:

        No change from above.

  * Simply change Project CRS to EPSG:100004 as defined earlier in this
    email.
    Result:

        Image orientation is now correct BUT
        Mouse coords readout shows -ve coord values, and these values
        still (numerically) DECREASE towards the West and South.

*Start with a "new" QGIS* (to pedantically make certain that no CRS 
functionality is left over from above tests)

  * Open QGIS, add _the same_ delimited text layer and specify its CRS
    to be EPSG:100004, keep Project CRS on the fly "off".
    Result:

        Image is inverted (A is South of L) and B & C are West of A.
        Coordinate values are both +ve numbers and mouse position coords
        decrease to West and South

  * Using above data, Set Project CRS (on the fly = YES) to same EPSG:100004
    Result:

        No change from above.

  * Simply change Project CRS to EPSG:2048 as defined earlier in this email.
    Result:

        Image orientation is now correct BUT
        Mouse coords readout shows -ve coord values, and these values
        still (numerically) DECREASE towards the West and South.

*To close:*

If I redo both the above tests but set Project CRS before importing the 
CSV layer, there is (of course) no change to the results.
Even though I was not expecting changes to some of the above test 
permutations, I did them all in order to pedantically wipe out any 
unexpected behaviour.


So, Why am I not able to work in my desired CRS orientation when using QGIS?
Is this a bug, or is it some feature that most likely may be worth 
implementing?

Regards to all,
Zoltan



-- 

===========================================
Zoltan Szecsei PrGISc [PGP0031]
Geograph (Pty) Ltd.
GIS and Photogrammetric Services

P.O. Box 7, Muizenberg 7950, South Africa.

Mobile: +27-83-6004028
Fax:    +27-86-6115323     www.geograph.co.za
===========================================

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