[FWTools] gdalbuildvrt does not support positive NS resolution

[spider plant]

Hi, I'm trying to stitch several map tiles together into a big image and
then warp it. However I'm getting error like this from gdalbuildvrt...

Warning 6: gdalbuildvrt does not support positive NS resolution. Skipping
map1.tiff

Searches for this error suggest its got something to do with north not being
up. But its not clear what this means. (My maps are in the northern
hemisphere).

I wondered if I am georeferencing the maps incorrectly. Am I right is saying
that top-left is (0,0) and the y-axis points downward for pixel coordinates
within the map? (I cant find anywhere where this is documented.) I also
tried y-axis pointing upward (giving -ve y-values) as this is what
quantumGIS seems to use.


COMMANDS I USE....

gdal_translate.exe -a_srs EPSG:27700
-gcp 0 400 241538.5 134628.66666667
-gcp 600 0 244713.5 136745.33333333
-gcp 0 0 241538.5 136745.33333333
-gcp 600 400 244713.5 134628.66666667
./map1.png ./map1.tiff

gdal_translate.exe -a_srs EPSG:27700
-gcp 0 400 241538.5 132512
-gcp 600 0 244713.5 134628.66666667
-gcp 0 0 241538.5 134628.66666667
-gcp 600 400 244713.5 132512
./map2.png ./map2.tiff

gdalbuildvrt -overwrite maps_merged.vrt map1.tiff map2.tiff


OUTPUT OF GDALINFO ON MAP1.TIFF....

Driver: GTiff/GeoTIFF
Files: map1.tiff
Size is 600, 400
Coordinate System is `'
GCP Projection =
PROJCS["OSGB 1936 / British National Grid",
    GEOGCS["OSGB 1936",
        DATUM["OSGB_1936",
            SPHEROID["Airy 1830",6377563.396,299.3249646000044,
                AUTHORITY["EPSG","7001"]],
            AUTHORITY["EPSG","6277"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4277"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",49],
    PARAMETER["central_meridian",-2],
    PARAMETER["scale_factor",0.9996012717],
    PARAMETER["false_easting",400000],
    PARAMETER["false_northing",-100000],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AUTHORITY["EPSG","27700"]]
GCP[  0]: Id=1, Info=
          (0,400) -> (241538.5,134628.66666667,0)
GCP[  1]: Id=2, Info=
          (600,0) -> (244713.5,136745.33333333,0)
GCP[  2]: Id=3, Info=
          (0,0) -> (241538.5,136745.33333333,0)
GCP[  3]: Id=4, Info=
          (600,400) -> (244713.5,134628.66666667,0)
Metadata:
  AREA_OR_POINT=Area
Image Structure Metadata:
  INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left  (    0.0,    0.0)
Lower Left  (    0.0,  400.0)
Upper Right (  600.0,    0.0)
Lower Right (  600.0,  400.0)
Center      (  300.0,  200.0)
Band 1 Block=600x3 Type=Byte, ColorInterp=Red
  Mask Flags: PER_DATASET ALPHA
Band 2 Block=600x3 Type=Byte, ColorInterp=Green
  Mask Flags: PER_DATASET ALPHA
Band 3 Block=600x3 Type=Byte, ColorInterp=Blue
  Mask Flags: PER_DATASET ALPHA
Band 4 Block=600x3 Type=Byte, ColorInterp=Alpha
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