gdal_merge.py can't handle images with inverse transforms

[simonf]

I'm trying to merge two files in MODIS sinusoidal projection whose Y axis runs north to south. gdalinfo output is:

Upper Left (-8388000.000, 4774000.000) (103d 2'18.11"W, 43d 6'5.53"N) Lower Left (-8388000.000, 4973000.000) (106d11'6.49"W, 44d53'33.07"N) Upper Right (-7989000.000, 4774000.000) ( 98d 8'13.34"W, 43d 6'5.53"N) Lower Right (-7989000.000, 4973000.000) (101d 8'2.86"W, 44d53'33.07"N)

Upper Left (-8388000.000, 4973000.000) (106d11'6.49"W, 44d53'33.07"N) Lower Left (-8388000.000, 5172000.000) (109d38'24.92"W, 46d40'58.58"N) Upper Right (-7989000.000, 4973000.000) (101d 8'2.86"W, 44d53'33.07"N) Lower Right (-7989000.000, 5172000.000) (104d25'29.62"W, 46d40'58.58"N)

gdal_merge.py refuses to merge them: ERROR 1: Attempt to create 798x0 dataset is illegal,sizes must be larger than zero. Creation failed, terminating gdal_merge.

I can specify the output box explicitly and force the issue with

-ul_lr -8388000.0 4774000.0 -7989000.0 4973000.0

Note that uly is less than lry.

The problem is that in the "if ulx is None" branch gdal_merge.py assumes that the axes run in the same direction as in WGS84.

This does not block anything work-related.

[simonf]

Actually, my image was inverted on the Y axis. Not sure if gdal_merge.py should handle such an abnormal case.

[Jukka Rahkonen]

I would say that images which are inverted on the Y axis must be very uncommon.