[gdal-dev] gdal_merge GeoTIFF output twice correct size

Frank Warmerdam warmerdam at pobox.com
Thu Mar 25 18:49:05 EDT 2010

Nathan Vander Wilt wrote:
> I used gdal_merge to combine four tiles into a perfectly square image.
> However, the resulting file was double the size on disk than I would expect
> based on its dimensions and bit depth. I opened it in a hex editor, and
> there is a huge pattern of data that repeats 0x00003075 followed by a ton of
> zeroes.
> Why is gdal_merge writing all this extra data? Does it have something to do
> with the "sparse" blocks discussed in this thread? 
> http://lists.osgeo.org/pipermail/gdal-dev/2008-October/018750.html
> I found how to perhaps compress these blocks with format options
> (http://gdal.org/frmt_gtiff.html) and could try that, but that may lead to
> compatibility issues. Why are these blocks in the file at all? Again, the
> merged image is square and is completely covered by the source data.


There are a variety of reasons the file could be larger than expected.

1) Is the file compressed?  If so, gdal_merge's rewriting of blocks
can result in them being moved to the end of the file if the block is
larger on the second write.

2) Is the file tiled?  Depending on tile and image size partially used
tiles on the right and bottom of an image can result in files being
significantly larger than expected.

A "tiffdump" report on the output file might help deduce what is going on.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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