Help with TIFFTAGs resolutions

[Frank Warmerdam]

Gambin Dejan wrote:
> Hi,
> 
> I have a situation that I need to understand, I hope someone can help:
> 
> I have a number of ortophoto images in tif format (with .tfw files).
> When I show them on my map they should be spatialy "connected" but the
> "empty" space shows between them. Here is the example gdalinfo result of
> the two horizontally adjacent images that should be "connected":
...
> Now, I have the similar set of ortophoto images that show correctly and
> they come from the same source (the National State Geodetic
> Administration). I have compared them and the difference is that this
> "correct" images have the size of 4500x6000 (exactly 1.6 bigger than the
> first one) and their TIFFTAG_XRESOLUTION and TIFFTAG_YRESOLUTION are 72
> (that is 45x1.6). 
> 
> My question is: who "made the mistake" and what do I have to do to show
> them correctly (they are shown correctly from a dwg file that is
> referencing them all)? Are the TFW files wrong for this resolution? Do I
> have to change the resolution, image size, or what?

Dejan,

It is unlikely that the DWG is utilizing the world files (.tfw) at all.
So if the DWG view is correct it is likely that the world files are
wrong.  If you think the origin's in the world files are correct you could
try changing the resolution in the world files to 0.8 from 0.5 and see if
things are properly placed.

The TIFFTAG_XRESOLUTION and TIFFTAG_YRESOLUTION are not used for
georeferenced placement or scaling by MapServer or other applications.

BTW, GDAL and MapServer will utilize internal GeoTIFF tags in preference
to the world files.  So if there is wrong internal geotiff info it
will get used instead of potentially correct worldfiles.  Keep this in
mind if the world files don't seem to agree with what gdalinfo is reporting.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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