[Geotiff] Re: Geotiff site

[Frank Warmerdam]

Gilad Ronnen wrote:
>  
> Hi Frank,
> 
> I hope it is ok to ask you questions, as nobody answers on geotiff list,
> and 
> You really helped us last time :)
> 
> Is it possible in Geotiff to use the corners as tiepoints without
> projection ?
> 
> For example, I have the corners of the image (720X576)
> a) UPPER LEFT: LON=-44.1569 degrees, LAT=58.8227 degrees
> b) UPPER RIGHT: LON=64.2775 degrees, LAT=58.8617 degrees
> c) LOWER LEFT: LON=-13.3162 degrees, LAT=30.5236 degrees
> d) LOWER RIGHT: LON=33.3307 degrees, LAT=30.5236 degrees
> 
> And I tried to make a metafile like this: (with 3 tiepoints - upperleft,
> upper right, and bottom right)
> as below, but it did not work for me.. Is it possible ?
> 
> Geotiff_Information:
>     Version: 1
>     Key_Revision: 1.0
>     Tagged_Information:
>       ModelTiepointTag (6,3):
>           0                0                0
>           -44.1169         58.8227           0	
>           720                0                0
>           64.2775         58.8227           0
>           720                576                0
>           33.3307         30.5236           0
>        End_Of_Tags.
>     Keyed_Information:
>     GTModelTypeGeoKey (Short,1): ModelTypeGeographic
>     GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
>     GeographicTypeGeoKey (Short,1): GCS_WGS_84
>     End_Of_Keys.
>  End_Of_Geotiff.

Gilad,

I tried setting up a file with your .geo description and it seems to work fine.
I even loaded it into OpenEV and it gave a reasonable stretch based on the
tiepoints.

In what way do you feel it isn't working for you?  Note that most applications
that have support for GeoTIFF will not do anything useful with multi-tiepoint
files.  Furthermore, anything that you can represent with 3 tiepoints can be
expressed as an affine transformation that you can save in GeoTIFF as a
GEOTRANSMATRIX.  I suspect this GEOTRANSMATRIX is more widely supported than
the multi-tiepoints approach, though it is certainly not as widely supported
as the usual tiepoint+pixel size which cannot represent the sheared case you
present.

I hope this helps.  I have cc:ed the list in case the info is of use to anyone
else.  I don't recall seeing your original email on the list but the system
keeps unsubscribing me every week so I can miss a few messages before I get
subscribed again.

Best regards,

-- 
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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