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Date: Tue, 16 Jan 2007 22:35:00 -0500
From: Frank Warmerdam <warmerdam@pobox.com>
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To: Craig Bruce <csbruce@cubewerx.com>
Subject: Re: [Tiff] Elevation Data
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Status:   

Craig Bruce wrote:
> I would like to write signed 16-bit elevation data to a TIFF image file,
> but according to the TIFF 6.0 specification, there doesn't appear to
> be a correct way to do this.  The specification allows the use of the
> SampleFormat to specify that the samples are signed 16-bit integers, but
> this range of values is incompatible with the PhotometricInterpretation
> tag, which must be present.  The obvious PhotometricInterpretation to
> use is BlackIsZero, but its range is defined as 0 to 2**nbits-1 (i.e.,
> unsigned integers).

Craig,

I just set photometric interpretation to min-is-black and leave it up
to the application to find a reasonable way of displaying the elevation
values.  Simple "photo" programs will likely not show the result well,
but general purpose scientific raster displaying problems will know
how to handle unusual data ranges.

> Also, is there a semantic difference between SMinSampleValue and
> MinSampleValue?  The spec explicitly states that MinSampleValue is
> just statistical and isn't part of a valid-sample-range definition,
> but it's much more coy about SMinSampleValue.  If it isn't part of a
> hard range definition, then how can one make an interoperable image with
> floating-point samples?

I have no wisdom here.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam@pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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