[Geotiff] Understanding the ModelTiePointTag

[Frank Warmerdam]

Tillerson, Clint wrote:
> I’m new to the GEOTIFF file structure and am trying to understand the 
> relationship between the values of the ModelTiePointTag and 
> ModelPixelScale.  Everything I read seems to be a quote from the 
> specifications without any real explanation.
> 
> I understand in a basic way the 6 values of a single tie point and the 
> scale values, but I need to better understand the relationship between 
> between the first two values of the tie point and the 4th and 5th values 
> (coordinates).
> 
> Here are my tag values from the TIFF file:
> 
> ModelTiePointTag = [0.5, 0.5, 0.0, 1047690.0, 1699530.0, 0.0]
> 
> ModelPixelScaleTag = [30.0, 30.0, 0.0]
> 
> I understand the 0.5 refers to half of a pixel.  A pixel in my example 
> is 30.0 meters.  0.5 * 30.0 = 15.0 m, but what does this offset mean 
> exactly, and how do you apply it to the coordinates?  The coordinates 
> are Albers which are meters. 

Clint,

(0.5,0.5) as the pixel line location means that the georeferenced location
is associated with the center of the top left pixel.  I'm not sure what
you mean about "applying the offset" to coordinates.  The tiepoints are
used to associate a location in the image with a georeferenced coordinate.

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