Understanding the ModelTiePointTag

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,
warmerdampobox.com
light and sound - activate the windows | http://pobox.com/~warmerd
am
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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