[Geotiff] Understanding the ModelTiePointTag

[Tillerson, Clint]

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.  

Thanks!
Clint
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