[Geotiff] ModelPixelScaleTag

[Gillian Walter]

Hi,

I have a question about the Geotiff ModelPixelScaleTag.  On page 25 of 
the Geotiff specification, there is a diagram indicating that either 
ModelPixelScaleTag and ModelTiepointTag, or ModelTransformationTag alone 
will be used to geocode an image.  However, the next page indicates that 
ModelPixelScaleTag and ModelTransformationTag are optional.  Is it valid 
to specify only the ModelTiepointTag and not ModelPixelScaleTag?  I was 
under the impression that it was, since the ModelPixelScaleTag and 
ModelTransformationTag parameters can't always accurately represent a 
dataset's geocoding (eg. slant range SAR imagery), but one of our 
customers disagrees, and I can see how the spec could be interpreted 
either way.

Gillian