[Geotiff] ModelPixelScaleTag

[Pascal Peuch]

I think that a reader should not consider that a Geotiff file is
georeferenced when only a list of tiepoints is provided unless it has some
external information on the geometry of the image.
When the Geotiff file contains only 3 (or 4 or 5 or more) tiepoints, you are
not granted that the file is rectified (orthogonal in the coordinate
system). You know the coordinates of 3 (or 4 or 5 or more) points of the
image and you know nothing on the other points. The image could be of very
raw geometry. A raw aerial photo with 3 or 4 tiepoints is not affine related
for example.
In such cases the reader software can nonetheless georeference the image
(depending on the accuracy requirements of the application) but it should
warn clearly the user.

On the other side, producers should not use the three tiepoints method to
specify an affine relation.

Pascal Peuch

----- Original Message -----
From: "Grissom, Ed" <ed.grissom at intergraph.com>
To: "Gillian Walter" <gillian.walter at atlantis-scientific.com>;
<geotiff at remotesensing.org>
Sent: Thursday, August 26, 2004 3:59 PM
Subject: RE: [Geotiff] ModelPixelScaleTag


>
> There are three ways to specify the raster-to-model relationship
>
> 1) point and scale    (no rotation, raster is orthogonal in coordinate
> system)
> 2) list of tie points (at least three points for affine relationship,
> unlimited for "mesh")
> 3) transform matrix.
>
> I would venture to say that specifying _only_ the tie points REQUIRES 3 or
> more points.  I would not consider a file georefernced with less than that
> (absent other info).
>
> --
> ed grissom
> ed.grissom at intergraph.com
>
>
>
> > -----Original Message-----
> > From: geotiff-bounces at remotesensing.org
> > [mailto:geotiff-bounces at remotesensing.org] On Behalf Of Gillian Walter
> > Sent: Thursday, August 26, 2004 9:24 AM
> > To: geotiff at remotesensing.org
> > Subject: [Geotiff] ModelPixelScaleTag
> >
> >
> > 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
> >
> > _______________________________________________
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> >
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