[Geotiff] ModelPixelScaleTag

[Grissom, Ed]

In a previous message, Pascal Peuch said:
> I think that a reader should not consider that a Geotiff file is
> georeferenced when only a list of tiepoints is provided [...]

Agreed, to a point.   I think that you are pointing out some of the pitfalls
that users or software developers might fall into, and I agree with all of
your comments on that.  However, the tie-point case is no less a valid
method of georeferencing than either of the other two. 

Bottom Line, readers must support the list of tie points and, given only 3
points, an affine relationship pretty much has to be inferred.  Warnings
might be a good idea, but are not required by GeoTIFF.  With 4 or a small
number more points, the affine relationship should be the first assumption,
but it can easily be tested and if the relationship is not affine then the
app needs to handle that case (including warnings, or asking the user, or
saying it does not support non-affine or something).  For a really large
number of points, a mesh might be the best first assumption. 



-- 
ed grissom
ed.grissom at intergraph.com



> -----Original Message-----
> From: Pascal Peuch [mailto:peuchpascal at wanadoo.fr] 
> Sent: Friday, December 31, 1999 5:40 PM
> To: Grissom, Ed; Gillian Walter; geotiff at remotesensing.org
> Subject: Re: [Geotiff] ModelPixelScaleTag
> 
> 
> 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
> > >
> > > _______________________________________________
> > > Geotiff mailing list
> > > Geotiff at remotesensing.org
> > > http://xserve.flids.com/mailman/listinfo/geotiff
> > >
> > _______________________________________________
> > Geotiff mailing list
> > Geotiff at remotesensing.org
> > http://xserve.flids.com/mailman/listinfo/geotiff