[Geotiff] ModelPixelScaleTag

Pascal Peuch peuchpascal at wanadoo.fr
Tue Sep 7 18:40:57 EDT 2004 

Ed,
I'm afraid we disagree.
I'm not sure we can reach a common definition of georeferencing but :
Three tiepoints loosely georeferences an image. And a raw image with three
known points is much less georeferenced than a rectified image. If I give
you a very very bad non conformal paper map with scale varying erratically
from 1:50000 to1:250000 on the sheet, with no grid and with the coordinates
of only three points on the map, even if you've got a good GPS receiver you
are going to get lost.

----- Original Message -----
From: "Grissom, Ed" <ed.grissom at intergraph.com>
To: "Pascal Peuch" <peuchpascal at wanadoo.fr>; <geotiff at remotesensing.org>
Sent: Wednesday, September 08, 2004 12:01 AM
Subject: RE: [Geotiff] ModelPixelScaleTag


>
> Pascal -
>
> Perhaps our disagreement is over the term "georeferencing".  I am more of
a
> programmer than a GIS expert, so it is possible that I am using the term
> incorrectly.
>
>
> GeoTIFF'ers --
>
> what is the definition of Georeferencing ?
> If tie-points do not georeference an image, what term should I be using
> instead  ?
>
>
>
>
> Also, I don't believe that the spec mandates application behavior.  You
did
> say "should" in the two cases quoted below, but if you meant "must" I
would
> have to disagree.
>
> > > > should warn clearly the user.
> > > >
> > > > On the other side, producers should not use the three tiepoints
>
> I agree that they should, but would not agree that it is required by the
> spec.
>
>
> --
> ed grissom
> ed.grissom at intergraph.com
>
>
>
> > -----Original Message-----
> > From: geotiff-bounces at remotesensing.org
> > [mailto:geotiff-bounces at remotesensing.org] On Behalf Of Pascal Peuch
> > Sent: Monday, September 06, 2004 6:28 PM
> > To: geotiff at remotesensing.org
> > Subject: Re: [Geotiff] ModelPixelScaleTag
> >
> >
> > Hello Ed,
> > You tend to make reading softwares as resilient as possible
> > and I understand your point of view. Nonetheles, we have to
> > access this issue with strict application of geotiff
> > specification unless we have defined specific interchange
> > conventions between a producer and a reader out of the scope
> > of general purpose Geotiff specification.
> >
> > The geotiff specification states that :
> > "However, tiepoints are only to be considered exact at the
> > points specified; thus defining such a set of bounding
> > tiepoints does not imply that the model space locations of
> > the interior of the image may be exactly computed by a linear
> > interpolation of these tiepoints."
> > and
> > "For orthorectification or mosaicking applications a large
> > number of  tiepoints may be specified on a mesh over the
> > raster image. However, the definition of associated grid
> > interpolation methods is not in the scope of the current
> > GeoTIFF spec."
> >
> > When providing three tiepoints, even if exists an affine
> > relation within these three tiepoints, the producer doesn't
> > tell the user that the raster space and the model space are
> > affine related. He only tells him the position of three points.
> >
> > Knowing and exchanging the earth position of one or more
> > points of the raster is one thing but knowing and exchanging
> > the relation between a raster and the earth is another thing.
> >
> > Geotiff provides three tags to define the relation between
> > model space and raster space : tiepoints, pixelscale and
> > Modeltransformation. With these three means and the set of
> > constraints of Geotiff specification, you can define the
> > three following types of images :
> >
> > 1/ relation between raster space and model space is a known
> > and simple mathematic relation :
> >
> >     1A this relation is X =ax+b, Y=a'y+b' (where x and y are
> > raster coordinates and X,Y are model coordinates). In other
> > words, the image is rectified, it has been resampled
> > according to a projection system.
> >
> >     1B this relation is X =ax+by+c, Y =a'x+b'y+c' and can't
> > be reduced to 1A case.
> >
> > 2/ The relation is not known or is too complex to be defined
> > with Geotiff means.
> >
> > If you are a producer :
> >
> >     The 1A/ case must be defined with One TiePoint and the
> > pixelscale. (This is the normal case for corrected satellite
> > images for example).
> >     The 1B/ case must be defined with Modeltransformationtag
> > (the case is not recommended by the spec and in my opinion
> > the case is not really usefull).
> >     In the 2/ case, you only can provide a certain number of
> > TiePoints. (this is the case of an unrectified aerial photo
> > or a scanned paper map without any geometric correction for example)
> >
> > If you are a reader :
> > 1/ If you find a tiepoint and a pixelscale, you know that the
> > image is 1A type. If there is more than one tiepoint, you can
> > check the consistency of the exceeding tiepoints and warn the
> > user if not.
> > 2/ if you find three tiepoints : you can check whether these
> > 3 points are related by an affine relation. If they are you
> > can make the assumption that the relation is this same affine
> > relation as well for all the other points of the raster. But
> > it is an ASSUMPTION. I understand that the users want their
> > software to read every Geotiff file they can encounter but
> > you just can't make such assumptions in any kinds of software
> > (military applications for example) without asking the user at least.
> >
> >
> > See also the note in chapter "3.2.1. Unrectified Aerial
> > photo, known tiepoints, in degrees."
> >
> > Remark: Since we have not specified the ModelPixelScaleTag,
> > clients reading this GeoTIFF file are not permitted to infer
> > that there is a simple linear relationship between the raster
> > data and the geographic model coordinate space. The only
> > points that are know to be exact are the ones specified in
> > the tiepoint tag.
> >
> >
> >
> > ----- Original Message -----
> > From: "Grissom, Ed" <ed.grissom at intergraph.com
> > <mailto:ed.grissom at intergraph.com> >
> > To: "Pascal Peuch" <peuchpascal at wanadoo.fr
> > <mailto:peuchpascal at wanadoo.fr> >; <geotiff at remotesensing.org
> > <mailto:geotiff at remotesensing.org> >
> > Sent: Friday, September 03, 2004 3:25 PM
> > Subject: RE: [Geotiff] ModelPixelScaleTag
> >
> >
> > > 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 <mailto: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 <mailto: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
> > <mailto:ed.grissom at intergraph.com> >
> > > > To: "Gillian Walter"
> > <gillian.walter at atlantis-scientific.com
> > <mailto:gillian.walter at atlantis-scientific.com> >;
> > > > <geotiff at remotesensing.org <mailto: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 <mailto:ed.grissom at intergraph.com>
> > > > >
> > > > >
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: geotiff-bounces at remotesensing.org
> > <mailto: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
> > <mailto: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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