[Geotiff] ModelPixelScaleTag
Grissom, Ed
ed.grissom at intergraph.com
Tue Sep 7 18:01:33 EDT 2004
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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