<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; charset=iso-8859-1">
<META content="MSHTML 6.00.2800.1106" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff>
<DIV><FONT face=Arial size=2>Hello Ed, </FONT></DIV>
<DIV><FONT face=Arial size=2>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.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial><FONT size=2>The geotiff specification states that :
</FONT></FONT></DIV>
<DIV><FONT face=Arial><FONT size=2>"</FONT><FONT size=2><EM>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."</EM></FONT></FONT></DIV>
<DIV><FONT face=Arial><FONT size=2><EM>and </EM></FONT></FONT></DIV>
<DIV><FONT face=Arial><FONT size=2><EM>"</EM></FONT><FONT face=Courier><FONT
face="Times New Roman" size=2><EM>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."</EM></FONT></FONT></FONT><FONT face=Arial><FONT face=Courier></DIV>
<P><FONT face=Arial size=2>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.</FONT></P>
<P><FONT face=Arial size=2>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.</FONT></P>
<P><FONT face=Arial size=2>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
:</FONT></P>
<P><FONT face=Arial size=2>1/ relation between raster space and model space is a
known and simple mathematic relation :</FONT></P>
<P><FONT face=Arial size=2> 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.</FONT></P>
<P><FONT face=Arial size=2> 1B this relation is X
=ax+by+c, Y =a'x+b'y+c' and can't be reduced to 1A case.</FONT></P>
<P><FONT face=Arial size=2>2/ The relation is not known or is too complex to be
defined with Géotiff means.</FONT></P>
<P><FONT face=Arial size=2>If you are a producer :</FONT></P></FONT></FONT>
<DIV><FONT face=Arial size=2> The 1A/ case
<STRONG>must</STRONG> be defined with One TiePoint and the pixelscale. (This is
the normal case for corrected satellite images for example).</FONT></DIV>
<DIV><FONT face=Arial size=2> 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).</FONT></DIV>
<DIV><FONT face=Arial size=2> 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)</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>If you are a reader :</FONT></DIV>
<DIV><FONT face=Arial size=2>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.</FONT></DIV>
<DIV><FONT face=Arial size=2>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 rela</FONT><FONT face=Arial size=2>tion 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
Géotiff 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.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<P><FONT face=Arial size=2>See also the note in chapter <B><FONT
face="Times New Roman" size=4>"3.2.1. Unrectified Aerial photo, known tiepoints,
in degrees."</P></B></FONT></FONT>
<DIV><FONT face=Arial size=2><FONT face="Times New Roman" size=2>
<P><EM>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.</EM></P></FONT></FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>----- Original Message ----- </FONT>
<DIV><FONT face=Arial size=2>From: "Grissom, Ed" <</FONT><A
href="mailto:ed.grissom@intergraph.com"><FONT face=Arial
size=2>ed.grissom@intergraph.com</FONT></A><FONT face=Arial
size=2>></FONT></DIV>
<DIV><FONT face=Arial size=2>To: "Pascal Peuch" <</FONT><A
href="mailto:peuchpascal@wanadoo.fr"><FONT face=Arial
size=2>peuchpascal@wanadoo.fr</FONT></A><FONT face=Arial size=2>>;
<</FONT><A href="mailto:geotiff@remotesensing.org"><FONT face=Arial
size=2>geotiff@remotesensing.org</FONT></A><FONT face=Arial
size=2>></FONT></DIV>
<DIV><FONT face=Arial size=2>Sent: Friday, September 03, 2004 3:25
PM</FONT></DIV>
<DIV><FONT face=Arial size=2>Subject: RE: [Geotiff]
ModelPixelScaleTag</FONT></DIV></DIV>
<DIV><FONT face=Arial><BR><FONT size=2></FONT></FONT></DIV><FONT face=Arial
size=2>> In a previous message, Pascal Peuch said:<BR>> > I think that
a reader should not consider that a Geotiff file is<BR>> > georeferenced
when only a list of tiepoints is provided [...]<BR>> <BR>> Agreed, to a
point. I think that you are pointing out some of the
pitfalls<BR>> that users or software developers might fall into, and I agree
with all of<BR>> your comments on that. However, the tie-point case is
no less a valid<BR>> method of georeferencing than either of the other two.
<BR>> <BR>> Bottom Line, readers must support the list of tie points and,
given only 3<BR>> points, an affine relationship pretty much has to be
inferred. Warnings<BR>> might be a good idea, but are not required by
GeoTIFF. With 4 or a small<BR>> number more points, the affine
relationship should be the first assumption,<BR>> but it can easily be tested
and if the relationship is not affine then the<BR>> app needs to handle that
case (including warnings, or asking the user, or<BR>> saying it does not
support non-affine or something). For a really large<BR>> number of
points, a mesh might be the best first assumption. <BR>> <BR>> <BR>>
<BR>> -- <BR>> ed grissom<BR>> </FONT><A
href="mailto:ed.grissom@intergraph.com"><FONT face=Arial
size=2>ed.grissom@intergraph.com</FONT></A><BR><FONT face=Arial size=2>>
<BR>> <BR>> <BR>> > -----Original Message-----<BR>> > From:
Pascal Peuch [mailto:peuchpascal@wanadoo.fr] <BR>> > Sent: Friday,
December 31, 1999 5:40 PM<BR>> > To: Grissom, Ed; Gillian Walter;
</FONT><A href="mailto:geotiff@remotesensing.org"><FONT face=Arial
size=2>geotiff@remotesensing.org</FONT></A><BR><FONT face=Arial size=2>> >
Subject: Re: [Geotiff] ModelPixelScaleTag<BR>> > <BR>> > <BR>>
> I think that a reader should not consider that a Geotiff file is<BR>>
> georeferenced when only a list of tiepoints is provided unless it<BR>>
> has some external information on the geometry of the image.
When<BR>> > the Geotiff file contains only 3 (or 4 or 5 or more)
tiepoints, you<BR>> > are not granted that the file is rectified
(orthogonal in the<BR>> > coordinate system). You know the coordinates of
3 (or 4 or 5 or<BR>> > more) points of the image and you know nothing on
the other<BR>> > points. The image could be of very raw geometry. A raw
aerial photo<BR>> > with 3 or 4 tiepoints is not affine related for
example. In such<BR>> > cases the reader software can nonetheless
georeference the image<BR>> > (depending on the accuracy requirements of
the application) but it<BR>> > should warn clearly the user.<BR>> >
<BR>> > On the other side, producers should not use the three
tiepoints<BR>> > method to specify an affine relation.<BR>> >
<BR>> > Pascal Peuch<BR>> > <BR>> > ----- Original Message
-----<BR>> > From: "Grissom, Ed" <</FONT><A
href="mailto:ed.grissom@intergraph.com"><FONT face=Arial
size=2>ed.grissom@intergraph.com</FONT></A><FONT face=Arial size=2>><BR>>
> To: "Gillian Walter" <</FONT><A
href="mailto:gillian.walter@atlantis-scientific.com"><FONT face=Arial
size=2>gillian.walter@atlantis-scientific.com</FONT></A><FONT face=Arial
size=2>>;<BR>> > <</FONT><A
href="mailto:geotiff@remotesensing.org"><FONT face=Arial
size=2>geotiff@remotesensing.org</FONT></A><FONT face=Arial size=2>><BR>>
> Sent: Thursday, August 26, 2004 3:59 PM<BR>> > Subject: RE: [Geotiff]
ModelPixelScaleTag<BR>> > <BR>> > <BR>> > ><BR>> >
> There are three ways to specify the raster-to-model relationship<BR>>
> ><BR>> > > 1) point and scale (no rotation, raster is
orthogonal in<BR>> >
>
coordinate system)<BR>> > > 2) list of tie points (at least three
points for affine<BR>> >
>
relationship, unlimited for "mesh")<BR>> > > 3) transform
matrix.<BR>> > ><BR>> > > I would venture to say that
specifying _only_ the tie points<BR>> > > REQUIRES 3 or more
points. I would not consider a file<BR>> > > georefernced with
less than that (absent other info).<BR>> > ><BR>> > >
--<BR>> > > ed grissom<BR>> > > </FONT><A
href="mailto:ed.grissom@intergraph.com"><FONT face=Arial
size=2>ed.grissom@intergraph.com</FONT></A><BR><FONT face=Arial size=2>> >
><BR>> > ><BR>> > ><BR>> > > > -----Original
Message-----<BR>> > > > From: </FONT><A
href="mailto:geotiff-bounces@remotesensing.org"><FONT face=Arial
size=2>geotiff-bounces@remotesensing.org</FONT></A><BR><FONT face=Arial
size=2>> > > > [mailto:geotiff-bounces@remotesensing.org] On Behalf
Of Gillian Walter<BR>> > > > Sent: Thursday, August 26, 2004 9:24
AM<BR>> > > > To: </FONT><A
href="mailto:geotiff@remotesensing.org"><FONT face=Arial
size=2>geotiff@remotesensing.org</FONT></A><BR><FONT face=Arial size=2>> >
> > Subject: [Geotiff] ModelPixelScaleTag<BR>> > > ><BR>>
> > ><BR>> > > > Hi,<BR>> > > ><BR>> >
> > I have a question about the Geotiff ModelPixelScaleTag. On
page<BR>> > > > 25 of the Geotiff specification, there is a diagram
indicating<BR>> > > > that either ModelPixelScaleTag and
ModelTiepointTag, or<BR>> > > > ModelTransformationTag alone will be
used to geocode an image.<BR>> > > > However, the next page
indicates that ModelPixelScaleTag and<BR>> > > >
ModelTransformationTag are optional. Is it valid to specify<BR>> >
> > only the ModelTiepointTag and not ModelPixelScaleTag? I
was<BR>> > > > under the impression that it was, since the
ModelPixelScaleTag<BR>> > > > and ModelTransformationTag parameters
can't always accurately<BR>> > > > represent a dataset's geocoding
(eg. slant range SAR imagery),<BR>> > > > but one of our customers
disagrees, and I can see how the spec<BR>> > > > could be
interpreted either way.<BR>> > > ><BR>> > > >
Gillian<BR>> > > ><BR>> > > >
_______________________________________________<BR>> > > > Geotiff
mailing list<BR>> > > > </FONT><A
href="mailto:Geotiff@remotesensing.org"><FONT face=Arial
size=2>Geotiff@remotesensing.org</FONT></A><BR><FONT face=Arial size=2>> >
> > </FONT><A
href="http://xserve.flids.com/mailman/listinfo/geotiff"><FONT face=Arial
size=2>http://xserve.flids.com/mailman/listinfo/geotiff</FONT></A><BR><FONT
face=Arial size=2>> > > ><BR>> > >
_______________________________________________<BR>> > > Geotiff
mailing list<BR>> > > </FONT><A
href="mailto:Geotiff@remotesensing.org"><FONT face=Arial
size=2>Geotiff@remotesensing.org</FONT></A><BR><FONT face=Arial size=2>> >
> </FONT><A href="http://xserve.flids.com/mailman/listinfo/geotiff"><FONT
face=Arial size=2>http://xserve.flids.com/mailman/listinfo/geotiff</FONT></A>
</BODY></HTML>