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<DIV><FONT face=Arial size=2>I am not sure I totally undestand your question,
but </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>>>>> It seems as if the tie points
always need to be specified in some sort of distance unit, which I don't know
and cannot readily compute. </FONT></DIV>
<DIV><FONT face=Arial size=2>It is not really a distance unit. It is coordinates
in a specific coordinate system. Yours is a projected coordinate system. You
have to store TiePoint according to this coordinate system. </FONT></DIV>
<DIV><FONT face=Arial size=2>There are no predefined 'units' for values stored
in ModelTiePointTag and ModelPixelScaleTag, These values are defined by the
coordinate system specified in the GeoKeyDirectoryTag.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>>>>>>So, is there any way to encode
the information such that the tie points are specified in lat/lon, but still use
a Projected CS with the other parameters specified? </FONT></DIV>
<DIV><FONT face=Arial size=2>I don't think so. Moreover, I can't figure out why
you want to do that?</FONT></DIV>
<DIV><FONT face=Arial size=2>Should I undestand that you know the tiepoint
coordinate only in lat/long and you don't know it in your projected system. If
so why just don't you transform it with GEOTRANS utility for example
?</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Looks like your image is "rectified". That is :
your georeferencing model is the following function. </FONT></DIV>
<DIV><FONT face=Arial size=2>X = ax + b</FONT></DIV>
<DIV><FONT face=Arial size=2>Y = a'x + b'</FONT></DIV>
<DIV><FONT face=Arial size=2>where (X,Y) are the coordinates of any point in a
projection P.</FONT></DIV>
<DIV><FONT face=Arial size=2>(x,y) are the pixel 'coordinates' of this
point</FONT></DIV>
<DIV><FONT face=Arial size=2>a and a' are derived from pixel size</FONT></DIV>
<DIV><FONT face=Arial size=2>b and b' are derived from the tiepoint</FONT></DIV>
<DIV><FONT face=Arial size=2>In your case P is a particular Lambert conformal
projection.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>The proces is thus :</FONT></DIV>
<DIV><FONT face=Arial size=2>1/</FONT></DIV>
<DIV><FONT face=Arial size=2>Look for your particular lambert projection
(depending on standard parallels) in EPSG database.</FONT></DIV>
<DIV><FONT face=Arial size=2>If it exists : just get the code from EPSG and
store it in the adequat geokey</FONT></DIV>
<DIV><FONT face=Arial size=2>If it doesn't exist, two solutions :</FONT></DIV>
<DIV><FONT face=Arial size=2> rectify your image in a
widespread projection (at least one existing in EPSG)</FONT></DIV>
<DIV><FONT face=Arial size=2> or</FONT></DIV>
<DIV><FONT face=Arial size=2> user-defined you projection in
geokeys (mind that a lot of readers software are not able to read that type of
images)</FONT></DIV>
<DIV><FONT face=Arial size=2>2/</FONT></DIV>
<DIV><FONT face=Arial size=2>store in the ModelTiePoint Tag, coordinates
(according to your coordinate system) of the upper left pixel (have a look at
pixelisArea definition in the specification)</FONT></DIV>
<DIV><FONT face=Arial size=2>3/</FONT></DIV>
<DIV><FONT face=Arial size=2>Store the pixel sizes (X and Y spacings in the unit
of your coordinate system) in the ModelPixelScale</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Hope this will help.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Pascal</FONT></DIV>
<BLOCKQUOTE
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=brent.shaw@wni.com href="mailto:brent.shaw@wni.com">Brent Shaw</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=geotiff@remotesensing.org
href="mailto:geotiff@remotesensing.org">geotiff@remotesensing.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, May 12, 2005 10:09
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> [Geotiff] Projection CS
Encoding</DIV>
<DIV><BR></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial size=2>Does anyone out
there have any experience encoding geotiffs that are referenced with a
standard transformation method (e.g., Lambert Conformal, Polar Stereographic,
Mercator) on a user-defined projection? For example, I can create an
image where I know:</FONT></SPAN></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial size=2>1.
Projection is Lambert Conformal </FONT></SPAN></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial size=2>2. I know
both standard parallels in degrees</FONT></SPAN></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial size=2>3. I know
the standard longitude in degrees</FONT></SPAN></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial size=2>4. I know
the spacing of the pixels along either of the standard latitudes in
meters</FONT></SPAN></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial size=2>5. I can
compute the lat/lon of any raster pixel in the image to use as a tie
points. Since I know items 1-4, I really should only need 1 tie point,
but I can compute any of the lat/lons with routines I already have.
</FONT></SPAN></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial size=2>What I
specifically don't understand how to use for this case is the ModelTiePointTag
and the PixelScaleTag. It seems as if the tie points always need to be
specified in some sort of distance unit, which I don't know and cannot readily
compute. So, is there anyway to encode the information such that the tie
points are specified in lat/lon, but still use a Projected CS with the other
parameters specified? </FONT></SPAN></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial size=2>Thanks for any
help anyone can provide.</FONT></SPAN></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial
size=2>Regards,</FONT></SPAN></DIV>
<DIV><SPAN class=576290020-12052005><FONT face=Arial
size=2>Brent</FONT></SPAN></DIV>
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