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<p>If you use the -tps flag of gdalwarp, it will strictly honour
your GCPs. Cf
<a class="moz-txt-link-freetext" href="https://gdal.org/programs/gdalwarp.html#cmdoption-gdalwarp-tps">https://gdal.org/programs/gdalwarp.html#cmdoption-gdalwarp-tps</a> and
<a class="moz-txt-link-freetext" href="https://gdal.org/api/gdal_alg.html#_CPPv424GDALCreateTPSTransformeriPK8GDAL_GCPi">https://gdal.org/api/gdal_alg.html#_CPPv424GDALCreateTPSTransformeriPK8GDAL_GCPi</a></p>
<p>"The <span class="highlighted">thin</span> <span
class="highlighted">plate</span> <span class="highlighted">spline</span>
transformer produces exact transformation at all control points
and smoothly varying transformations between control points with
greatest influence from local control points. It is suitable for
for many applications not well modeled by polynomial
transformations."</p>
<div class="moz-cite-prefix">Le 29/11/2023 à 16:05, Joe Lovick via
gdal-dev a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:ddd53b19-a9ab-43bb-8083-0f225626ad01@anatexis.com">
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<div>Could someone explain how GCP's placed into a VRT file
work, i was expecting a linear/bi-linear, or polynomial fit
between the points for the simplest EPSG:4326 projection.<br>
</div>
<div><br>
</div>
<div>however that is not what i see, adjusting a single point,
effects the projection of all corner points.<br>
</div>
<div><br>
</div>
<div>for example,<br>
</div>
<div><br>
</div>
<div>a vrt file that starts...<br>
</div>
<div><br>
</div>
<div><VRTDataset rasterXSize="5328.0"
rasterYSize="2160.0"><br>
</div>
</div>
<div> <GCPList Projection="EPSG:4326"><br>
</div>
<div><GCP Id="1" Info="a" Pixel="0.0" Line="2160.0"
X="-93.6369420815179" Y="41.9822570038769" /><br>
</div>
<div><GCP Id="2" Info="b" Pixel="5328.0" Line="2160.0"
X="-93.636924110907" Y="41.9822308438819" /><br>
</div>
<div><GCP Id="3" Info="c" Pixel="5328.0" Line="0.0"
X="-93.6369153669777" Y="41.9822348674463" /><br>
</div>
<div><GCP Id="4" Info="d" Pixel="0.0" Line="0.0"
X="-93.636927955436" Y="41.9822605751492" /><br>
</div>
<div></GCPList><br>
</div>
<div>.....<br>
</div>
<div><br>
</div>
<div>if i adjust the 0,0 point to be further east, when i look at
the resulting remaped image, all corner image points have
shifted, rather than just the single corner location for 0,0<br>
</div>
<div><br>
</div>
<div>what am i missing here?<br>
</div>
<div><br>
</div>
<div>many thanks<br>
</div>
<div>Joe</div>
<div><br>
</div>
<div style="font-size: 15px;">
<div><br>
</div>
</div>
<div id="cm_signature">
<div>via <a
href="https://cloudmagic.com/k/d/mailapp?ct=dw&cv=10.0.71&pv=10.0&source=email_footer_2"
moz-do-not-send="true">Newton Mail</a></div>
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<pre class="moz-quote-pre" wrap="">_______________________________________________
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</blockquote>
<pre class="moz-signature" cols="72">--
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My software is free, but my time generally not.</pre>
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