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<blockquote type="cite"
cite="mid:CAL520Rk_8zcWq_AYoU0PnzkLnAauG1C4RmUkhmazaSB4zzkbPg@mail.gmail.com">
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<div>Anyway a proper pipeline I use for years is:</div>
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<div><font face="monospace">+proj=pipeline +step +inv
+proj=krovak +lat_0=49.5 +lon_0=24.8333333333333
+alpha=30.2881397527778 +k=0.9999 +x_0=0 +y_0=0
+ellps=bessel +step +inv +proj=hgridshift
+grids=Slovakia_JTSK03_to_JTSK.gsb +step +proj=krovak
+lat_0=49.5 +lon_0=24.8333333333333
+alpha=30.2881397527778 +k=0.9999 +x_0=0 +y_0=0
+ellps=bessel +step +inv +proj=krovak +lat_0=49.5
+lon_0=24.8333333333333 +alpha=30.2881397527778 +k=0.9999
+x_0=0 +y_0=0 +ellps=bessel +step +proj=push +v_3 +step
+proj=cart +ellps=bessel +step +proj=helmert +x=485.021
+y=169.465 +z=483.839 +rx=-7.786342 +ry=-4.397554
+rz=-4.102655 +s=0 +convention=coordinate_frame +step +inv
+proj=cart +ellps=WGS84 +step +proj=pop +v_3 +step
+proj=webmerc +lat_0=0 +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84<br>
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<div>It is the official pipeline provided by our national
cartography office, together
with Slovakia_JTSK03_to_JTSK.gsb file. Without this pipeline
the offset varies up to more than 10 meters. With the
pipeline the transformed geotata matches that from state
office's WMS 🙂</div>
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</blockquote>
<p>It makes slightly more sense, but still seems dubious to me</p>
<p>The part "+step +inv +proj=krovak +lat_0=49.5
+lon_0=24.8333333333333 +alpha=30.2881397527778 +k=0.9999 +x_0=0
+y_0=0 +ellps=bessel +step +inv +proj=hgridshift
+grids=Slovakia_JTSK03_to_JTSK.gsb +step +proj=krovak +lat_0=49.5
+lon_0=24.8333333333333 +alpha=30.2881397527778 +k=0.9999 +x_0=0
+y_0=0 +ellps=bessel" is what you would want to apply to go *from*
"S-JTSK / Krovak East North" to "S-JTSK [JTSK03] / Krovak East
North"</p>
<p>and then the second part uses the expected Helmert tranformation
from "S-JTSK [JTSK03] / Krovak East North" to WGS 84, so this
pipeline is to go from "S-JTSK / Krovak East North" to WebMercator
WGS 84.</p>
<p>Perhaps your dataset is mislabelled as being in "S-JTSK [JTSK03]
/ Krovak East North" whereas it is in "S-JTSK / Krovak East
North" ?<br>
</p>
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<pre class="moz-signature" cols="72">--
<a class="moz-txt-link-freetext" href="http://www.spatialys.com">http://www.spatialys.com</a>
My software is free, but my time generally not.
Butcher of all kinds of standards, open or closed formats. At the end, this is just about bytes.</pre>
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