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Maybe I misunderstood your question, but you *should* expect
different length, area and direction calculations when converting
from one projection to another. Every projection distorts. Either
distance, area, or direction, or some combination. A local
projection is designed to minimize distortion for some small section
of the planet. So survey measurements in that region will closely
match map calculations. But trying to calculate from a layer that's
projected to some other projection will always give different
results.<br>
<br>
(If you already understand the above, I apologize for restating the
obvious...)<br>
<br>
<div class="moz-cite-prefix">On 05/18/2018 01:14 PM, Michel Stuyts
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:027B53E50C22524F8DB7A9FA67F30A9301B235AAAF@S-Elsschot.edegem.be">
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US">Thank you very much for the explanation.
<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US">When I use
<i>length(transform($geometry,layer_property(@layer_id,'crs'),'EPSG:31370'))</i>
in Belgium the results are very similar to the result of the
Measure Tool, but if use this expression in New York the
result is different from the result of the Measure Tool. It
is the other way around if I use <i>length(transform($geometry,layer_property(@layer_id,'crs'),'EPSG:102316')).</i>
What kind of projection does the Measure Tool use to
calculate a length? Because the results of the Measure Tool
seem to be very similar to local projections.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US">Michel<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:9.0pt;font-family:"Calibri",sans-serif;color:#1F497D"
lang="EN-US"><br>
<br>
</span><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"
lang="EN-US"><o:p></o:p></span></p>
<div>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif"
lang="NL">Van:</span></b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif"
lang="NL"> Julien Guilloux
[<a class="moz-txt-link-freetext" href="mailto:julien.guilloux@ecrins-parcnational.fr">mailto:julien.guilloux@ecrins-parcnational.fr</a>]
<br>
<b>Verzonden:</b> vrijdag 18 mei 2018 11:01<br>
<b>Aan:</b> Michel Stuyts
<a class="moz-txt-link-rfc2396E" href="mailto:michel.stuyts@edegem.be"><michel.stuyts@edegem.be></a>; Springfield Harrison
<a class="moz-txt-link-rfc2396E" href="mailto:stellargps@gmail.com"><stellargps@gmail.com></a>; qgis-user
<a class="moz-txt-link-rfc2396E" href="mailto:qgis-user@lists.osgeo.org"><qgis-user@lists.osgeo.org></a><br>
<b>Onderwerp:</b> Re: [Qgis-user] results Measure Tool
vs. length()<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black">Hi
Michel,<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black">Never
use epsg 3857 for measurement beacuse it is not a
"projection"...
<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black">this
"pseudo projection mercator" 3857, called often "Popular
Visualisation" is pseudo.... Because only use for
visualization applications.
<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black">example
in France : 3200 km for a road for example, instead of
2100 km in a real projection (as local zone UTM or a
Lambert projection)<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black">So,
you can use qgis, esri, postgis etc... the measurment
with 3857 will be always bad, due to a "false" calcul
from origine 0,0 (equateur cross greenwich) and "false"
ellipoide etc....<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black">best
regards,<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black">Julien<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black">have
a look on :<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black"><a class="moz-txt-link-freetext" href="http://spatialreference.org/ref/sr-org/6864/">http://spatialreference.org/ref/sr-org/6864/</a><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
</div>
<div>
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align="center"><span
style="font-family:"Arial",sans-serif;color:black">
<hr size="2" align="center" width="100%">
</span></div>
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<p class="MsoNormal"><b><span
style="font-size:10.0pt;font-family:"Arial",sans-serif">Julien-Pierre
GUILLOUX</span></b><span
style="font-size:10.0pt;font-family:"Arial",sans-serif"><br>
Chargé de mission Eau, Forêt, Natura 2000,
coordinateur des porter-à-connaissance<br>
<br>
<b><span style="color:#999999">Parc national des
Écrins</span></b><span style="color:#999999"><br>
04 92 40 20 56 / 06 99 77 38 26<br>
<a href="http://www.ecrins-parcnational.fr/"
target="_blank" moz-do-not-send="true"><span
style="color:#999999;text-decoration:none">www.ecrins-parcnational.fr</span></a>
</span><o:p></o:p></span></p>
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<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
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</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918</pre>
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