Hi Michel, Never use epsg 3857 for measurement beacuse it is not a "projection"... this "pseudo projection mercator" 3857, called often "Popular Visualisation " is pseudo.... Because only use for visualization applications.
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) 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.... best regards, Julien have a look on : http://spatialreference.org/ref/sr-org/6864/ Julien-Pierre GUILLOUX Chargé de mission Eau, Forêt, Natura 2000, coordinateur des porter-à-connaissance Parc national des Écrins 04 92 40 20 56 / 06 99 77 38 26 [ http://www.ecrins-parcnational.fr/ | www.ecrins-parcnational.fr ] De: "Michel Stuyts" <michel.stu...@edegem.be> À: "Springfield Harrison" <stellar...@gmail.com> Cc: "qgis-user" <qgis-user@lists.osgeo.org> Envoyé: Vendredi 18 Mai 2018 08:44:01 Objet: Re: [Qgis-user] results Measure Tool vs. length() $length gives the same result as length($geometry) Michel Van: Springfield Harrison [mailto:stellar...@gmail.com] Verzonden: vrijdag 18 mei 2018 2:52 Aan: Michel Stuyts <michel.stu...@edegem.be> CC: qgis-user <qgis-user@lists.osgeo.org> Onderwerp: Re: [Qgis-user] results Measure Tool vs. length() Interesting and alarming observation. And how do those results compare with output from the $length variable? Cheers . . . . . Springfield Samsung Tab 4 On May 17, 2018 5:26 AM, "Michel Stuyts" < [ mailto:michel.stu...@edegem.be | michel.stu...@edegem.be ] > wrote: In the length() function QGIS uses the CRS unit of the layer to calculate the result. When I use for example a layer in a local CRS like EPSG:31370 or EPSG:5439 and a layer in EPSG:3857 (that all use meters as unit) the results are very different (+40% difference). Changing the project CRS makes no difference. When I check the length using the Measure Tool the results are very similar to the local CRS. How can I measure the length of a line and use it as a label independent from the layer CRS and get results similar to the results of the Measure Tool? Michel _______________________________________________ Qgis-user mailing list [ mailto:Qgis-user@lists.osgeo.org | Qgis-user@lists.osgeo.org ] List info: [ https://lists.osgeo.org/mailman/listinfo/qgis-user | https://lists.osgeo.org/mailman/listinfo/qgis-user ] Unsubscribe: [ https://lists.osgeo.org/mailman/listinfo/qgis-user | https://lists.osgeo.org/mailman/listinfo/qgis-user ] _______________________________________________ Qgis-user mailing list Qgis-user@lists.osgeo.org List info: https://lists.osgeo.org/mailman/listinfo/qgis-user Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
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