[Qgis-user] results Measure Tool vs. length()

[Julien Guilloux]

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.stuyts at edegem.be> 
À: "Springfield Harrison" <stellargps at gmail.com> 
Cc: "qgis-user" <qgis-user at 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:stellargps at gmail.com] 
Verzonden: vrijdag 18 mei 2018 2:52 
Aan: Michel Stuyts <michel.stuyts at edegem.be> 
CC: qgis-user <qgis-user at 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.stuyts at edegem.be | michel.stuyts at 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 



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