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

[Michel Stuyts]

Thank you very much for the explanation.

When I use length(transform($geometry,layer_property(@layer_id,'crs'),'EPSG:31370')) in Belgium the results are very similar to the result of the Measure Tool, but if use this expression in New York the result is different from the result of the Measure Tool.  It is the other way around if I use length(transform($geometry,layer_property(@layer_id,'crs'),'EPSG:102316')). 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.


Michel





Van: Julien Guilloux [mailto:julien.guilloux at ecrins-parcnational.fr]
Verzonden: vrijdag 18 mei 2018 11:01
Aan: Michel Stuyts <michel.stuyts at edegem.be>; Springfield Harrison <stellargps at gmail.com>; qgis-user <qgis-user at lists.osgeo.org>
Onderwerp: Re: [Qgis-user] results Measure Tool vs. length()

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/

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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
www.ecrins-parcnational.fr<http://www.ecrins-parcnational.fr/>


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