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

[Micha Silver]

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. (If you already understand the above, I apologize for restating the obvious...) On 05/18/2018 01:14 PM, Michel Stuyts wrote: 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.guill...@ecrins-parcnational.fr] Verzonden: vrijdag 18 mei 2018 11:01 Aan: Michel Stuyts <michel.stu...@edegem.be>; Springfield Harrison <stellar...@gmail.com>; qgis-user <qgis-user@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/     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   _______________________________________________ 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 -- Micha Silver Ben Gurion Univ. Sde Boker, Remote Sensing Lab cell: +972-523-665918

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