Hi, Your calculations are based on a sphere. The different crs use a geoids. It would help if you could say what crs you are using and the differences between what you get and what you expect.
Nicolas Cadieux https://gitlab.com/njacadieux > Le 6 avr. 2022 à 13:13, Jorge Gustavo Rocha via Qgis-user > <[email protected]> a écrit : > > Hi, > > Thank you all for the comments. > > As far as I can see from my tests, the behavior is not related with the CRS > in use. I've used more than one meter based projection to make sure the areas > were fine. I use the advanced digitizing tools to create segments with the > exact distance. Using projected CRS, the layouts are always fine. > > The problem is related to the map area calculated for a giving scale, in the > composer, when a lat/long coordinate system is in use. > > If I make a very simple calculation of the area to render at a giving scale, > my calculations work fine. > > The width does not depend on the latitude, and should be: > > layout_width_mm * scale / 1000.0 > > The height depends on the latitude. To compute it, I use the cosine of the > latitude: > > cosine = cos( center_latitude * PI / 180 ); > > layout_height_mm * scale / 1000.0 / cosine; > > Based on this simple math, after converting these distances to lat/log, I can > compute the "correct" ( ie expected) extend of the map in the layout. If I > setup this extent manually, it works fine. > > But the extent calculated by QGIS is different. I'll have to check the code > to see why the logic if different from this one. Probably I am simplifying > the calculations too much. > > I'll return back after reading the code. > > Regards, > > Jorge Gustavo > >> On 06/04/22 15:10, Greg Troxel via Qgis-user wrote: >> Patrick Dunford via Qgis-user <[email protected]> writes: >> >>> On 6/04/22 11:34, Greg Troxel via Qgis-user wrote: >>>> Jorge Gustavo Rocha via Qgis-user <[email protected]> writes: >>>> >>>> 2) Using a geographic CRS, the same 150mm x 150mm map, at the same >>>>> 1:10000 scale, the area is bigger then the 1500m polygon. The result >>>>> is attached https://nextcloud.geomaster.pt/index.php/s/H2eAytsPANyxn6Y >>>>> >>>>> On both layouts the scale bar widget is working properly. The >>>>> distances (and areas) are properly calculated in QGIS interface. I >>>>> have set the GRS 1980 ellipsoid for distance and area calculations. >>>> What does it mean to use a geographic CRS for a print layout? Do you >>>> really expect longitude on the x axis and latitude on the y axis? The >>>> value of meters/degree is different for x and y, unless you are at or >>>> near the equator. (Living at 42N-ish, I'm very aware of this.) >>> This may be the case for projections like EPSG:3857 that are based on >>> a spherical projection of the Earth but is it the case for all >>> projections? >> By "geographic CRS", I interpet that to mean one that is latitude and >> longitude. That is not really a "projection". >> >> Yes, EPSG:3857 has an incorrect spherical assumption, and I'd have to >> calculate, but I would not expect massive scale distortion, where >> massive is casually visually apparent. >> >> Perhaps Jorge can be precise about which CRS codes are in use. >> >> _______________________________________________ >> Qgis-user mailing list >> [email protected] >> List info: https://lists.osgeo.org/mailman/listinfo/qgis-user >> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user > _______________________________________________ > Qgis-user mailing list > [email protected] > List info: https://lists.osgeo.org/mailman/listinfo/qgis-user > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
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