[Qgis-user] Help with scales in print layouts for geographic CRS

[Nicolas Cadieux]

Hi,

Great question!  I am not the best person to answer it as this is a question for the Proj4 project  people. They do geodesy and generally, a conversation with them bring me back to my worse math class experiences! I should have listened, I say to myself!

Anyways, CRS are not all created the same.  If you read the link below, you will see that a CRS can minimize distorsion for the area, distance, shape, direction and bearing but not all of them at once.  Google for the projected coordinates system you used.  You may find what quality it aims to minimize distortion.  If not, Google for the type of projection it’s based on (conformal, equal-area…)
https://en.wikipedia.org/wiki/Map_projection

As others have pointed, a degree of longitude does not measure the same length as a degree of latitude.  I would expect scales to reflect latitude distances for the latitude of your map.  If this is not the case, then they are possibly using a short cut and using a latitude distance at the equator.  This would be a good question for the developers.

Whatever test you do, make sure your polygon in the projected CRS does measure the correct 1000x1000m or 1500x1500m in length.  The easiest way to do this is to draw the rectangle using advanced digitizing modes is a local UTM zone.  You can also use a WKT polygon you created in a csv file.

 (https://en.wikipedia.org/wiki/Well-known_text_representation_of_geometry) 

Then,  « densify » (processing tools box) the rectangle to add tons of nodes to the lines.  ( A rectangle with only 4 corner nodes cannot be accurately reprojected on a sphere.) Then, reproject the layer to a projected CRS you want to test.  Make sure all the layers are in the same crs as the project, same thing for the map space.  You need to make sure QGIS does not do any reprojection on the fly without you knowing it.

Nicolas Cadieux
https://gitlab.com/njacadieux

> Le 5 avr. 2022 à 11:00, Jorge Gustavo Rocha via Qgis-user <qgis-user at lists.osgeo.org> a écrit :
> 
> Hi,
> 
> I need you help to understand how scale works with geographic coordinate systems.
> 
> For projected coordinates systems, the layouts seems to work as expected.
> 
> This is my use case:
> 
> I've draw a polygon with 1000m width and height, and another with 1500, width and height, sharing the upper left corner.
> 
> 1) Using a projected CRS, if I create a layout with a 150mm x 150mm map, the 1500m polygon fits perfectly on the print area, setting a 1:10000 scale. That's what I expected. The results is https://nextcloud.geomaster.pt/index.php/s/TKpkBaqty8BdLL8
> 
> 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.
> 
> My question is: why the second layout does not fit the 1500m polygon properly? The second layout shows approximately a 2000m square, instead of a 1500m square.
> 
> Regards,
> 
> Jorge Gustavo
> 
> 
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