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

[Nicolas Cadieux]

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 <qgis-user at lists.osgeo.org> 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 <qgis-user at lists.osgeo.org> writes:
>> 
>>> On 6/04/22 11:34, Greg Troxel via Qgis-user wrote:
>>>> Jorge Gustavo Rocha via Qgis-user <qgis-user at lists.osgeo.org> 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.
>> 
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