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

[Greg Troxel]

Jorge Gustavo Rocha via Qgis-user <qgis-user at lists.osgeo.org> writes:

> I need you help to understand how scale works with geographic
> coordinate systems.

A good question.

> This is my use case:

It seems like your minimal example to show what doesn't work like you
think it should, instead of what you want to do, but that's good -
simpler is better for discussing.

> I've draw a polygon with 1000m width and height, and another with
> 1500, width and height, sharing the upper left corner.

Presumably you have some project CRS, and a layer in which you drew the
polygon and it has a layer CRS.  You then say width and height, which is
a map concept more than an on-the-ground concept.  But I'll assume you
are using UTM or similar and have a square in UTM coordinates.

Presumably also the 1000m is measured in your layer or project CRS.

> 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

That makes sense.

> 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.)

> 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.

I would expect Scale to relate units in the project CRS to dimensions on
the layout.  But, there's perhaps some implicit conversion to meters
from geographic coordinates.  I wonder if that's happening, and if so if
it's a bug -- it feels like a bug to me.

I also wonder if geographic coordinates somehow use different scaling on
each axis to make it seem more conformal than it is.

Overall, it feels like a bug to use geographic coordinates in a print
layout.