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

[Nicolas Cadieux]

Hi Nyall,

That would be a "nice to have"!

Nicolas

On 2022-04-06 9:13 p.m., Nyall Dawson via Qgis-user wrote:
> On Thu, 7 Apr 2022 at 10:26, Greg Troxel <gdt at lexort.com> wrote:
>>
>> Nyall Dawson <nyall.dawson at gmail.com> writes:
>>
>>> In all cases (projected OR geographic) the scalebar logic is:
>>>
>>> 1. Create a horizontal line across the width of the layout map
>>> 2. Calculate the length of that line using great circle/ellipsoidal
>>> calculations, based on the project's ellipsoid settings.
>>> 3. Compare the length of the ellipsoidal line vs the map width to
>>> calculate the corresponding scale
>> That is a very straightforward approach with easy-to-understand
>> semantics.
>>
>>> So qgis scalebar calculations are ALWAYS based on great
>>> circle/ellipsoidal lengths*.
>> And more precisely, a true geodesic between two points varying in x and
>> with constant y in the layout CRS.  For north-up (at least ish)
>> projections, that means a line of constant latitude.
>>
>> For a lat/lon CRS in the layout, at mid latitudes, it's going to mean
>> that the scale in the y axis is smaller by about 110km/70km; a square in
>> lat/lon is taller than it is wide (and plus it's not a square, but
>> that's less obvious).  Equivalently, a square on the ground has a larger
>> difference in longitude than it does in latitude.  Or maybe I have that
>> backward, but using lat/lon in a layout does not preserve shapes.
>>
>> This is what I was trying to get at: a single concept of scale is really
>> only valid for a projection that has the same scale in x and y.  Which I
>> think is true iff the projection is conformal.  And it's only truly
>> valid if the scale doesn't change over the layout, which is probably
>> ~never exactly true, but many projections aim to have it remain close
>> enough to be treated as constant (e.g. UTM).
>>
>> Which is a long way of getting around to "if you are using lat/lon as
>> the axes in your layout, either you are doing something odd on purpose
>> or you probably should rethink your approach."  If you really do mean
>> it, I would find the explanation of why interesting.
> All correct, indeed!
>
> I've been thinking about introducing a new warning when exporting
> layouts, which would look something like this:
>
> 1. For each scale bar, look at the referenced map
> 2. Measure the scale using multiple lines, each across the
> top/middle/bottom horizontally and left/center/right vertically
> 3. Compare these scale values, and if any differ by more than XXX %
> then show a warning to the user. Something like "The scale for map 1
> varies from 1:xxxx to 1:yyyyy across different parts of the map.
> Consider using an alternative form of representing the map scale (such
> as a grid overlay), as the scale bar may be misleading for this map."
>
> What do you think?
>
> Nyall
>
>
>>> * Unless the project itself is set to not use ellipsoidal calculations
>>> and only use planar calculations, in the project properties dialog
>> Presumably those calculations are in the project CRS's xy space, and
>> would typically (when used sensibly) be grid distance in UTM, some State
>> Plane Coordinate System, or similar.
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-- 
Nicolas Cadieux
https://gitlab.com/njacadieux