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

[Greg Troxel]

Nyall Dawson <nyall.dawson at gmail.com> writes:

> 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."

UTM and Lambert Conformal Conic (1SP) have a central scale factor chosen
to make the scale error ratio straddle 1.0, to more or less minimize the
max error.  LCC 2SP is essentially the same but specified differently.

Given that, I'd suggest making the map scale bar be some sort of
geometric mean of horizontal scale, to minimize the max error in a rough
way (without getting too complicated).

Then, when considering large-scale 3857, I'd suggest making the scale
bar be the geometric mean of horizontal and vertical.

So my first suggestion is:

  calculate horiztonal scale top/middle/bottom

  calculate vertical scale left/middle/right

  set scale bar to geometric mean of min and max of the 6 scales

  do this calculation anyway even if there is no scale bar

As for warnings, I don't think we want to warn about 3857 class scale
errors.  So

  If any of the 6 scales are more different from what would be obtained
  with 3857 at 70 N or S, warn if there is a scalebar.  (Maybe that's
  not right, but something like that).

  If any are more different than 1.1x of what Mercator would be, warn,
  even if there is not a scale bar.


I think the big points are

   to warn on the lat/lon non-projection, which is basically always a
   wrong thing to do

   to warn if the scale bar is unreliable at the level of a human using
   a ruler on a paper map


and avoid complaints for commonly-accepted projections for
continent-scale maps.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 194 bytes
Desc: not available
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20220407/63beced9/attachment.sig>