Bug report #17239
Reported map scale representative fraction is incorrect
|Affected QGIS version:||2.18.2||Regression?:||No|
|Operating System:||macOS 10.10.5 (14F27)||Easy fix?:||No|
|Pull Request or Patch supplied:||No||Resolution:||no timely feedback|
|Crashes QGIS or corrupts data:||No||Copied to github as #:||25137|
I first noticed this when trying to match up screenshots taken off of Google Maps, OpenStreetMap, and Mapzen with screenshots out of my QGIS project.
When referencing http://kelsocartography.com/blog/?p=2407 I can see web map zoom 5 is 1:18,489,297 map scale.
When I type that scale into the QGIS document I instead get something that is NOT at that zoom. It should be exactly zoom 5 and 1:18,489,297 map scale.
I can always fix this by scaling the QGIS screenshot up by 133%. That fraction reminded me of 96/72. I remember way back in Windows 95 days that OS used a pixels per inch (ppi) of 96 while most everyone else uses 72 ppi.
But when I set my scale to 1:13,901,727 instead for zoom 5 (applying the 133% ratio) then they match up correctly!
Tom Chadwin @tomchadwin looked into this on Gitter and suggests that 96DPI is being suggested as a default in a bunch of places, but didn't know enough to investigate further:
I suspect there may be other HiDPI issues at play here (as Anita Graser @anitagraser points to): #11103
But since the map scale is fundamentally broken and a critical part of "truth" in mapping, this seems like an important piece to track separately.
For instance, I already enable the QGIS app to run into HiDPI already by toggling the Finder bit to force it to run in HiDPI and it works fine. It's not a 72 ppi versus 144 ppi issue, it's the bad math with 96 ppi.
Please let me know if I can add additional details :) I use QGIS daily and it's a pleasure to use, keep up the good work!