[Qgis-user] Meters at Scale in QGIS 3 -- incorrect behavior IMO

[Mark Newbold]

I'm new to this mailing list. I couldn't find a way to search the list archives, so forgive me if 
this is a duplicate.

The "Meters at Scale" sizing option looks like a nice addition in QGIS 3. I would like to use it as 
a CRS-independent alternative to "map units".

I don't find "Meters at Scale" documented, but what I think it /should/ do is set a size in actual 
meters (on the earth), independent of latitude and independent of CRS.

But that's not what's happening. To duplicate what I'm looking at:

 1. Create a new QGIS project.
 2. Set the project CRS to WGS 84, EPSG:4326.
 3. Make a new temporary scratch layer with MultiLineString / MultiCurve geometry, with CRS of
    EPSG:4326 - WGS 84.
 4. Add a horizontal line feature near 48 degrees north latitude. Style the line as "Simple line"
    with a "Stroke width" of 100 "Meters at Scale".
 5. Zoom in on the line and measure its width with the Measure tool. It measures around 149.5
    meters. Shouldn't it be 100 meters wide? The 149.5 meters width seems to be 100/cos(48°).
 6. Now, just for fun, change the project CRS to WGS 84 / Pseudo Mercator, EPSG:3857. When you hit
    "Apply", the line snaps to a smaller width, which measures as 66.8 meters. It went from too
    large to too small. The 66.8 meters width seems to be 100*cos(48°) -- now multiplied by the
    cosine rather than divided. I don't know where that's coming from.

If I follow the above steps, but draw the line at the equator instead at 48 degrees north latitude, 
its width measures 100 meters, with either project CRS.

I also tried it with point geometry, styled with a Simple marker -- a square sized at 100 "Meters at 
Scale". Same result.

So it seems like "Meters at Scale" isn't working right. For one thing, I think its sizing should be 
independent of the project CRS. I'm hesitant to start using it in my QGIS projects, for fear that 
this is a bug and when it is fixed, my maps will suddenly change, and I'll have to go back through 
them and fix the styles.

Regards,
--Mark

-- 
Mark Newbold
Port Angeles, Washington, USA
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20180430/5448eb91/attachment.html>