Bug report #20489
QGIS composer export - issue with projection, scale & scalebar ?
|Affected QGIS version:||2.18.25||Regression?:||No|
|Operating System:||Easy fix?:||No|
|Pull Request or Patch supplied:||No||Resolution:||end of life|
|Crashes QGIS or corrupts data:||No||Copied to github as #:||28309|
I am facing what seems to be an issue, or probably something well known by anyone but me...
I have created 2 very simple QGIS projects (attached with data)
- 1 vector polygon grid 1km X 1km (GeoJSON), created with the Vector processing tool.
- 1 OpenStreetMap TMS background layer (EPSG:3857)
- 1 A4 composer with a map taking full size, and a scale bar
- 1 project in EPSG:3857 & its brother in EPSG:2154 (French proj)
- In the composer, the scale bar does not fit with the 1km grid, in both projects.
- After defining the same scale (1:10 000) in the composers, the view are really different in the 2 projects (see attached PDF)
I know projections are biased representations of the reality, but as a user perspective, this 2 behaviours really seem like issues. Every time a user prints a map and take the A4 paper, he is condident he will be able to use his ruler to measure things and convert to real distances according to the scale. Or report N times the scale bar segments to get distances.
I am like someone who just discovers the Earth is not flat...
NB: I have tested this behaviour only in 2.18.
#2 Updated by Michael Douchin over 2 years ago
I have no solution at present, because I first need to understand what is considered correct or not in the GIS community, and perhaps I missed something here...
In my humble opinion:
- a Scale bar in the composer should always be trusted on a paper printed export. Even if the printer has rescaled the layout (for example by adding margins), the scale bar should fit completely with the data underneath, for any projection: in the attached projects, 1 km in the scalebar must fit exactly with 1 side of a 1km square. Just like the distance measure tool in QGIS canvas does (when I measure a distance in the QGIS project between 2 grids, in the 3857 and 2154 projects, QGIS shows me 1000 m, which is correct and expected).
- if the printer machine does not rescale the page (by adding margins), the user should trust the scale of the printed map (physical map on paper, I mean ;-) ), if the projection has been correctly chosen. If I have printed a 1:10 000 map, 1cm on the A4 paper map should always represent 10 000 cm = 100 metres. I know I cannot trust a scale in a screen, because of the screen resolution, but I think GIS users believe they can trust a scale in a printed map (at big scales such as 1:10000). It can be different for the mercator EPSG:3857 version, because this projection is worlwide and known for not respecting distances but angles. But the French IGN projection is more local, and the result PDF seems wrong, probably because of the scale bar.
I am completely aware that the Earth is not flat and that the act of projecting data has consequences. It is very clear to me that measuring the width of Antartica in a Mercator map with a ruler and multiplying it by the scale will lead to very wrong distance.
In the 2 attached PDF we have a "real world" square grid layer visible in the layout, and this rendering compared to the scale bar rendering with the background OSM layer make me feel as a chicken in front of a knife (French expression...)
#3 Updated by Michael Douchin over 2 years ago
- File export_qgis_explanations.pdf added
Ok, I finally understand the "issue". There is no QGIS issue, but a lack of explanation I think.
- scale bar segments in "maps units" are calculated in the QGIS canvas SCR, in cartesian Wich is ok to match data in the same projection, but can feel awkward.
- scale bar segments in meter are calculated by using the spheroid, wich leads to correct and reliable representation (IMHO)
- 1 grid generated in EPSG:3857 in grey
- 1 grid generated in EPSG:2154 in red
- Map canvas in EPSG:3857
I cannot tell if the 1:10000 scale, as set in the map composer option, and visible in the numeric scale, is calculated in map units or not ?
#5 Updated by Giovanni Manghi about 2 years ago
- Resolution set to end of life
- Status changed from Feedback to Closed
End of life notice: QGIS 2.18 LTR
QGIS 3.4 has recently become our new Long Term Release (LTR) version. This is a major step in our history – a long term release version based on the massive updates, library upgrades and improvements that we carried out in the course of the 2.x to 3x upgrade cycle.
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