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I am new in QGIS, and I can't quite figure out how grid and scale work. Should my "Project CRS" and my "Grid CRS" be the same or not?

I played around with settings and after some time and managed to get what I think is correct grid (based on some comparison on existing maps). But still, I'm not sure how to set it up on other maps I plan to make and Google isn't helping.

My problem is - my scale is now off. For example, I know that distance from Zagreb to Sisak is around 50km, and based on my map it says around 70/80. Some other examples are even more extreme. What is the problem? Why is my scale now working?

I georeferenced my image perfectly - it matches with the OpenStreetMap layer beneath perfectly - but my scale is still off. Projection is World Mercator.

example

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The difference in results is because of the properties of the Mercator projection. With Mercator, the measurements will only provide reliable results on the vertical (longitude) axis, not horizontal (latitude).

If you need to measure distances on your map, you should choose another projection.

I would suggest the following projections that can be used to measure distance in your region of interest (in no particular order):

  1. Bonne
  2. Equidistant conic
  3. Lambert conformal conic
  4. Albers

I have checked and in all of them, the distance between Zagreb and Sisak is about 47 km.

As to the grid lines - yes, they should, in theory, be in the same CRS as the map, but any GIS software will reproject it on the fly for you. However, do not expect the lines to be straight vertical and horizontal - the chosen projection will dictate the orientation of latitudes and meridians.

  • Oh, never thought about that. How is that than the world maps in Mercator projection have working scale? And what would be best projection to use if I want accurate distance on my map for this kind of scale, and also for smaller scale? – Filip Frankopan Jul 9 at 21:08
  • Wikipedia has a good introduction to map projections. Have a look at the section about Equidistant projections, and also the section called Which projection is best? – csk Jul 9 at 21:25
  • Tnq! Will look into it. And what about grids? Do I always use WGS 1984 (EPSG:4326) CRS, or do my map CRS and grid CRS must be same? – Filip Frankopan Jul 9 at 21:30
  • Equidistant projections (contrary to their name) do not provide least distortions in measuring distances. The distortion is 0 between the 'focal points' of the projection, but it can be significantly higher in other places on the map. [This link] (observablehq.com/@toja/five-map-projections-for-europe) explains about several traditionally used projections for your region. I will them to the answer. – Mecklenburg Pomeranian Jul 9 at 21:46

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