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Is it recommended to always project layers before printing a map?

I use QGIS and if I create a Print Layout often the map appears to be visually fine even if the Project Projection System is a Geographic one (e.g. WGS84).

Is this data automatically projected in GIS software before print or do I have a distorted map in the end?

10

You always get a distorted map, that how projection work (by distorting a 3D surface to project it to a 2D surface).

You dont absolutely need to choose any projection : if your data are in geographic coordinate QGIS will use a projection to display it (see this for more detail), But depending on what you are mapping it may be better to choose yourself the projection (some preserve area, some preserve angle other try to balance all deformation to be mostly good looking).

As being said in MrXsquared answer, if you want your map to be in a specific projection you wont need to reproject your data, you could just change the project CRS or the map item CRS in print composer and QGIS will project the data on the fly to give you the desired output.

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(adding just to flesh out details alluded to in other answers)

QGIS uses a coordinate system for any map you show (canvas) or print (layout). Based on your settings under Settings / Options / CRS, this may default to a projected CRS you specify, the CRS of the first layer you add, or -- as a last resort -- the unprojected CRS EPSG:4326 - WGS 84, which is just x=longitude and y=latitude. So if you don't change anything, and don't (re)project any layers, you will end up with this implied "choice", whatever it may be.

All projections create distortions. The "Geographic", i.e. (non)-projection EPSG:4326, in particular, when used as (x,y) coordinates for a visual map, tremendously elongates apparent distances E-W far away from the equator. So it's unlikely to be a good choice for anything other than a very small-scale map right at the equator if you care at all about whether it "looks like what you experience on the ground". You should choose something better as the project CRS and/or the CRS for the map on your layout.

A word on terminology that may be causing confusion here. QGIS cheerfully reprojects layers on the fly, so (as others have said) you don't need to create a copy of a layer reprojected to your desired (project) CRS just to work on a good map, or even to do basic geospatial analysis. However, projections impact measurement (length, area, scale...) and projection mismatches can get in the way of processing involving multiple layers, especially if you use algorithms from other platforms that may be imperfectly integrated. So I would manually reproject if you're planning to work with a layer in a meaningful way (vector or raster).

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No, there is no need to do that. Your data is reprojected on the fly to your Projects CRS (you can set it at the bottom right corner). For visualization purposes it is not necessary to reproject your data - this would be different if you wish to process and work with your data.

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Printing a map is a cartography action, and there are cartography standards, and yes, you must adopt a projected CRS for the canvas or the layout before printing a map.

It is also a standard that you must put a linear scale in your map, and it is a big mistake to put linear scales in geographic CRSes. A geographic CRS is not a projected CRS, it does not involve any linear dimension.

If you must to distribute data you can do it in a geographic CRS. If you must to show it before the distribution, I don't like it but you can show it with a pseudo Plate Carrée conversion. But if you will print a map, please use a projected CRS, clarify in the map the datum and projection used, and put a linear scale on it.

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