My QGIS project has various raster and (complex) vector layers from different sources, in different CRSes. Using reprojection on the fly, I don't have to reproject them. But should I, especially for the layers I've decided are part of my base map set, for performance or other reasons?

In particular, is the performance overhead associated with ongoing on-the-fly reprojection of a shading (DEM-derived) raster layer, or a complex and dense vector layer such as contours or vectorized waterbodies, worth manually reprojecting and saving in the displayed CRS?

Editing to add: my goal is a usable, responsive map that will continue to be improved with adding new information, rather than analysis of embedded GIS data.

closed as too broad by Vince, ahmadhanb, Jochen Schwarze, PolyGeo Mar 4 at 8:29

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If you are simply creating a map for display, then I would not worry about reprojecting the layers. Yes, there is some computing overhead when reprojecting on the fly, especially when you have many layers with differing projections. This used to be a much greater issue with computers in the past.

With that being said, I will Always, without fail, save my layers as another layer with the desired, common, Cartesian projection if I am going to perform spatial operations on or with them. I want to have an apples to apples comparison without the possibility of getting erroneous results due to on the fly reprojection. I also explicitly name the reprojected layers, including either EPSG number, or Coordinate System information.

This is just a habit I have had for over thirty years, and it has worked for me very well.

The advantages to this approach for me are: I keep the original data intact, and unaltered. I get consistent results, as do my clients/employers. It is one less thing for me to troubleshoot if the results are not what I expected.

The only real disadvantages I have found are: More time upfront building the project, hard drive space (Not really an issue, but it could be.) Getting people I work for, and with to adopt this as part of their routine so we are all on the same page.

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