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I'm using QGIS 3.4.12 and wanted to make a map with the Craig Retro-azimuthal projection.
The projection is not implemented in Proj4 and I couldn't find information about implementing new projections in the QGIS documentation. Is there anyway to "Install" the projection in QGIS?
QGIS uses the PROJ library for coordinate transformations and projections. Unfortunately, the Craig Retro-azimuthal projection is not implemented in the PROJ library.
Implementing it for use in QGIS requires adding the necessary code (written in C) to the PROJ library, compiling the PROJ library, and then compiling a new version of QGIS linked to the new version of the PROJ library. Here are examples of how other projections were added to the PROJ library: Tobler-Mercator projection, Equal Earth projection. Your best bet might be to open a feature request.
The Craig Retro-azimuthal projection is implemented in other programs:
You should first check the web site epsg.io to see if you can find the projection you want. If you can, it is a fair chance that QGIS can load it. If it is not a recognized projection, you can create a custom projection and save it for re-use but only if you know the parameters of the projection you require, or you can derive the parameters from known points. Look up Proj4 specifications on the web to see the sorts of information you need.
See (among others) this post: Defining new custom projections in QGIS?
And this post: Create custom projection using either QGIS or ArcGIS Desktop?
