Help with QGIS and georeferencing

[Skalimoi]

Hey there,

I'm trying to create a QGIS project with my world map in order to calculate distances and coordinates correctly for my future maps. I've used G.Projector before, but I've come to realise that I need to keep every image I use in high resolution, and every other program I use distorts my resources. So well, I have the following:

- A complete equirectangular map of my planet (image below):
total.png
- Maps of specific continents and regions with more detail, also in equirectangular projection (as they're just cuts from the original map)

As my planet is of a different size than Earth, I know that I need to make a custom CRS for it. I know the total circumference of the planet, and the radius on the equator and the poles (semi/major, semi/minor). So far I've come up with this:

+proj=eqc +a=3509525 +b=3468220 +datum=WGS84 +lat_ts=0 +lon_0=0 +rf=800 +units=m +no_defs

However, I don't think it's correct, but I don't know how to fix it either. I need help with the following:

- How can I make a custom CRS for my planet that actually works?
- How do I georeference the whole equirectangular map then? And how would I georeference the other sections of my map?
- If I were to make a map of a certain region, would I need more custom CRS's in case I wanted to use a different projection for it?

I think I'm going crazy over this, so any help would be greatly appreciated!

[Redrobes]

I don't know much about CRS on GQIS but I was under the impression that preexisting datums like WGS84 define ellipsoids which include the world size as part of them. I found this link:

https://gis.stackexchange.com/questi...tasy-world-map

which seem to show the CRS without the datum in it. But generally I would be interested in what other people comment on this topic too.

[Skalimoi]

Thanks for your reply, Redrobes! Following that link I managed to create a custom CRS that actually works, by deleting the parameters "+datum=WGS84", "+rf=800", and replacing the +a and +b with +R, which is the radius of the custom ellipsoid. Then, in order to georeference the equirectangular map, I defined the four corners of the map this way:

Upper left: -180, 90
Lower left: -180, -90
Upper right: 180, 90
Lower right: 180, -90

The resulting CRS doesn't have a predefined datum but is more than enough for any worldbuilding shenanigans.

Just in case anyone has the same problem.

[morne]

This is great! My world is half the size of Earth, and I've hadn't ever figured out how to specify a custom world radius for my projections. If you want a named projection in QGIS to match your custom CRS, you can use:

gdalsrsinfo --single-line map.tif

And copy the WKT string for your CRS.

In QGIS, go to Settings -> Custom Projections. Give your CRS a name and paste in the WKT string.

You can now set your layer's CRS by right clicking the layer -> Set CRS -> Set Layer CRS. Search for your Custom CRS in the Filter box.

I am using QGIS 3.14 for this by the way.