Most GIS Software doesn't have a built-in map projection for Middle Earth, or Pern, or any other fictional world that we want to create a map for. But they do give us some good tools to create one. The first thing to consider is how big is the world you want to map. Is is the size of Earth - that makes it really easy, just use an Earth projection. More likely it is a little smaller. I find a lot of authors when they come up with their own maps of their worlds don't want to create such a large space that they have trouble filling it all, so they tend to go smaller. In tools like ArcGIS or QGIS they have predefined coordinate systems available for all of the planets and moons in our solar system. A good starting point is Mars which is about half the size of Earth. You can copy the coordinate system and rename it. Creating a new Datum and Projection are pretty challenging tasks and could potentially involve a lot of math, so I'd personally suggest you stick to the predefined choices. Here are the radii in kilometres of your most likely candidates:
If you do want to create your own datum, that can be done too. I've selected a custom projection and decided to make a world halfway between Mars and Earth in size, at around 4500 km radius:
You will probably want a Projected Coordinate system for making maps, as well as your new Geographic Coordinate System so you can create one of those too. This one may take a little experimentation to get it to appear the way you want to once you have some data created.
That's all for today - next time we'll start creating data.
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