I am making a map using an image of another map. This map exists in a completely flat space, and therefore I do not want to georeference the map at all. Instead, I want to tell arcGIS to use a X/Y coordinate system that I define myself (the origin will be somewhere in the middle of the map). After doing this, I will input feature points on certain coordinates that I already have saved. However, I am having a great deal of difficulty figuring out how to create my own XY coordinate system which is independent of Earth-based projections, and I am new to GIS so any help with this puzzle would be greatly appreciated!